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EM 1110-2-1908 30 June 1995 US Army Corps of Engineers ENGINEERING AND DESIGN Instrumentation of Embankment Dams and Levees ENGINEER MANUAL

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Page 1: USACE instrumentation manual

EM 1110-2-190830 June 1995

US Army Corpsof Engineers

ENGINEERING AND DESIGN

Instrumentation of EmbankmentDams and Levees

ENGINEER MANUAL

Page 2: USACE instrumentation manual

AVAILABILITY

Copies of this and other U.S. Army Corps of Engineerspublications are available from National Technical InformationService, 5285 Port Royal Road, Springfield, VA 22161. Phone(703)487-4650.

Government agencies can order directlyu from the U.S. ArmyCorps of Engineers Publications Depot, 2803 52nd Avenue,Hyattsville, MD 20781-1102. Phone (301)436-2065. U.S.Army Corps of Engineers personnel should use Engineer Form0-1687.

UPDATES

For a list of all U.S. Army Corps of Engineers publications andtheir most recent publication dates, refer to Engineer Pamphlet25-1-1, Index of Publications, Forms and Reports.

Page 3: USACE instrumentation manual

DEPARTMENT OF THE ARMY EM 1110-2-1908U.S. Army Corps of Engineers

CECW-EG Washington, DC 20314-1000

ManualNo. 1110-2-1908 30 June 1995

Engineering and DesignINSTRUMENTATION OF EMBANKMENT DAMS AND LEVEES

1. Purpose. This manual provides guidance to Corps of Engineers personnel who are responsible formonitoring and analyzing embankment dams and levees.

2. Applicability. This manual applies to HQUSACE elements, major subordinate commands (MSC),districts, laboratories, and field operating activities (FOA) involved with planning, design, construction,installation, monitoring, analysis, and maintenance of instrumentations systems in Corps of Engineersembankment dams and levees.

FOR THE COMMANDER:

____________________________________________________________________________________This manual supersedes EM 1110-2-1908 Part 1 dated 31 Aug 1971 and Part 2 dated 19 Nov 1976.

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DEPARTMENT OF THE ARMY EM 1110-2-1908U.S. Army Corps of Engineers

CECW-EG Washington, DC 20314-1000

ManualNo. 1110-2-1908 30 June 1995

Engineering and DesignINSTRUMENTATION OF EMBANKMENT DAMS AND LEVEES

Table of Contents

Subject Paragraph Page Subject Paragraph Page

Chapter 1IntroductionPurpose . . . . . . . . . . . . . . . . . . . . . . 1-1 1-1Applicability . . . . . . . . . . . . . . . . . . . 1-2 1-1References . . . . . . . . . . . . . . . . . . . . 1-3 1-1Focus . . . . . . . . . . . . . . . . . . . . . . . . 1-4 1-1Approach . . . . . . . . . . . . . . . . . . . . . 1-5 1-2Scope. . . . . . . . . . . . . . . . . . . . . . . . 1-6 1-2Format of the Manual . . . . . . . . . . . . 1-7 1-2Use of the Manual . . . . . . . . . . . . . . 1-8 1-3Implementation of an Instrumentation

Program . . . . . . . . . . . . . . . . . . . . . 1-9 1-3

Chapter 2Behavior of Embankments and AbutmentsIntroduction . . . . . . . . . . . . . . . . . . . 2-1 2-1Basics of Soil Structure. . . . . . . . . . . 2-2 2-1Groundwater Level and Pore Water

Pressure. . . . . . . . . . . . . . . . . . . . . 2-3 2-2Time Lag in Groundwater

Observations. . . . . . . . . . . . . . . . . . 2-4 2-4Mechanisms That Control Behavior . . . 2-5 2-5

Chapter 3Instrumentation Concepts, Objectives,and System Design ConsiderationsIntroduction . . . . . . . . . . . . . . . . . . . 3-1 3-1Unique Characteristics of Geotechnical

Engineering . . . . . . . . . . . . . . . . . . 3-2 3-1Characteristics of Geotechnical

Instrumentation. . . . . . . . . . . . . . . . 3-3 3-1Instrumentation Objectives. . . . . . . . . 3-4 3-1Basic Instrumentation Concepts. . . . . . 3-5 3-2Instrumentation System Planning. . . . . 3-6 3-3Procurement . . . . . . . . . . . . . . . . . . . 3-7 3-7Personnel Qualifications and

Responsibilities. . . . . . . . . . . . . . . . 3-8 3-7

Chapter 4Summary of Measurement MethodsIntroduction . . . . . . . . . . . . . . . . . . . . 4-1 4-1Instrumentation Measurement

Methods . . . . . . . . . . . . . . . . . . . . . . 4-2 4-1Measurement of Piezometric Pressure . . . 4-3 4-4Measurement of Deformation. . . . . . . . 4-4 4-9Measurement of Total Stress. . . . . . . . . 4-5 4-16Measurement of Temperature. . . . . . . . 4-6 4-18Measurement of Seismic Events. . . . . . 4-7 4-19Measurement of Seepage Emerging

Downstream . . . . . . . . . . . . . . . . . . . 4-8 4-22

Chapter 5Automation ConsiderationsIntroduction . . . . . . . . . . . . . . . . . . . . 5-1 5-1Applications . . . . . . . . . . . . . . . . . . . . 5-2 5-1Advantages and Limitations of

Automation . . . . . . . . . . . . . . . . . . . . 5-3 5-1Description of Automated Systems. . . . . 5-4 5-2Planning a System. . . . . . . . . . . . . . . . 5-5 5-4Designing an Automation System. . . . . 5-6 5-5Implementation . . . . . . . . . . . . . . . . . . 5-7 5-5Data Acquisition, Management,

Analysis, and Reporting. . . . . . . . . . . 5-8 5-5Maintenance . . . . . . . . . . . . . . . . . . . . 5-9 5-5

Chapter 6InstallationIntroduction . . . . . . . . . . . . . . . . . . . . 6-1 6-1Personnel Issues. . . . . . . . . . . . . . . . . 6-2 6-1Contracting Issues. . . . . . . . . . . . . . . . 6-3 6-1Instrumentation of New Structures. . . . . 6-4 6-1Instrumentation of Existing

Structures. . . . . . . . . . . . . . . . . . . . . 6-5 6-2Drilling Fluids . . . . . . . . . . . . . . . . . . . 6-6 6-2General Installation Procedures. . . . . . . 6-7 6-2

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Subject Paragraph Page Subject Paragraph Page

Installation Procedures forPiezometers in Boreholes. . . . . . . . . 6-8 6-4

Installation Procedures for OtherInstruments. . . . . . . . . . . . . . . . . . . 6-9 6-5

Backfilling Boreholes. . . . . . . . . . . . . 6-10 6-5Protective Housings. . . . . . . . . . . . . . 6-11 6-5Documentation . . . . . . . . . . . . . . . . . 6-12 6-6

Chapter 7Data Management, Analysis, and ReportingIntroduction . . . . . . . . . . . . . . . . . . . 7-1 7-1Data Management. . . . . . . . . . . . . . . 7-2 7-1Engineering Analysis. . . . . . . . . . . . . 7-3 7-5Formal Reporting and Documentation . 7-4 7-6

Chapter 8Instrument MaintenanceIntroduction . . . . . . . . . . . . . . . . . . . 8-1 8-1Importance of Maintenance and

Recalibration. . . . . . . . . . . . . . . . . . 8-2 8-1Recalibration During Service Life. . . . 8-3 8-2Maintenance During Service Life. . . . . 8-4 8-2Instruments Requiring Specific

Maintenance. . . . . . . . . . . . . . . . . . 8-5 8-3Automation Equipment. . . . . . . . . . . . 8-6 8-5

Chapter 9Continual Reassessment forLong-Term MonitoringIntroduction . . . . . . . . . . . . . . . . . . . 9-1 9-1

Long-term vs Construction-Related Instruments . . . . . . . . . . . . . 9-2 9-1

Primary, Secondary, and TertiaryInstruments. . . . . . . . . . . . . . . . . . . . 9-3 9-1

The Evolving InstrumentationSystem. . . . . . . . . . . . . . . . . . . . . . . 9-4 9-1

Steps for Continual Reassessment. . . . . 9-5 9-1Selection of Parameters to Monitor. . . . 9-6 9-3Management. . . . . . . . . . . . . . . . . . . . 9-7 9-4

Appendix AReferencesRequired Publications. . . . . . . . . . . . . . A-1 A-1Related Publications. . . . . . . . . . . . . . . A-2 A-1

Appendix BDrilling MethodsIntroduction . . . . . . . . . . . . . . . . . . . . B-1 B-1Hollow Stem Auger Drilling . . . . . . . . . B-2 B-1Cable Tool Drilling . . . . . . . . . . . . . . . B-3 B-1Direct Mud (Water) Rotary Drilling. . . . B-4 B-1Air Drilling Systems . . . . . . . . . . . . . . B-5 B-1Core Drilling . . . . . . . . . . . . . . . . . . . . B-6 B-4Dual-Wall Reverse Circulation

Drilling . . . . . . . . . . . . . . . . . . . . . . B-7 B-4Wash Boring . . . . . . . . . . . . . . . . . . . B-8 B-4Other Drilling Methods. . . . . . . . . . . . . B-9 B-4

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Chapter 1Introduction

1-1. Purpose

This manual provides guidance to Corps of Engineers(CE) personnel who are responsible for monitoring andanalyzing embankment dams and levees.

1-2. Applicability

This manual applies to HQUSACE elements, major subor-dinate commands (MSC), districts, laboratories, and fieldoperating activities (FOA) involved with planning, design,construction, installation, monitoring, analysis, and main-tenance of instrumentations systems in Corps of Engineersembankment dams and levees.

1-3. References

a. A variety of sources was used to compile theinformation presented herein, including publications ofprofessional societies, and guidance from consultants andfrom that developed by the Corps of Engineers and otherfederal agencies. References specifically applicable tochapter topics are included in that respective chapter text.A listing of all references is included as Appendix A.

b. Permission to use copyrighted material wasobtained. The following sources are acknowledged.

(1) Figures 2-2, 2-3, 4-1 through 4-12, 4-16 through4-26, 6-1, B-5, and B-6, Tables 4-1 through 4-4 and 9-1,a portion of the text in Chapter 4, and the lists in Sec-tions 6-7c and 6-12 were obtained from the followingsource:

Dunnicliff, John. 1988. Geotechnical Instrumen-tation for Monitoring Field Performance. Copy-right ©1988 by John Wiley & Sons, Inc.Reprinted by permission of John Wiley & Sons,Inc.

(2) Figure 2-4 was obtained from the followingsource:

______________________________This manual supersedes EM 1110-2-1908 Part 1 dated31 Aug 1971 and Part 2 dated 19 Nov 1976.

Terzaghi, K., and Peck, R. B. 1967. SoilMechanics in Engineering Practice. 2nd.Copyright © 1967 by John Wiley & Sons, Inc.Reprinted by permission of John Wiley & Sons,Inc.

(3) Figure 4-31 was obtained from the followingsource:

GEONOR, Inc. Reprinted by permission ofGEONOR, Inc.

(4) Figures B-1, B-2, B-3, B-4, B-8, and B-9 wereobtained from the following source:

Handbook of Suggested Practices for the Designand Installation of Ground Water MonitoringWells. 1989. Reprinted by permission of theNational Ground Water Association.

(5) Figure B-7 was obtained from the followingsource:

Driscoll, Fletcher G. 1986. Groundwater andWells. 2nd. Copyright © Johnson Division,SES, Inc. Reprinted by permission ofWheelbractor/ Johnson Screens.

(6) Tables 3-1 and 3-2 were obtained from the fol-lowing source:

Dunnicliff, John. 1990. Twenty-Five Steps toSuccessful Performance Monitoring of Dams.Volume 9, Number 4. Hydro Review. Copy-right, HCI Publications, 1990. Reprinted bypermission of HCI Publications.

1-4. Focus

This revision focuses on issues that are of current concernin the Corps’ mission of dam safety which includes moni-toring embankment dams and levees. Design and con-struction of new projects have been decreasing. Themajority of Corps dams have been instrumented, and thebasic instrumentation concepts for monitoring seepage,pressure, and movement have been emphasized over theyears in several CE regulations, directives, and programs.Geotechnical personnel are generally familiar with thebehavior of instrument systems for which they are respon-sible. The current challenge for personnel responsible forembankment dam and levee safety is increased, and

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continuous attention must be paid to operation and main-tenance of existing embankment structures and instrumentsystems. Therefore, a significant amount of engineeringjudgment must be applied to major rehabilitation, assess-ment of system performance, replacement, maintenance,and other situations that have become unique to eachembankment structure and instrument system. Thismanual provides information and guidance that is neededfor modifying existing systems as well as for new systeminstallation for new construction.

1-5. Approach

Every instrument system is unique and a significantamount of engineering judgment must be applied to each.It is not possible to provide blanket instructions and/orspecific procedures that are universally applicable to everysituation that may be encountered. Therefore, this manualemphasizes the use of judgment rather than providing arecipe to follow. Whether analyzing data to determine thecompetency of a sensor, planning a new system for arehabilitation project, or selecting a sensor for retrofittinga system with automation, responsible personnel must takethe initiative to observe the conditions that require atten-tion, to recognize the need for some action to be taken, toapply available information and properly assess whataction is appropriate, and, finally, to follow through withimplementing the efforts that are necessary.

1-6. Scope

The manual addresses all aspects of instrumentationincluding the traditional methods of monitoring, theimportant geotechnical concepts that must be understood,and the growing concerns of rehabilitation, replacement,and maintenance. New initiatives such as automation arealso addressed. The manual also addresses features thatinfluence the performance of embankment dams andlevees. Therefore, information presented can be appliedto abutments, foundations, and the reservoir rim. Instru-mentation of concrete and steel structures is notaddressed. Refer to EM 1110-2-4300 andTR REMR-CS-5 for guidance for these features. Instru-mentation for research and investigative purposes also isnot specifically included.

1-7. Format of the Manual

The manual begins with a description of geotechnicalaspects of embankment dams and levees, and brieflydiscusses important concepts of applying instrumentation.This is followed by addressing various methods of moni-toring, the potential for automation, and the installation

that is appropriate. The last portion of the manualaddresses operation and maintenance issues including datamanagement and analysis, maintenance, and continualreassessment over the life of the project. For quick refer-ence, a summary of each successive chapter follows:

a. Chapter 2 - Behavior of Embankments and Abut-ments. The chapter begins by discussing the geotechnicalaspects of soil behavior, including soil structure, the pres-ence and buildup of pressures and stresses, and the effectsof them on the embankment materials. It also identifiesspecific mechanisms that are inherent to embankment damperformance and that are key to accessing behavior. Thischapter contains fundamental information which is essen-tial to understanding the parameters that are monitoredand the environment in which instruments perform.

b. Chapter 3 - Instrumentation Concepts, Objec-tives, and System Design Considerations.This chapterbuilds on geotechnical knowledge, identifies parameters inembankment structures that may require monitoring, andoffers an approach to planning and designing systems toachieve that purpose. It includes important characteristicsof instrument systems that must be considered and recom-mendations for designing them such that all criticalaspects of the process are addressed. The suggestedqualifications of the personnel that are responsible forimplementing various related instrumentation tasks arealso presented.

c. Chapter 4 - Summary of MeasurementMethods. The most commonly used devices and technol-ogies for embankment monitoring are discussed. Theperformance characteristics are described as well as thepreferred uses and applications. This information isneeded for the selection of appropriate sensors for variousapplications.

d. Chapter 5 - Automation Considerations.Currentguidance for automating new or existing systems is givenin this chapter. Automation presents an increasinglyviable alternative to traditional manual instrument sys-tems. Certain characteristics of automation, however, areunique and require special attention. Information in thischapter should be kept in mind as other instrumentationaspects (e.g., data analysis, installation, sensor selection,and maintenance) in other chapters are discussed.

e. Chapter 6 - Installation. This chapter discussesseveral issues that are critical to obtaining reliable andfunctional instrument systems including contractor quali-fications, quality control, instrument protection,

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acceptance tests, drilling methods, placement of sensors,seals, and filters, and documentation.

f. Chapter 7 - Data Management, Analysis, andReporting. This chapter stresses the importance of acquir-ing valid and complete field information, and the thoroughinterpretation and meaningful presentation of the data.Other related topics include data validity, selection ofscales for plotting, and use of data to prompt the mainte-nance of instruments. It also contains valuable sugges-tions for various ways of analyzing raw as well asprocessed data and managing computerized databases.

g. Chapter 8 - Instrument Maintenance.Testing andcalibration of components are critical to instrument perfor-mance and data validity. Chapter 8 suggests specific basicattention that is required for portable, retrievable, andembedded components of the instrument system. Histori-cal documentation, maintenance programs, as well asinstruments that require more than the basic maintenanceof portable, retrievable, and embedded components arealso discussed.

h. Chapter 9 - Continual Reassessment for Long-Term Monitoring. The performance of embankment damsand levees must continue for the life of the project.Therefore, comprehensive programs must be establishedto assure continued attention in the overall context of thebehavior of the embankment structures. Changes in per-formance with time, remedial modifications to the struc-ture in the course of useful life, and the inevitable damageor loss of operating instruments requires significant judg-ment regarding abandonment, rehabilitation, replacement,or upgrade of instruments. This chapter addresses theseissues.

1-8. Use of the Manual

There are three primary uses of this manual: planningand design of instrument systems, continuous use ofinstrument systems, and making decisions regarding the

modification or replacement of instrument systems. Plan-ners and designers are reminded of theoretical and practi-cal concepts and are assisted with acquiring a reliable andappropriate system. Personnel responsible for the use ofthe systems are provided guidance regarding dataretrieval, analysis, and maintenance. Dams and levees,and associated instrument systems, require increasedattention with age and use. Practical approaches to modi-fication and replacement are provided.

1-9. Implementation of an InstrumentationProgram

An instrumentation program is a comprehensive approachthat assures that all aspects of instrumentation from plan-ning and design through maintenance and rehabilitationare commensurate with the overall purpose. An instru-mentation program is an important contributing effort tothe much larger concept of dam safety. As such, the char-acteristics of the instrumentation program must be consis-tent with the other entities of dam safety such as damsafety training, emergency response, periodic inspections,remedial studies, and structure modifications. In the over-all context of understanding and responding to the behav-ior of the dams and levees, particularly with agingstructures and increasing maintenance, it is most impor-tant to apply the quality and quantity of attention that isrequired. Qualified personnel, quality equipment, andtimely information and assessment must be encouragedand supported. Without this level of attention and com-mitment, the importance of all entities of dam safety willdeteriorate, wasted effort and expense will result, andareas that depend on input from the instrumentation pro-gram will also suffer.

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Chapter 2Behavior of Embankments andAbutments

2-1. Introduction

A basic familiarity with geotechnical concepts is neces-sary to understand the application of instrumentationdevices and analyses of instrumentation data. Key geo-technical aspects of embankment dam and levee behaviorare described in this chapter. These geotechnical aspectsare related primarily to pore water pressure and deforma-tion. For a detailed treatment of these geotechnical top-ics, the reader is referred to other engineer manuals andgeotechnical textbooks (e.g., Terzaghi and Peck 1967, andHoltz and Kovacs 1981). For detailed information ondesign and construction considerations refer to EM 1110-2-2300 and EM 1110-2-1913.

2-2. Basics of Soil Structure

a. Soil structure. Soil as it exists in the ground oras compacted in embankment fills consists of solid parti-cles between which are spaces calledvoids, pores, or porespaces. The pore spaces may be filled with either gas,usually air, or water. When all the voids are filled withwater, the soil issaturated. If any gas is present in thepore spaces, the soil is calledunsaturated. The termpartially saturatedor vadose zone is used by some whenreferring to unsaturated soils. Diagrams of both saturatedand unsaturated soil structure are shown in Figure 2-1.

b. Types of soils.Soils are categorized into twobroad groups:cohesionless soil and cohesive soil.Cohesionless soils are those soils that consist of particlesof rocks or minerals having individual grains usuallyvisible to the eye. These materials, sands and gravels, aregranular and nonplastic, meaning that they do not sticktogether when unconfined, and have little or no strengthwhen air dried. Usually these soils have not been alteredby chemical decomposition. Inorganic silt is fine grainedwith little or no plasticity and is considered a cohesionlessmaterial. Cohesive soils such as clays consist of fine-textured materials with microscopic and submicroscopicparticles resulting from chemical decomposition of rocks.These materials have some strength when unconfined andair dried. Water affects the interaction between the min-eral grains and gives plasticity or cohesion between theparticles.

c. Stress and pressure.In geotechnical engineer-ing, the termsstressandpressureare defined as force perunit area, with typical units of pounds per square inch(lb/in.2) or pascals (Pa). Usually, pressure is a generalterm for force per unit area and stress is the force per unitarea that existswithin a mass. In the practice of geotech-nical engineering, these terms are sometimes used inter-changeably. The total force applied to or acting within agiven area is known as thetotal stress. That componentof the total stress that is transmitted by grain-to-graincontact within the soil mass is called theeffective stress.The component of the total stress transmitted through thepore water is called thepore water pressure, or the neu-tral stress. The total stress consists of the sum of theeffective stress and the pore water pressure. This rela-tionship is known as Terzaghi’s principle of effectivestress. In unsaturated soils, a component of the totalstress is transmitted to the pore gas portion of the voidsand is called thepore gas pressure. If pore gas pressureexists it will always be greater than the pore water pres-sure (Dunnicliff 1988). All the components of the totalstress are applied over the same area as the total stress.

d. Consolidation. When a load is applied orincreased on saturated soil layers, the increased force(total stress) is initially carried by the pore water pressure.The process of transferring the total stress to effectivestress and decreasing the pore water pressure by squeez-ing out the water is known asconsolidation. Figure 2-2illustrates what happens to the pore water pressure, effec-tive stress, and volume of the soil as a total stress isapplied. The change in volume shown in Figure 2-2b,that occurs as the total stress is transferred from the porewater pressure to the effective stress, generates verticaldeformation that results insettlement. The amount bywhich the pore water pressure exceeds the equilibriumpore pressure is known as theexcess pore water pressure.The decrease of the excess pore water pressure is calleddissipation and is a function of thepermeability of thematerial. Settlement results from the dissipation of excesspore water pressure and reorientation of the soil particlesdue to the additional load. Permeability is a measure ofthe rate at which water can move through the soil anddetermines how rapidly settlement occurs. When a soilhas never been subjected to an effective stress greaterthan the existing overburden pressure, the soil isnormallyconsolidated. A soil that has been subjected to an effec-tive stress greater than the existing overburden pressure isan overconsolidated soil. Soils that have been subjectedto glacier loadings are generally overconsolidated soils.

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Figure 2-1. Saturated and unsaturated soil structure

Movement of the soil to resist sliding can also generateexcess pore water pressure. The effective stress is relatedto the ability of the soil to resist sliding and itsshearstrength. Thus, gains in shear strength during the consol-idation process can be monitored by measuring pore waterpressure. An example of this is the monitoring of porewater pressure as levees are constructed on softfoundations.

2-3. Groundwater Level and Pore Water Pressure

a. Hydrostatic piezometric conditions. Thegroundwater levelor table is defined as the elevation thatthe free water surface assumes in permeable soils or rockbecause of equilibrium with atmospheric pressure in ahole extending a short distance below the capillary zone,as shown in Figure 2-3(a). The water-bearing stratum

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Figure 2-2. Response of effective stress, pore water,and soil volume to an applied total stress (Dunnicliff1988)

containing the groundwater is called anaquifer. Thecapillary zone is defined as the interval between the freewater surface and the limiting height above which watercannot be drawn by capillarity. Normal behavior ofgroundwater conditions occurs when the pore water pres-sure increases hydrostatically with depth below thegroundwater level. In this condition the pore water pres-sure, also called thehydrostatic pressure, can be calcu-lated by multiplying the unit weight of water by thevertical distance from the point of interest (screen loca-tion, sensor location, etc.) to the groundwater surface.

b. Pore water pressure.Figure 2-3 illustrates thegroundwater condition soon after a layer of material isplaced over existing soil layers and before consolidation iscomplete. Thus, excess pore water pressure exists in theclay and the groundwater is no longer in equilibrium.The five perforated pipes in Figure 2-3 are installed suchthat the soil is in intimate contact with the outsides of thepipes. The second pipe (pipe b) is perforated throughoutits length while the other pipes are perforated only nearthe bottom. Due to the high permeability of the sand, theexcess pore water pressure in the sand is dissipated imme-diately. Thus, pipe (a) indicates the groundwater level.Pipe (b) indicates the groundwater level because the per-meability of the sand is such that the excess pore water

Figure 2-3. Groundwater level and pore water pressure when there is flow of groundwater (Dunnicliff 1988)

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pressure from the clay will be dissipated into the sand.Pipes (c) and (d) indicate the pore water pressures in theclay at two locations. More dissipation of the excess porewater pressure has occurred in pipe (c) than in pipe (d)because the drainage path for excess pore water pressureis shorter and the rate of dissipation is greater in pipe (c)than in pipe (d). Pipe (b) in Figure 2-3 is anobservationwell, because there are no subsurface seals that prevent avertical connection between multiple strata. Apiezometeris a measuring device that is sealed within the ground sothat it responds only to groundwater pressure around itselfand not to groundwater pressure at other elevations.Pipes (a), (c), and (d) are called piezometers because theyindicate pore water pressure at one location (they aresealed above and below the perforated locations) and notto the groundwater pressure at other elevations. Thepiezometric elevationor piezometric levelis the elevationto which water will rise in a piezometer.

c. Positive pore water pressures.Pore water pres-sure that is above atmospheric pressure is calledpositivepore water pressure. Pore water pressure can beincreased by applying a compressive force to the soil orwhen a shearing force is applied to a soil that decreasesits volume while preventing dissipation of pore waterpressure. Excess pore water pressure resulting from anytype of stress change can also be calledinduced porewater pressure.

d. Negative pore water pressures.Negative porewater pressureoccurs when the pore water pressure isless than atmospheric pressure. This condition can occurwhen a compressive load is removed or when a denselypacked soil is sheared and increases in volume.

e. Non-hydrostatic groundwater conditions.Porewater pressure does not always increase hydrostaticallywith depth below the groundwater level. These ground-water conditions include:

(1) Perched water table.Perched water tablesarecaused when a permeable material overlies a relativelyimpermeable strata above the main groundwater level andretains some groundwater. A piezometer placed in aperched water table will indicate an elevated surface asillustrated in Figure 2-3, pipe (e).

(2) Artesian pressure.Artesian pressuresare foundin strata that are confined between impervious strata andare connected to a water source at a higher elevation. Awell drilled to an artesian aquifer having a pore waterpressure above the ground surface will flow withoutpumping and is called a free-flowing artesian well.

Artesian conditions are shown in Figure 2-3, pipes (c)and (d).

f. Variation in piezometric levels and pressures.Piezometric levels and pressures are rarely constant overan extended period of time. Natural forces such as pre-cipitation, evaporation, atmospheric pressure, and seepagemay cause wide variations in the groundwater level.

2-4. Time Lag in Groundwater Observations

a. Hydrostatic time lag.Most piezometers requiresome movement of pore water to or from the device toactivate the measuring mechanism. When pore waterpressures change, the time required for water to flow to orfrom the piezometer to create equalization is called thehydrostatic time lag. The hydrostatic time lag is depend-ent primarily on the permeability of the soil, type anddimensions of the piezometer, and the change in porewater pressures. The volume of flow required for pres-sure equalization at a diaphragm piezometer is extremelysmall, and the hydrostatic time lag is very short. For anopen standpipe piezometer, the time lag may be reducedby providing a large intake area and reducing the diameterof the riser pipe, thereby reducing flow required for pres-sure equalization. The pressure of gas bubbles (natural orresult of corrosion) can also cause a time lag. Hydrostatictime lag is not significant when piezometers are installedin highly pervious soils such as coarse sands. Thosepiezometers that have a long time lag are described ashaving aslow response time. The time required to estab-lish equalization of pore water pressures after installing orflushing a piezometer is called thestress adjustment timelag, sometimes referred to as theinstallation time lag.

b. Determination of time lag.An estimate of thehydrostatic time lag aids in selecting the proper type ofpiezometer for given subsurface conditions at a given site.The order of magnitude of the time required for 90%response of several types of piezometers installed inhomogeneous soils can be determined from Figure 2-4.As stated in Dunnicliff (1988), the 90% response is con-sidered reasonable for most practical purposes since the100% response time is infinite. The response time ofopen standpipe piezometers can be estimated from equa-tions derived by Penman (1960). As an example,

(2-1)t 3.3 × 106 d 2 ln [L/D 1 (L/D)2 ]kL

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Figure 2-4. Approximate response time for varioustypes of piezometers (after Terzaghi and Peck 1967)

where

t = time required for 90% response, days

d = inside diameter of standpipe, centimeters (cm)

L = length of intake filter (or sand zone around thefilter), centimeters (cm)

D = diameter of intake filter (or sand zone), centi-meters (cm)

k = permeability of soil, centimeters per second(cm/sec)

Similar equations and other procedures have been devel-oped by Hvorslev (1951) and others. It is often moredesirable to measure time lag in the field by comparingpool fluctuation with desired piezometer readings as illus-trated in Figure 2-5.

2-5. Mechanisms That Control Behavior

The engineering behavior of soils is controlled by eitherhydraulic, stress-deformation, or strength mechanisms.When a soil is subjected to excess pore water pressures,the water flows through the pore spaces creating frictionand a resistance to flow. The permeability of the soil is a

Figure 2-5. Time lag measurement from time-historyplot of reservoir level and piezometer

function of this friction and resistance to flow. Whenwater is flowing through soil, the water exerts a force onthe soil particles in the direction of flow. Also if the soilis saturated, water acts on the soil with a buoyant forcewhether or not the water is moving. Stress-deformationcharacteristics of cohesionless soils are caused byrearrangement of the relative positions of grains as sheardeformation occurs. For cohesive soils, the stress-deformation characteristics are governed by the timerequired for water to flow through the pore spaces in thesoil and for subsequent volume changes to occur. Shearstrength is governed by the nature, size, and shape of thesoil grains, packing density, and effective stresses withinthe soil. Shear failure occurs when the applied stressesincrease beyond those that can be sustained by the soil.

a. Embankments and foundations.Earth and rock-fill embankments may experience failure by the followingmechanisms: overtopping, instability, and internalseepage. Overtopping results when the reservoir or riverlevel exceeds the embankment height. This condition canoccur due to incorrect estimations of water volume or asthe result of some other mechanism (i.e., slope instability)causing a displacement of a large mass of water.

(1) Embankment and foundation instability. Whenthe available shearing resistance along a potential surfaceof sliding of an embankment slope is greater than theshear stress or driving force on that surface a stable sloperesults. Any increase in pore water pressure along thepotential surface of sliding decreases the shearing resis-tance and the factor of safety against sliding. If the foun-dation is stronger than the embankment soil, the slope willgenerally move within the embankment. If the

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embankment is overlying a soft foundation, the propertiesof the foundation material will determine if stability is aproblem. The embankment loading could cause a move-ment through the foundation or a movement along a weakfoundation layer. In addition, the embankment could causesettlement and lateral bulging of the foundation. If thefoundation consists of loose uniform cohesionless mate-rial, an earthquake can cause liquefaction and slidingwithin the foundation.

(2) Piping. Seepage can cause piping (internal ero-sion) when the velocity of the flowing water is sufficientto move or carry soil particles. If the soil has some cohe-sion, a small tunnel or pipe could form at the downstreamexit of the seepage path. As the soil is eroded, the pipelengthens inside the embankment. Once piping begins,there is less resistance to flow, the flow in the pipeincreases, and piping accelerates. Silt and fine sands aremost prone to piping. In a well-designed embankment,piping is prevented by using filters or drains to ensurethat material from upstream cannot migrate downstream.

(3) Transverse cracking. The load of the embank-ment can cause foundation settlement and some settlement

and lateral movement in the compacted fill. Uniformsettlement of the embankment or foundation is usually nota problem. However, when the abutments are steep,settlement may cause cracking transverse to the axis ofthe dam.

b. Soil around structures and abutments.Whensoil is placed around structural elements or against abut-ments, behavior mechanisms of concern include seepage,stability, and sliding of the structural element. The inter-face between the soil and either the abutments or a struc-tural element is a potential location for abnormal seepage.Special care and compaction effort are needed in this areato ensure that the potential for piping is minimized. Inaddition, whenever material is placed against abutments orother existing surface, the potential exists for weak shearsurface planes to develop, which could become a shearsurface. Good compaction along the interface shouldprevent this type of mechanism. Sliding of a structuralelement is a function of the force caused by theactiveearth pressurepushing on the element minus the shearingresistance along the base of the element and the forcecaused by thepassive earth pressure.

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Chapter 3Instrumentation Concepts, Objectives,and System Design Considerations

3-1. Introduction

This chapter expands on geotechnical concepts discussedin the previous chapter, identifies parameters in embank-ment structures and foundations that may need to bemonitored, and provides an approach to the developmentof an appropriate instrumentation system. ER 1110-2-110provides policy guidance on instrumentation for safetyevaluations of civil works projects. Instrumentation nec-essary to assist in the evaluation of the safety of embank-ments is considered an integral part of the geotechnicaldesign.

3-2. Unique Characteristics of GeotechnicalEngineering

Geotechnical engineering of embankment projects requiresthe exploration and analysis of a wide variety of earth androck materials. These materials must be considered foradequacy as foundations and for use in structures. Sinceearth and rock are created by natural processes, unlikeother engineering materials such as steel and concrete,they seldom exhibit uniform properties. There is risk inevery project that unexpected conditions will be encoun-tered. The inability of exploration programs to detect inadvance all potential significant properties and conditionsin natural deposits requires designers to make assumptionsand generalizations that may be at variance with actualfield conditions. However, even though geotechnicaldesign and construction of embankments are subject touncertainties, visual observations supported by quantitativemeasurements obtained from appropriate instrumentationcan provide engineers with information for checking andverifying design assumptions. Visual observations com-bined with instrumentation data provide the basis forassessing embankment and foundation performance andsafety during actual operations.

3-3. Characteristics of GeotechnicalInstrumentation

Field instrumentation is more vital to the practice of geo-technical engineering than to most other branches of engi-neering, in which designers have greater control over thematerials utilized for construction. Therefore, geotechni-cal engineers must have more than casual knowledge ofinstrumentation. However, geotechnical engineers must

recognize that, although instrumentation is a valuable toolthat can be utilized for monitoring performance andsafety, it is not a stand-alone solution to monitoringembankment performance. The determination of the needfor instrumentation must be kept in perspective. In thewords of Dr. Ralph Peck (as cited by Dunnicliff 1988)“Every instrument on a project should be selected andplaced to assist with answering a specific question; ifthere is no question, there should be no instrumentation.”Instrumentation cannot guarantee good design, trouble-freeconstruction, or long-term maintenance-free operation.The wrong type of instruments placed in inappropriatelocations can provide information that may be confusing,or divert attention away from other signs of potentialdistress. It is not appropriate to mandate instrumentationat every dam or levee with the expectation that someunknown defect will be revealed during monitoring andprovide a warning of an impending failure. Instrumentscannot indicate signs of pending deterioration or failureunless they happen to be placed at the right location.Geotechnical instrumentation is not intended to be a solebasis for embankment evaluation; it is intended to providedata for evaluation within a comprehensive embankmentsafety inspection and surveillance program.

3-4. Instrumentation Objectives

The principal objectives of a geotechnical instrumentationplan may be generally grouped into four categories: first,analytical assessment; second, prediction of future perfor-mance; third, legal evaluation; and fourth, developmentand verification of future designs. Instrumentationachieves these objectives by providing quantitative data toassess groundwater pressure, deformation, total stress,temperature, seismic events, leakage, and water levels.Total movements as well as relative movements betweenzones of an embankment and its foundation may alsoneed to be monitored. A wide variety of instruments maybe utilized in a comprehensive monitoring program toensure that all critical conditions for a given project arecovered sufficiently. The most commonly used geotech-nical instrumentation is described in Chapter 4.

a. Analytical assessment.Analysis of data obtainedfrom geotechnical instrumentation may be utilized toverify design parameters, verify design assumptions andconstruction techniques, analyze adverse events, andverify apparent satisfactory performance as discussedbelow.

(1) Verification of design parameters. Instrumenta-tion can be utilized to verify design parameters with

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observations of actual performance, thereby enablingengineers to determine the suitability of the design.Instrumentation also aids engineers in modifying designsby incorporating the effects of actual field conditions.The design of earth and rockfill embankments generallyentails a rigorous and sometimes complex study of forces,which must be based on conservative geotechnicalassumptions concerning material characteristics and struc-tural behavior. These generalized assumptions can leavesome unknown conditions, variations, and uncertainties inthe design. Observations from instrumentation systems,and an assessment of structural performance of anembankment, can help resolve these unknowns, allowingnecessary refinements and improvements to be made inthe design.

(2) Verification of design assumptions and construc-tion techniques. Experience has shown that most new ormodified designs and construction techniques are notreadily accepted until proven satisfactory on the basis ofactual performance. Data obtained from instrumentationcan aid in evaluating the suitability of new or modifiedtechniques.

(3) Analysis of adverse events. When a failure, apartial failure, a severe distress condition, a visually notednon-severe change in shape, appearance, or seepage hasoccurred at a dam or levee project, data from instrumenta-tion can be extremely valuable in the determination of thespecific cause or causes of the event. Also, instrumenta-tion is often installed prior to, or during, remedial work ata site to determine the effectiveness of the improvementsand the effect of the treatment on existing conditions.

(4) Verification of apparent satisfactory performance.It is just as necessary to confirm satisfactory performanceof a project as it is to identify areas of concern. Positiveindications of satisfactory performance are very reassuringto evaluating engineers and operators of a dam or leveeproject. Instrumentation data can prove to be valuableshould some future variation in historic data occur, signal-ing a potential problem. Also, records of satisfactoryperformance are valuable for use in future design efforts.

b. Prediction of future performance.Instrumentationdata should be used in such a manner that informed, validpredictions of future behavior of an embankment can bemade. Such predictions may vary from indicating contin-ued satisfactory performance under normal operatingconditions to an indication of potential future distresswhich may become threatening to life or safety, andnecessitate remedial action. Often earth and rockfillembankments constructed for flood-control purposes

remain dry, or maintain only very low level conservationor recreation pools, except during infrequent flood events.As a result, these embankments may have existed foryears without ever experiencing maximum design condi-tions. However, instrumentation data obtained duringintermediate flood events can be projected to predictperformance during potential maximum flood stage orreservoir levels.

c. Legal evaluation.Valid instrumentation data canbe valuable for potential litigation relative to constructionclaims. It can also be valuable for evaluation of laterclaims relative to changed groundwater conditions down-stream of a dam or landward of a levee project. In manycases, damage claims arising from adverse events can beof such great monetary value that the cost of providinginstrumentation can be justified on this basis alone.Instrumentation data can be utilized as an aid in determin-ing causes or extent of adverse events so that variouslegal claims can be evaluated.

d. Development and verification of future designs.Analysis of the performance of existing dams and levees,and instrumentation data generated during operation, canbe used to advance the state-of-the-art of design and con-struction. Instrumentation data from existing projects canpromote safer and more economical design and construc-tion of future earth and rockfill embankments.

3-5. Basic Instrumentation Concepts

A determination of the number, type, and location ofinstruments required at a dam or levee can only beaddressed effectively by combining experience, commonsense, and intuition. Embankments represent uniquesituations and require individual solutions to their instru-mentation requirements. The instrumentation systemdesign, therefore, needs to be conceived with care andconsideration for the site-specific geotechnical conditionspresent in the embankment, foundation, abutments, andreservoir rim. Substantial geotechnical reasons, such asunique design or difficult foundation conditions, severedownstream hazard, visually observed problems or con-cerns, remoteness of location, normally unmanned opera-tion, or other concerns justify providing instrumentation.It also must be recognized that instruments are actuallydiscontinuities, or nonrepresentative objects, introducedinto soil/rock structure systems. Their presence, or theflows or displacements required to generate an observa-tion, may alter the parameters which are being measured.Engineers installing field instrumentation must understandthe fundamental physics and mechanics involved, and how

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the various available instruments will perform under theconditions to which they will be subjected.

3-6. Instrumentation System Planning

Planning an embankment instrumentation system requiresthe consideration of many factors, and a team effort of thedesigners (or those responsible for evaluating existingprojects) and personnel having expertise in the applicationof geotechnical instrumentation. Developing an instru-mentation system should begin with a definition of anobjective and proceed through a comprehensive series oflogical steps that include all aspects of the system. Aseries of recommended steps is provided below. Thissystematic planning is based on the approach recom-mended by Dunnicliff (1988 and 1990). The steps arelisted in Table 3-1.

a. Prediction of mechanisms that control behavior.During the design of a new project or analysis of an exist-ing project, geotechnical engineers should identify siteweaknesses and areas of sensitivity. Based on theseitems, hypotheses regarding hydraulic, stress deformation,or strength mechanisms that are likely to affect projectbehavior for various conditions should be developed. Theinstrumentation program should then be planned aroundthe hypotheses. For example, a soft foundation materialmay lead to a concern for stability and settlement; there-fore, instrumentation is required to monitor pore waterpressures and consolidation. Or, if abutment materialscause concern for excessive seepage, instrumentation may

be selected to monitor flow quantities and solids contentin the seepage.

b. Definition of purpose of instrumentation.Theprimary purpose of instrumentation is to provide datauseful for determining whether or not an embankment orfoundation is behaving in accordance with engineeringpredictions. When an embankment has special foundationconditions or uncommon design features, instrumentationcan help in determining if design concepts for these con-ditions or features are being realized during constructionand operation. A comprehensive monitoring programmust be developed for the special conditions and featuresat that site. If an embankment has no special foundationconditions or uncommon design features, the need forinstrumentation is less certain. Many conventionalembankments of moderate height on good foundationshave only minimal instrumentation.

c. Definition of geotechnical questions.Everyinstrument on, in, or near an embankment should beselected and placed to assist in answering a specific con-cern. Before addressing measurement methods, a listingshould be made of geotechnical questions that are likelyto arise during the design, construction, or operationphases. For the various phases, questions should coverinitial site conditions, performance during construction,performance during first filling, performance during draw-down, and long-term performance. Dunnicliff (1990)listed examples of possible geotechnical questions

Table 3-1Steps for Developing an Instrumentation System (Dunnicliff 1990)

Step Element of Plan

a Prediction of mechanisms that control behaviorb Definition of purpose of instrumentationc Definition of geotechnical questionsd Selection of parameters to monitore Prediction of magnitudes of changef Selection of instrument locationsg Selection of instrumentsh Determination of need for automationi Planning for recording of factors which influence measurementsj Establishment of procedures for ensuring data validityk Determination of costsl Planning installationm Planning long-term protectionn Planning regular calibration and maintenanceo Planning data collection and managementp Coordination of resourcesq Determination of life cycle costs

Table material adapted with permission from Hydro Review magazine, Kansas City, MO.

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associated with the appropriate features and parameters(see Table 3-2).

d. Selection of parameters to monitor.Typicalparameters that can be monitored include groundwaterpressure, deformation, total stress, temperature, seismicevents, leakage, and water levels. Engineers need toconsider which parameters are most significant for aparticular project.

e. Prediction of magnitudes of changes.Predictionsof magnitudes of anticipated change are necessary so thatrequired instrument ranges and sensitivities, or accuracies,can be selected. An estimate of the maximum possiblevalue, or maximum value of interest, leads to a selectionof instrument range. An estimate of the minimum valueof interest leads to a selection of instrument sensitivityand accuracy. Where measurements are for constructionor safety purposes, predeterminations can be made ofnumerical values that indicate the need for immediateanalysis or remedial action (hazard warning levels).Wherever possible a positive means should be developedfor remediating any problem that may be disclosed duringconstruction by results of instrumentation data. If theobservations demonstrate that remedial action is neces-sary, the action should be based on appropriate, pre-viously anticipated plans. Construction personnel shouldhave contingency plans to follow in the event that hazardwarning levels are reached at the site. For example,embankment construction on a site with a soft foundationwould require a predetermination of hazard warning

levels. The planned remedial action might include awaiting period to allow for dissipation of pore water pres-sure and associated gain in strength of the foundationprior to proceeding with construction. Design and con-struction personnel should maintain an open channel ofcommunication so that remedial action plans can be dis-cussed at any time.

f. Selection of instrument locations.Locations forinstruments should be determined based on predictedbehavior of the site. The locations should be compatiblewith the geotechnical concerns and the method of analysisthat will be used when interpreting the data. A practicalapproach to selecting instrument locations includes:(1) identify zones of particular concern such as struc-turally weak areas that are most heavily loaded, and locateappropriate instrumentation, (2) select zones that can berepresented by typical cross sections where predictedbehavior is considered representative of behavior as awhole (Typically, one cross section will be at or near themaximum height of the dam, and one or two other sec-tions will be at appropriate locations.), (3) identify zoneswhere there is discontinuity in the foundation or abut-ments, (4) install some additional instruments at otherpotentially critical secondary locations to serve as indicesof comparable behavior, and (5) locate rows of surveymonuments at intervals in the longitudinal direction atappropriate elevations. If behavior at one or more of thesecondary locations appears to be significantly differentfrom the primary sections, engineers should also provideadditional instruments at these secondary locations. When

Table 3-2Examples of Possible Geotechnical Questions (Dunnicliff 1990)

Questions to Answer Features to Assess Parameters to Monitor

What are initial site conditions? Foundation Pore water pressureAbutments HydrologyDrainage basin Meteorology

How does the embankment perform Foundation Pore water pressureduring construction? Embankment Horizontal/vertical movement

Abutments

How does the embankment perform All features and adjacent terrain Pore water pressureduring first filling? Horizontal/vertical movement

SeepageDissolved solids

How does the embankment perform Upstream face Pore water pressureduring drawdown? Adjacent natural slopes Slope stability

How does the embankment perform All features All parametersduring long-term operation?

Table material adapted with permission from Hydro Review magazine, Kansas City, MO.

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selecting all locations, the survivability of instrumentsshould be considered. Damage during construction to theinstrument, riser, or cable should be prevented by gooddesign and temporary and permanent protection of surfaceexposure. Protection against vandalism should also bepart of the design.

g. Selection of instruments.Reliability is the mostdesirable feature in the selection of monitoringinstruments. This is to ensure that dependable data ofadequate accuracy can be obtained throughout the periodwhen the information is needed. Often there is a ten-dency to seek unnecessarily high accuracy and whenaccuracy and reliability are in conflict, high accuracyshould be sacrificed for high reliability. High accuracyoften requires excessive delicacy and fragility. Usuallythe most reliable instrumentation devices are the simplestdevices. Where a choice exists, the simpler device islikely to have more success. The performance record ofcommercially available instruments should be considered.There is an inherent risk in using any electronic instru-ment that does not have a satisfactory track record. How-ever, in some cases, simple instruments are notsatisfactory, and more complex devices must be provided.For instance, an open standpipe piezometer may be thesimplest instrument for observing a piezometric level, butthe point at which the pore water pressure needs to bemeasured may be located where direct vertical access isimpossible. In this case a less simple instrument will berequired to make measurements at a remote location. Ingeneral, transducers can be placed in the following orderof decreasing simplicity and reliability: optical, mechani-cal, hydraulic, pneumatic, electrical. When consideringcosts, the lowest initial cost of an instrument should neverdictate the selection of an instrument. Instead, a compari-son of the overall cost of procurement, calibration, instal-lation, life cycle maintenance, reading, and dataprocessing of the available instruments (plus a consider-ation of replacement cost if there is some risk the instru-ment should go bad) should be made. The leastexpensive instrument is not likely to result in the leastoverall cost because it may be less reliable. The cost ofinstruments themselves is usually a minor part of the totalcost. Engineers must also keep in mind that instrumentsthemselves can cause problems. The installation ofinstruments, even under the best of circumstances, canintroduce discontinuities into embankments and founda-tions. Horizontal or vertical protecting risers or cablesleading from instrumentation can have areas of poor com-paction and preferential paths for seepage. Avoidhorizontal conduits in the primary direction of seepageflow. Potential weaknesses introduced by an installationshould be balanced against the potential benefits from the

observations. Engineers need to develop an adequatelevel of understanding of the instruments that they select,and they will often benefit from discussing their applica-tion with a representative of the manufacturer or with aninstrumentation consultant before selecting instruments.During discussions, users should convey as much infor-mation as possible about the planned application and seekout any limitations of the proposed instruments.

h. Determination of need for automation.Auto-mated data acquisition systems (ADAS) have become avaluable means of collecting geotechnical instrumentationdata. Developments in the field of electronics have madeit possible to install and operate remote ADAS that pro-vide accurate, reliable, and effective real time data collec-tion. With increased emphasis on dam safety and thecontinued decrease in manpower, the advantages of pro-viding automatic systems are numerous. Considerationshould be given to providing ADAS at new projects andto retrofitting at existing projects. However, it must beunderstood that while ADAS can provide valuable data ina timely manner, they are only to be considered as anintegral part of an overall dam safety program. AnADAS does not replace visual observations and the soundevaluation of instrumentation data provided. Furtherdiscussions on ADAS are presented in Chapter 5.

i. Planning for recording of factors which influ-ence measurements.Measurements alone do not producesatisfactory assessments. A successful geotechnical moni-toring program relates instrumentation measurements tocauses. Therefore, plans should be developed to maintainrecords of all factors that might cause changes in themeasured parameters. For example, it is necessary torecord construction details and progress, geology andsubsurface conditions, and results of visual observationsof expected and unusual behavior. Data such as reservoirand tailwater levels, rainfall amounts, ambient and watertemperatures, barometric pressure, and seismic eventsshould also be included. Chapter 7 discusses data man-agement, analysis, and reporting of instrumentationreadings.

j. Establishment of procedures for ensuring datavalidity. Personnel responsible for monitoring instrumen-tation must be able to determine if the instruments arefunctioning correctly. This can sometimes be determinedthrough visual observations or with backup systems. Abackup system can be useful to confirm, or deny, thebehavior indicated by some component of the primarysystem, even when the backup accuracy is significantlyless accurate than the primary system. For example,optical surveys can be used to confirm apparent slope

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movements detected by inclinometers. Or, electronic orpneumatic pressure transducers used for long-term moni-toring of pore water pressure can be supplemented bytwin-tube hydraulic piezometers as backup. Data can alsobe evaluated for validity by examining consistency. Forexample, where consolidation is being monitored, dissipa-tion of pore water pressure should be consistent withmeasured settlement, and increases in pore water pressureshould be consistent with added loading. Repeatabilitycan also indicate data validity. It is often worthwhile totake many readings over a short period of time to deter-mine whether a lack of normal repeatability indicatessuspect data. Wherever possible, the most dependablemethod of ensuring collection of dependable data is toprovide the simplest instrumentation system available thatwill provide the desired data.

k. Determination of costs.Funding for instrumenta-tion of a project should be planned during the designstage of a new project or the planning stage of an existingproject evaluation. Early planning will identify theamount of funds needed, what funding is available, and ifadditional funding sources are needed. When the need forinstrumentation is properly and correctly established, andwhen the program is properly planned, there should besufficient justification for funding. Instrumentation doesnot have to reduce costs to be justified. Instrumentationis valuable in proving that the design is correct, or insome cases, instrumentation might show that the design isinadequate, increasing construction costs. However, thevalue of ensured safety and the avoidance of failure,saving the cost of damages and/or cost of repairs, makesinstrumentation programs cost-effective. Cost estimatesshould take into consideration the cost of instruments,installation, calibration, automation, long-term protection,maintenance, and data collection and management.

l. Planning installation. Proper installation is criti-cal to achieving reliable performance and obtainingdesired information. Installation procedures should beplanned well in advance of scheduled installation dates,following guidelines presented in Chapter 6. Writtenstep-by-step procedures should be prepared, making useof the manufacturer’s instruction manual and thedesigner’s knowledge of specific site geological condi-tions. The written procedures should include a detailedlisting of required materials and tools, and installationrecord sheets should be prepared for documenting factorsthat may influence measured data. Whenever possible,instrumentation should be installed by experiencedGovernment personnel, rather than by contract personnel.For projects under construction, installation plans shouldbe coordinated with the construction contractor and

arrangements made for access, safety of personnel, andboth temporary and permanent protection of instrumentsand monuments from damage. An installation scheduleshould be prepared that is consistent with the constructionschedule.

m. Planning long-term protection.Considerationshould be made for protection of instrumentation over thelong-term. Survivability should be included in determin-ing the original location of all instruments with respect totraffic patterns, operation of project maintenance equip-ment, and access to project visitation. Where necessary,installations should be buried out of sight in watertightcontainers, or adequate protection should be provided atthe surface. Buried cable locations and other subsurfacelocations should be clearly documented on as-built draw-ings to prohibit future damage. In some isolated areas, itmay be necessary to provide bulletproof housing. Appro-priate parts and components should be available forreplacement of instruments that may become inoperativefor one reason or another over a long period of time.

n. Planning regular calibration and maintenance.The design of an instrumentation system must take intoconsideration the fact that regular calibration and mainte-nance of hardware will be required over the life of theproject. During the design of the system, procedures andschedules should be developed for regular maintenance ofreadout units and any components which are accessible.Calibration and maintenance are discussed in Chapter 8.

o. Planning data collection and management.Procedures for collecting, processing, presenting,interpreting, and reporting instrumentation data should bedeveloped prior to installation of the system. The effortrequired for these tasks should not be underestimated.Too often data collection files become filled with largequantities of partially processed and undigested databecause sufficient time and funds were not appropriatedfor activities required after the instruments were installed.With the development of computerized data collection,processing, and presentation procedures, overall manualeffort has been greatly reduced. However, the limitationsof computerization must be recognized. No computerizedsystem can replace engineering judgment, and engineersmust make a special effort to ensure that measured effectsare correlated with probable causes. The tasks of inter-pretation, judgment, and implementation must be per-formed by competent personnel rather than computers.Data management and analysis are discussed in Chapter 7.

p. Coordination of resources.Tasks to be consid-ered during the monitoring program include planning

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regular calibration and maintenance of instruments, instru-ment replacement procurement, installation, and calibra-tion, data collection, and data management. When tasksare assigned, the person with the greatest vested interestin the data should be given direct responsibility for pro-ducing the data accurately. Instrumentation personnelneed to have a background in the fundamentals of geo-technical engineering, and be reliable, patient, and moti-vated. In addition, they need to demonstrate attention todetail. Qualifications for personnel are presented inparagraph 3-8.

q. Determination of life cycle costs.Once planningfor the system is completed, the cost estimate for thetasks suggested in paragraph 3-6k should be updated.Care must be taken to ensure that sufficient funds areprovided to cover all aspects of the program, includinginstrument maintenance and the data collection and pro-cessing costs for the life of the project.

3-7. Procurement

The specialized nature of instrumentation facilities and thecare required in the procurement, installation, and calibra-tion of equipment demand that these features of work beretained under close operational control of qualified Corpsof Engineers personnel. Direct procurement by the Gov-ernment of instruments, cable, tubing, and indicating orrecording equipment, and similar items not normallyencountered in construction work, is recommended.

3-8. Personnel Qualifications andResponsibilities

A variety of tasks must be accomplished for an instru-mentation program to be successful. The tasks includeinstrumentation installation, calibration, automation, long-term protection, maintenance, and data collection andmanagement. Generally, these responsibilities can bedelegated among individuals with diverse skills and back-grounds, or the same individual can be responsible forvarious tasks. Responsibilities should be very clear, com-munication being the single most important link amongthe various personnel (Table 3-3). The overall scope ofthe work should be supervised by a senior-level geotech-nical or instrumentation engineer.

a. Project inspection personnel.Inspection of theproject features should be accomplished on a routine basisduring both construction and project operation. Theactual schedules should vary depending on the criticalityof the particular feature. A project engineer, project

superintendent, or maintenance foreman are the mostlikely candidates, but anyone at the project with propertraining can accomplish the inspections. As an example,the inspector should look for evidence of seepage or dis-tress in the embankment or abutments, particularlysprings, seeps, or boils developing along the downstreamtoe of the embankment, or zones of settlement, caving,bulging, or other distress in the embankment. Remarksshould be noted on a preprinted form, and if deemednecessary, the appropriate District personnel should benotified immediately.

b. Instrumentation and/or automation equipmentinstallation personnel.Installation should be coordinatedby the senior-level geotechnical engineer, since properinstallation of instruments and automation equipmentrequires specific knowledge and requirements.

c. Data collection personnel.It is highly recom-mended that permanent project employees be assigneddata collection responsibilities. These employees shouldbe properly instructed by the senior-level geotechnical orinstrumentation engineer, and receive training on theimportance of the tasks. Project personnel responsible forcollecting data are too often temporary or summer hireswho have limited knowledge of the geotechnical questionsof concern. If permanent personnel are assigned toaccomplish this task, they too often do not understand orenjoy the work, thereby affecting data quality.Repeatedly training part-time employees is difficult andtime-consuming. There are three options when consider-ing personnel to read manual instruments. The task ofreading the manual instruments can often be combinedwith the maintenance of automation equipment if the datacollection is partially automated. A second option is tohire a permanent employee whose job description includesthe reading of manual instrumentation on a District proj-ect or area-wide basis. Ideally, a single individual at theDistrict or project level should make the manual readings,since this allows for consistency of data, and also allowsthe individual to become familiar with the historicalbehavior of the instruments. A third option is to contractthe work out, but this is not generally recommendedbecause of the possible turnover in the contractor’s read-ing personnel.

d. Instrumentation and automation equipment main-tenance personnel.The maintenance of instrumentationand automation equipment is becoming increasingly morecomplex due to the partial automation of many Corpsprojects. A discussion on the personnel requirementsrelating to maintenance is included in paragraph 8-2.

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Table 3-3Personnel Responsibilities

Title Responsibility

Senior-level geotechnical or Coordinates and supervises the entire instrumentation program, including design of instrumen-instrumentation engineer tation and automation equipment, selection, procurement, and installation. Provides training to

instrumentation personnel and assists in personnel decisions. Should assist in data interpreta-tion and analysis and provide technical assistance or guidance to other geotechnical engineersresponsible for final data analysis.

Project inspection personnel Inspect the project during both construction and operation. Make notes of changes (seepage,boils, caving, etc.) and notify District personnel if necessary.

Instrumentation and/or automation Responsible for initial instrumentation and installation; assure initial readings are correct andequipment installation personnel make necessary initial calibrations.

Data collection personnel Collect data from manually read instrumentation. May also enter data into the computer.

Instrumentation and automation Maintain instrumentation and automation throughout life of the project. Recalibrate and replaceequipment maintenance personnel instruments as necessary.

Data entry personnel Enter collected data into the computer. May also be responsible for collecting data from thefield.

Data management, reporting, Process data from all District projects. Work with senior-level geotechnical or instrumentationand plotting personnel engineer. Work with various software programs, data transfer from remote sites, programming,

creating reports, and submitting data plots.

Instrumentation program manager See senior-level geotechnical or instrumentation engineer.

Review and analysis personnel Review and analyze final data to assure the project is performing in accordance with design.Also notify maintenance personnel of inaccurate readings or malfunctions.

e. Data entry personnel.The task of data entry isoften combined with other tasks. Data entry can be at theproject site by data collection personnel, or in the Districtoffice by data management personnel. Data entry can behandled by non-geotechnical personnel, such as informa-tion management personnel. However, since initial errorchecking should be accomplished at the data entry level, itis highly recommended that a person familiar with theinstruments enters all data.

f. Data management, reporting, and plotting per-sonnel. This position is generally filled at the Districtlevel by an engineering technician with strong computerskills, who processes the data from all District projects.This person must work closely with the senior-level geo-technical or instrumentation engineer who is responsiblefor coordination of the entire instrumentation program.Tasks include interrelating with numerous software pro-grams, data transfer from remote sites, programming,creating reports, and submitting data plots. It is alsonecessary that the individual be trained in data collectionand field maintenance so that he or she better understandsthe instruments and automation equipment in use.

g. Instrumentation program manager.The overallmanagement of an instrumentation program should besupervised by a single instrumentation or geotechnicalengineer, whose responsibilities should include instrumen-tation and automation equipment design, selection, pro-curement, and installation. He or she should coordinateall of the tasks listed above, provide training to the instru-mentation personnel, and assist in personnel decisions.As a minimum, this person should assist in data interpre-tation and analysis, and provide technical assistance orguidance to other geotechnical engineers responsible forfinal data analysis.

h. Review and analysis personnel.Senior-levelgeotechnical or instrumentation engineers must beinvolved in the final review and analysis of all instrumen-tation data. It is possible that this task can be accom-plished by the instrumentation program manager, but otherengineers must be involved so that multiple reviews arebeing accomplished. The program manager and the tech-nical reviewers should establish data review schedules andformulate reports on a consistent and frequent basis.

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Chapter 4Summary of Measurement Methods

4-1. Introduction

This chapter provides a brief summary of the measure-ment methods that are most frequently used to instrumentembankment dams and levees. Methods for measuringpiezometric pressure, deformation, total stress, tempera-ture, seismic events, seepage, and water level areincluded. Methods for measuring joint movement, upliftpressure, strain and stress with load cells, strain gages,and concrete stress cells are structural parameter measure-ments which are outside the scope of this manual and arecovered in EM 1110-2-4300. More detailed informationof measurement methods is included in Dunnicliff (1988).

4-2. Instrumentation Measurement Methods

Most electronic instrumentation measurement methodsconsist of three components: a transducer, a data acquisi-tion system, and a linkage between these two components.A transducer is a component that converts a physicalchange into a corresponding electrical output signal. Dataacquisition systems range from simple portable readoutunits to complex automatic systems. Later in this chapterdevices for measurement of various parameters aredescribed. It will be seen that there is significant overlapamong the various devices. For example, vibrating wiredevices are used for measuring piezometric pressure,deformation, and total stress. To avoid repetition, thisparagraph provides a brief description of some of themost frequently used devices. Users are warned to bewary of unproven new devices. New developmentsshould be tested extensively, in the laboratory and field,before being accepted for incorporation as the sole meas-urement device in an embankment dam or levee. Othertechnologies such as vibrating strip capacitance andacoustics transducer may have potential but have not beensufficiently field-tested for geotechnical applications andare not discussed in this manual.

a. Pneumatic devices.Pneumatic devices are usedfor pneumatic piezometers, earth pressure cells, and liquidlevel settlement gages. Most modern devices are of thetype shown in Figure 4-1, for which a measurement ismade under a condition of no gas flow. The pressureP isthe pressure of interest. An increasing gas pressure isapplied to the inlet tube and, while the gas pressure is lessthan P, it merely builds up in the inlet tube. When thegas pressure exceedsP, the diaphragm deflects, allowinggas to circulate behind the diaphragm into the outlet tube,

and flow is recognized using a gas flow detector. Thegas supply is then shut off at the inlet valve, and anypressure in the tubes greater thanP bleeds away, suchthat the diaphragm returns to its original position whenthe pressure in the inlet tube equalsP. This pressure isread on a Bourdon tube or electrical pressure gage. Manydetailed issues need to be considered when selecting apneumatic device, including the sensitivity of the readingto diaphragm displacement, gas flow, tubing length anddiameter, type of tubing, tubing fittings, gas, and pressuregage.

b. Vibrating wire devices.Vibrating wire devicesare used in pressure sensors for piezometers, earth pres-sure cells, and liquid level settlement gages, and innumerous deformation gages. In a vibrating wire device alength of steel wire is clamped at its ends and tensionedso that it is free to vibrate at its natural frequency. Aswith a piano string, the frequency of vibration of the wirevaries with the wire tension. Thus with small relativemovements between the two end clamps of the vibratingwire device, the frequency of the vibration of the wirevaries. The wire can therefore be used as a pressuresensor as shown in Figure 4-2. The wire is plucked mag-netically by an electrical coil attached near the wire at itsmidpoint, and either this same coil or a second coil isused to measure the period or frequency of vibration.Frequency(f) is dependent on the bending of the dia-phragm, hence on the pressureP. Many detailed issuesneed to be considered when selecting a vibrating wiredevice, including the method of clamping the wire, pre-venting corrosion or seepage, and pretreating the trans-ducer to prevent significant zero drift. The attached wireis under near maximum tension at zero pressure. Thistension applies the greatest demand on the clamping andannealing of wire, a condition that may cause creepingand slippage of the wire at the clamps, which results in afrequency reduction unrelated to strain. This is com-monly known as drift of the baseline pressure or zerodrift. With vibrating wire transducers undesirable effectsinvolving signal cable resistance, contact resistance, elec-trical signal seepage to ground, or length of signal cableare negligible. Very long cable lengths are acceptable.

c. Electrical resistance strain gage devices.

(1) Electrical resistance strain gages have been usedin many measurement devices. An electrical resistancestrain gage is a conductor with the basic property thatresistance changes in direct proportion to change inlength. The relationship between resistance change∆Rand length change∆L is given by the gage factor (GF),where

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Figure 4-1. Schematic of pneumatic device (Dunnicliff 1988)

Figure 4-2. Schematic of vibrating wire pressure sensor (Dunnicliff 1988)

∆RR

∆LL

× GF

Output from the gages is normally measured using aWheatstone bridge circuit. Electrical resistance straingages can be packaged asbonded wire, unbonded wire(Figure 4-3), bonded foil (Figure 4-4), and weldablegages.

(2) The measured resistance change can be stronglyinfluenced by signal cable length, contact, moisture, tem-perature, and leakage to ground. However, correction forthese influences can be made by measuring the resistance

of the various system components (cable, contact, etc.)and subtracting the resistance from the total resistance.Various companies now manufacture low-current signaltransducers (4- to 20-milliamp range) that are unaffectedby resistance problems.

d. Electrical transducers for measuring lineardisplacement.

(1) A linear variable differential transformer(LVDT) (Figure 4-5) consists of a movable magnetic corepassing through one primary and two secondary coils. AnAC voltage is applied to the primary coil, thereby

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Figure 4-3. Schematic of unbonded wire resistancestrain gage (Dunnicliff 1988)

Figure 4-4. Uniaxial bonded foil resistance strain gage(Dunnicliff 1988)

Figure 4-5. Schematic of linear variable differentialtransformer (LVDT) (Dunnicliff 1988)

inducing an AC voltage in each secondary coil, with amagnitude that depends on the proximity of the magneticcore to each secondary coil.

(2) A direct current differential transformer(DCDT)is similar to an LVDT, except that unwanted cable effectsassociated with LVDTs are avoided by using DC voltages,requiring miniaturizing the electrical circuitry and placingadditional components within the transducer housing.

(3) A linear potentiometeris a device with a mov-able slider, usually called a wiper, that makes electricalcontact along a fixed resistance strip. As shown in Fig-ure 4-6, a regulated DC voltage is applied to the two endsof the resistance strip and the voltage or resistancebetweenB and C is measured as the output signal. Thevoltage betweenA and C varies as the wiper moves frompoint A to point B.

Figure 4-6. Schematic of linear potentiometer(Dunnicliff 1988)

(4) A vibrating wire transducer (paragraph 4-2b) canbe adapted for measuring linear displacement by includinga coil spring in series with the vibrating wire, thereby“softening” the system to give it adequate range, andproviding the advantages of a frequency signal.

e. Other electrical systems.

(1) Force balance accelerometersare used as tiltsensors in inclinometers. The device consists of a masssuspended in the magnetic field of a position detector(Figure 4-7). When the mass is subjected to a gravityforce along its sensitive axis, it tries to move, and themotion induces a current change in the position detector.This current change is fed back through a servo-amplifierto a restoring coil, which imparts an electromagnetic forceto the mass that is equal and opposite to the initiatinggravity force. The mass is thus held in balance and doesnot move. The current through the restoring coil is

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Figure 4-7. Schematic of force balance accelerometer(Dunnicliff 1988)

measured by the voltage across a precision resistor. Thisvoltage is directly proportional to the input force.

(2) The magnet/reed switchsystem is used in probeextensometers. It is an on/off position detector, arrangedto indicate when the reed switch is in a certain positionwith respect to a ring magnet, as shown in Figure 4-8.The switch contacts are normally open and one of thereeds must be magnetically susceptible. When the switchenters a sufficiently strong magnetic field, the reed con-tacts snap closed and remain closed as long as they stayin the magnetic field. The closed contacts actuate abuzzer or indicator light in a portable readout unit.

(3) Induction coil transducersare also used in probeextensometers. An electrical coil is powered to create amagnetic field around the coil. When this coil is placedinside a steel wire ring (with no external electrical con-nection), a voltage is induced in the ring, which in turnalters the current in the coil because its inductancechanges. The current in the coil is a maximum when thecoil is centered inside the ring; thus, by measuring currentin the coil, the transducer can be used as a proximitysensor.

(4) Sonic transducerscan be used to monitor waterlevel in open standpipe piezometers and weir stillingbasins. The transducer is mounted above the water

Figure 4-8. Magnet/reed switch (Dunnicliff 1988)

surface. Sound pulses travel to the water surface and arereflected back to the transducer. Distance to the watersurface is determined from the measured pulse time andthe known velocity of sound waves, corrected for errorsinduced by temperature change.

4-3. Measurement of Piezometric Pressure

Definitions of the terms groundwater level, pore waterpressure, pore gas pressure, piezometric level, and otherterms associated with measurement of piezometric pres-sure are given in Chapter 2. The most common devicefor measuring these parameters is the piezometer.

a. Applications. Applications for piezometers fallinto two general categories: first, for monitoring thepattern of water flow and second, to provide an index ofsoil strength. Examples in the first category includedetermining piezometric pressure conditions prior to con-struction, monitoring seepage, and effectiveness of drains,relief wells, and cutoffs. In the second category, monitor-ing of pore water pressure allows an estimate of effectivestress to be made, and thus an assessment of strength.Examples include monitoring dissipation of pore waterpressure during consolidation of foundation and fill mate-rial and the effect of rapid drawdown. Applications forobservation wells are very limited because they create avertical connection between strata. Observation wells andvarious types of piezometers are described in the

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subsequent paragraphs, together with recommendations fortheir use. Under certain conditions piezometers may needto be protected against freezing.

b. Observation wells.Figure 4-9 shows a schematicof an observation well. The elevation of the water sur-face in the riser pipe is determined by sounding with awater level indicator.

Figure 4-9. Schematic of observation well (Dunnicliff1988)

c. Open standpipe piezometers.Figure 4-10 showsa schematic of an open standpipe piezometer (also knownas a Casagrande piezometer) installed in a borehole. Thecomponents are identical in principle to components of anobservation well, with the addition of subsurface sealswhich isolate the zone of interest. Readings can be madeby sounding with a water level indicator, with a pressuretransducer placed in the standpipe below the lowest piezo-metric level, or with a sonic transducer.

d. Twin-tube hydraulic piezometers.

(1) The twin-tube hydraulic piezometer is shownschematically in Figure 4-11. The piezometric elevationis determined by adding the pressure gage reading to the

Figure 4-10. Schematic of open standpipe piezometerinstalled in a borehole (Dunnicliff 1988)

elevation of the pressure gages. If both plastic tubes arecompletely filled with liquid, both pressure gages shouldindicate the same pressure. However, if gas has enteredthe system (through the filter, tubing, or fittings) the gaswill cause an inaccurate pressure reading on one or bothgages. The gas must be removed by flushing. Dualgages therefore indicate the need for flushing andre-calibration.

(2) Twin-tube hydraulic piezometer systems havebeen developed in the United States by the U.S. Bureauof Reclamation and in England by Imperial College,London. Each system has been used widely in embank-ment dams throughout the world with variable success.The system developed in England appears to have a bettersuccess record than the system developed in the UnitedStates, and successful long-term use of twin-tube hydrau-lic piezometers requires close adherence to many provendetails (Dunnicliff 1988).

e. Pneumatic piezometers.Pneumatic piezometersare based on the device shown in Figure 4-1. A filter isadded to separate the flexible diaphragm from the materialin which the piezometer is to be installed, and the

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Figure 4-11. Schematic of twin-tube hydraulic piezometer installed in fill (Dunnicliff 1988)

installation arrangements can be similar to those shown inFigure 4-10. A special type of pneumatic piezometer isavailable for installation by pushing into foundation mate-rial, rather than by sealing as shown in Figure 4-10. Theinstrument shown in Figure 4-12 is applicable for moni-toring consolidation pore water pressures below levees, incases where vertical compression of the foundation mate-rial is large. The piezometer is pushed below the bottomof a borehole, and the borehole is filled with a soft ben-tonitic grout. Great care must be taken during installationto avoid damaging the lead connection to the sensor.

f. Vibrating wire piezometers.Vibrating wirepiezometers are based on the pressure sensor shown inFigure 4-2. A filter is added, and permanent embeddedinstallation arrangements can be similar to those shownfor an open standpipe piezometer in Figure 4-10. Specialheavy-walled versions are available for installation incompacted fills, the heavy wall ensuring that the instru-ment responds only to changes in pore water pressure,and not to total stresses acting on the housing. Specialversions are also available, similar to the pneumaticpiezometer shown in Figure 4-12, for monitoring consoli-dation pore water pressures below levees where verticalcompression of the foundation material is large.

g. Electrical resistance piezometers.Electrical resis-tance piezometers, based on the strain gages shown inFigures 4-3 and 4-4, have been used in embankment damsand levees. The vent tubes have been known to blockand invalidate readings as shown in Figure 4-13.

Figure 4-12. Pneumatic piezometer for installation bypushing in place below bottom of a borehole in verysoft clay (Dunnicliff 1988)

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Figure 4-13. Fully encapsulated submersible pressure transducer (courtesy of Druck, Inc.)

h. Type of filter. All piezometers include an intakefilter. The filter separates the pore fluid from the struc-ture of the soil in which the piezometer is installed andmust be strong enough to avoid damage during installa-tion and to resist the total stresses without undue deforma-tion. Filters can be classified in two general categories:high air entryand low air entry.

(1) Filters keep fluids in equilibrium by balancing thepressure differential with surface tension forces at thegas/water interface. The smaller the radius of curvatureof the menisciat the interface, the larger can be the pres-sure difference between water and gas. Since the mini-mum radius of curvature of themenisciis dictated by thepore diameter in the filter, the finer the filter, the greatercan be the pressure differential. Theair entry valueorbubbling pressureof the filter is defined as the pressuredifferential at which blow-through of gas occurs. Thus, afilter with a high air entry value (or high bubbling pres-sure) is a fine filter that will allow a high pressure differ-ential before blow-through occurs.

(2) Low air entry filters are coarse filters that readilyallow passage of both gas and water, and should be usedfor all piezometer types that are installed in saturated soilsand for open standpipe piezometers installed in unsatu-rated soils. Typical low air entry filters have a porediameter of 0.02-0.08 mm, 20-80 microns (0.001-0.003 in.), and air entry values range from 3-30 kPa (0.4to 4.0 lb/in.2). Filters should be saturated when installed.They can readily be saturated with water prior to in-stallation by soaking or by passing water through thepores.

(3) High air entry filters are fine filters that must beused when piezometers (except open standpipe piezom-eters) are installed in unsaturated soil, such as the com-pacted core of an embankment dam, with the intent ofmeasuring pore water pressure as opposed to pore gaspressure, in an attempt to keep gas out of the measuringsystem. These filters typically have a pore diameter of0.001 mm, 1 micron (4 × 10-5 in.) and an air entry valueof at least 100 kPa (15 lb/in.2). Saturation of high airentry filters requires a much more controlled procedure,entailing removal of the filter from the piezometer, plac-ing the dry filter in a container, and applying a vacuum.The filter should then be allowed to flood gradually withwarm de-aired water.

i. Recommended instruments for measuring piezo-metric pressure in saturated soil.Advantages and limita-tions of instruments for measuring piezometric level aresummarized in Table 4-1.

(1) Reliability and durability are often of greaterimportance than sensitivity and high accuracy. Therefore,the designer’s intent for the use of the instrument is cru-cial to the selection of the type of instrument. The factthat the actual head may be in error by 300 mm (1 ft), asa result of time lag, may not matter in some cases, pro-vided the piezometer is functioning properly. Piezometerinstallations with transducers may require corrections forbarometric pressure if high accuracy is needed.

(2) As indicated in paragraph 4-3a and Table 4-1,observation wells should be used only rarely. For meas-urement of piezometric pressure in saturated soil, an open

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Table 4-1Instruments for Measuring Piezometric Pressure

Instrument Type Advantages Limitationsa

Observation well Easy installationField readable

Provides vertical connection between strataand should only be used in continuouslypermeable strata

Open standpipe piezometer ReliableLong successful performance recordSelf-de-airing if inside diameter of

standpipe is adequateIntegrity of seal can be checked after

installationCan be used to determine permeabilityReadings can be made by installing

pressure transducer or sonic sounder instandpipe

Time lag can be a factorSubject to damage by construction

equipment and by vertical compression ofsoil around standpipe

Extension of standpipe throughembankment fill interrupts constructionand may cause inferior compaction

Possible freezing problemsPorous filter can plug owing to repeated

water inflow and outflow

Twin-tube hydraulicpiezometer

Buried components have no moving partsReliable when maintainedLong successful performance recordWhen installed in fill, integrity can be

checked after installationPiezometer cavity can be flushedCan be used to determine permeabilityShort time lagCan be used to read negative pore water

pressures

Application generally limited to long-termmonitoring of pore water pressure inembankment dams

Elaborate terminal arrangements neededTubing must not be significantly above

minimum piezometric elevationPeriodic flushing is requiredPossible freezing problemsAttention to many details is necessary

Pneumatic piezometer(Embedded)

Short time lagCalibrated part of system accessibleMinimum interference to construction; level

of tubes and readout independent of levelof tip

No freezing problems

Requires a gas supplyInstallation, calibration, and maintenance

require care

Vibrating wire piezometer(Embedded)

Easy to readShort time lagMinimum interference to construction; level

of lead wires and readout independent oflevel of tip

Lead wire effects minimalCan be used to read negative pore water

pressuresNo freezing problems

Potential for zero drift (Specialmanufacturing techniques required tominimize zero driftb)

Need for lightning protection should beevaluated

Electrical resistance piezometer(Embedded)

Easy to readShort time lagMinimum interference to construction; level

of lead wires and readout independent oflevel of tip

Can be used to read negative pore waterpressures

No freezing problems

Potential lead wire effects unless convertedto 4 to 20 milliamps

Errors caused by moisture and corrosionare possible

Need for lightning protection should beevaluated

a Diaphragm piezometer readings indicate the head above the piezometer, and the elevation of the piezometer must be measured orestimated if piezometric elevation is required. All diaphragm piezometers, except those provided with a vent to the atmosphere, aresensitive to barometric pressure changes. If piezometer pipes, tubes, or cables are carried up through fill, there will be significant inter-ruption to construction and the probability of inferior compaction.b See Dunnicliff (1988)

Source: Dunnicliff (1988)

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standpipe piezometer is the first choice and should beused when applicable. Limitations associated withextending the standpipe through embankment fill normallyprevent their use within the fill of an embankment dam orlevee (see paragraph 6-4). When any of these limitationsare unacceptable, a choice must be made among theremaining piezometer types.

(3) For short-term applications, defined as applica-tions that require reliable data for a few years (forexample during the typical construction period), thechoice is generally between pneumatic and vibrating wirepiezometers. The choice will depend on the factors listedin Table 4-1, on the user’s own confidence in one or theother type, and on a comparison of cost of the total moni-toring program.

(4) For long-term applications, selection criteria aresimilar, but because of their basic simplicity and reliabil-ity, twin-tube hydraulic piezometers and Casagrandepiezometers have become attractive options.

(5) For monitoring consolidation pore water pressuresbelow levees, in cases where vertical compression of thefoundation material is large, the push-in pneumatic orvibrating wire piezometers (paragraphs 4-3e and 4-3f) aregood choices. Push-in versions of open standpipepiezometers are also available.

(6) When the economics of alternative piezometersare being evaluated, thetotal cost should be determined,considering costs of instrument procurement, calibration,installation, maintenance, monitoring, and data processing.The cost of the instrument itself is rarely the controllingfactor and should never dominate the choice.

j. Recommended instruments for measuring porewater pressure in unsaturated soil.If the pores in a soilcontain both water and gas, such as in the compacted claycore of an embankment dam or in an organic soil deposit,the pore gas pressure will be greater than the pore waterpressure (Chapter 2). In fine-grained soils the pressuredifference can be substantial, and special techniques arerequired to ensure measurement of pore water pressurerather than pore gas pressure. For all piezometer typesother than open standpipe piezometers, these techniquesinclude use of high air entry filters, saturated as describedin paragraph 4-3h, with the filter in contact with theunsaturated soil. Intimate contact will not be achieved ifthe filter is on the flat end of a cylindrical piezometer; thefilter must be on the side or on a conical end. Thepiezometer should not be installed in a sand pocket.

(1) Piezometer selection criteria are similar to thosegiven in paragraph 4-3i for saturated soil. For short-termapplications, the choice will generally be among openstandpipe, pneumatic, and vibrating wire piezometers.

(2) For long-term applications the longevity of filtersaturation is uncertain because gas may enter the filter bydiffusion. The compacted fill in an embankment dammay remain unsaturated for a prolonged period after thereservoir is filled, and in fact the fill may never becomepermanently saturated by reservoir water. Increase ofwater pressure causes air to go into solution, and the air isthen removed only when there is enough flow through thefill to bring in a supply of less saturated water. Thepressure and time required to obtain saturation depend onthe soil type, degree of compaction, and degree of initialsaturation. Pore gas pressure may therefore remain signi-ficantly higher than pore water pressure for a substantiallength of time, perhaps permanently. Pneumatic andvibrating wire piezometers therefore cannot be relied uponfor monitoring long-term porewater pressures. However,twin-tube hydraulic piezometers allow for flushing of thefilter and cavity with de-aired liquid, thereby ensuring thatpore water pressure continues to be measured. Thechoice for long-term reliable measurement of pore waterpressure is therefore between open standpipe and twin-tube hydraulic piezometers.

4-4. Measurement of Deformation

Instruments for measuring deformation can be grouped inthe categories listed in Table 4-2. Definitions of eachcategory, together with an indication of typical applica-tions, are given below.

a. Surveying methods.Surveying methods are usedto monitor the magnitude and rate of horizontal and verti-cal deformations of the surface monuments on and at thetoes of embankment dams and levees. When subsurfacedeformation measuring instruments are installed, survey-ing methods are also often used to relate instrument meas-urements to a reference datum.

(1) Surveying methods include optical leveling,taping, traverse lines, measuring offsets from a baseline,triangulation, electronic distance measurement, trigono-metric leveling, photogrammetric methods, and the satel-lite-based global positioning system. Details are beyondthe scope of this manual and are included, together withadditional references, in Dunnicliff (1988).

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Table 4-2Categories of Instruments for Measuring Deformation

Type of Measured Deformation

Category ↔ —̇ —

SURVEYING METHODSOptical and other methodsBenchmarksHorizontal control stationsSurface measuring points

• • • •

PROBE EXTENSOMETERSMechanical probe gagesElectrical probe gagesCombined probe extensometers and

inclinometer casings

• • • •

FIXED EMBANKMENT EXTENSOMETERSSettlement platformsBuried platesGages with electrical linear

displacement transducers

• • • •

SUBSURFACE SETTLEMENT POINTS • •

FIXED BOREHOLE EXTENSOMETERSSingle-point extensometersMultipoint extensometers

• • • •

INCLINOMETERSProbe inclinometersIn-place inclinometers

• • • • •

LIQUID LEVEL GAGESSingle-point gagesFull-profile gages

• •

Key: ↔ horizontal deformation rotational deformationvertical deformation —̇ surface deformationaxial deformation (↔ or or in between) — subsurface deformation

Source: Dunnicliff (1988)

(2) All surveying methods must be referenced to astable reference datum: abenchmarkfor vertical defor-mation measurements and ahorizontal control stationforhorizontal deformation measurements. Great care must betaken to ensure stability of reference datums, for exampleby installing deep benchmarks (below frost depth) inboreholes drilled to an immovable stratum.

(3) Surface measuring points (points on the surfacethat are used for survey observations, and that may move)

must be stable and robust points that will survive through-out the project life, and must be isolated from the influ-ence of frost heave and seasonal moisture changes.Figure 4-14 shows a typical measuring point for monitor-ing settlement on the surface of an embankment dam.

b. Probe extensometers. Probe extensometersaredefined in this manual as devices for monitoring thechanging distance between two or more points along a

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Figure 4-14. Measuring point for monitoring settlementon surface of embankment dams

common axis, by passing a probe through a pipe. Meas-uring points along the pipe are identified mechanically orelectrically by the probe, and the distance between pointsis determined by measurements of probe position. Fordetermination of absolute deformation data, either onemeasuring point must be at a location not subject todeformation, or its position with respect to a referencedatum must be periodically determined by surveyingmethods. The pipe may be vertical, providingmeasurements of settlement or heave, or may be horizon-tal, providing lateral deformation measurements, forexample in downstream shells of embankment dams.

(1) Probe extensometers are used for monitoringvertical compression of the fill or foundations of embank-ment dams and levees. Four types of probe exten-someters are described below.

(2) The crossarm gage was developed by theU.S. Bureau of Reclamation for installation during con-struction of embankment dams. As shown in Figure 4-15,it consists of a series of telescoping pipe sections withalternate sections anchored to the embankment by hori-zontal steel channel crossarms. The crossarms ensure thatthe pipes move together an amount equal to compressionof the intervening fill. Depths to the measuring point atthe lower end of each interior pipe are sounded by aprobe with spring-loaded sensing pawls, lowered on asteel tape. The probe is lowered just beyond each interiorpipe in turn and raised until the pawls latch against the

Figure 4-15. Crossarm gage: (a) Schematic of pipearrangement and (b) measurement probe

lower end. On reaching the bottom of the pipes, thepawls retract and lock within the body of the probe. Thecrossarms limit the application to embankments, becausethe gage cannot be installed in boreholes in foundationmaterial. It is therefore applicable only for newconstruction.

(3) A mechanical probe, similar to the measurementprobe shown in Figure 4-15(b), can be lowered withintelescoping inclinometer casing (paragraph 4-4f) to deter-mine the depths to bottom ends of casing sections. Inembankments, the casing is forced to follow the pattern ofsoil compression by attaching settlement collars to itsoutside. Settlement collars are not possible for boreholeinstallations; thus, the telescoping of casing in boreholeswill not necessarily conform with soil compression, anddata may not be correct.

(4) The induction coil gageconsists of an embeddedtelescoping pipe surrounded by steel rings or plates at therequired measuring points. The reading device consists ofa primary coil housed within a probe and an attachedsignal cable connected to a current indicator. Readings

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are made by traversing the probe along the pipe and not-ing the tape graduation when output current is a maxi-mum. A schematic of a commercial version is shown inFigure 4-16. As will be seen from the figure, boreholeinstallations require that the grout must be very carefullyselected to ensure that the measurement system deformsaxially in exact conformance with the soil, and this isoften not possible when there are variable strata or wherevertical compression is large, such as in levee founda-tions. When the gage is installed horizontally in thedownstream shell of an embankment dam, with steelplates around a telescoping pipe, it is referred to as ahorizontal plate gage. This arrangement can be used onlywhen drainage is provided downstream of the core. Asan alternative to the probe, multiple transducers can beconnected by rods and left in place within the pipe, andinched along the pipe to take readings, thereby increasingmeasurement accuracy.

Figure 4-16. Schematic of Slope Indicator Companysondex probe extensometer installed in a borehole(Dunnicliff 1988)

(5) The magnet/reed switch gageis based on thetransducer shown in Figure 4-8, arranged as in Fig-ure 4-17. Thespider magnetshave spring anchors thatensure conformance with soil deformation, hence allowingthe gage to be used where vertical compression is large.

Figure 4-17. Schematic of probe extensometer withmagnet/reed switch transducer, installed in a borehole(Dunnicliff 1988)

c. Fixed embankment extensometers. Fixedembankment extensometersare defined in this manual asdevices placed in embankment fill as filling proceeds formonitoring the changing distance between two or morepoints along a common axis without use of a movableprobe. They are used for monitoring settlement, horizon-tal deformation, or strain. Three types are describedbelow.

(1) Settlement platformscan be used for monitoringthe settlement of levee foundation material. A settlementplatform consists of a square plate of steel, wood, orconcrete placed at a known elevation on the originalground surface, to which a riser pipe of known elevationis attached (Figure 4-18). Optical leveling measurementsto the top of the riser provide a record of plate elevations.Although a simple and frequently used device, the riserpipe tends to interfere with fill placement, and can readilybe damaged if not protected.

(2) A buried plate is identical to the steel or con-crete base plate of a settlement platform, and its use over-comes problems associated with the coupled riser pipe.To take an elevation reading on the plate, a vertical bore-hole is drilled, jetted, or augered from an accurately sur-veyed surface position, the plate located, and a depthmeasurement made. An accurate record must of coursebe made of the initial location of the plate in plan andelevation, and the plate must be large and level.

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Figure 4-18. Typical settlement platform (Dunnicliff1988)

(3) Gages with electrical linear displacement trans-ducers (Figure 4-19) are used for monitoring horizontalstrain in embankment dams. The transducers can be anyof the types described in paragraph 4-2d.

d. Subsurface settlement points. Subsurface settle-ment pointsare used for monitoring consolidation settle-ments in embankment and foundation material. Thedevice consists essentially of a riser pipe anchored at thebottom of a vertical borehole and an outer casing to iso-late the riser pipe from downdrag forces caused by settle-ment of soil above the anchor. Settlement of the anchoris determined by measuring the elevation of the top of theriser pipe, using surveying methods. Figure 4-20 showsone example.

e. Fixed borehole extensometers.

(1) Fixed borehole extensometersare defined in thismanual as devices installed in boreholes in soil or rockfor monitoring the changing distance between two ormore points along the axis of a borehole, without use of a

Figure 4-19. Schematic of fixed embankmentextensometer with electrical linear displacementtransducers (Dunnicliff 1988)

Figure 4-20. Schematic of spiral-foot subsurface settle-ment point (Dunnicliff 1988)

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movable probe. When the location of one measurementpoint is determined with respect to a fixed referencedatum, the devices also provide absolute deformation data.

(2) Many types of fixed borehole extensometers areavailable, the primary variables being anchor type, SPBXor MPBX, transducer type, and extensometer head. Atypical application is monitoring deformations behind thefaces of excavated slopes. The operating principle isshown in Figure 4-21. The distance from the face of thecollar anchor to the end of the rod is measured usingeither a mechanical or an electrical transducer (para-graph 4-2d). The device shown is a single-point boreholeextensometer (SPBX), but several downhole anchors canbe located in a single borehole, each with an attached rodfrom the downhole anchor to the collar anchor, to create amultipoint borehole extensometer (MPBX). MPBXs areused to monitor the deformation or strain pattern alongthe axis of an appropriately oriented borehole, for exam-ple, so that potential failure zones can be located anddangerous deep-seated movements separated from surfacespalling.

Figure 4-21. Operating principle of fixed boreholeextensometer (Dunnicliff 1988)

f. Inclinometers. Inclinometers are defined asdevices for monitoring deformation parallel and normal tothe axis of a flexible pipe by means of a probe passingalong the pipe. The probe contains two gravity-sensingtransducers (usually a force balance accelerometer, as

shown in Figure 4-7) designed to measure inclination withrespect to the vertical. The pipe may be installed eitherin a borehole or in fill, and in most applications isinstalled in a near-vertical alignment, so that the inclinom-eter provides data for defining subsurface horizontaldeformation.

(1) Typical applications for inclinometers includemonitoring the extent and rate of horizontal movement ofembankment dams and levees, both in the fill and founda-tion, and monitoring slope stability. As shown in Fig-ure 4-22, an inclinometer system consists of fourcomponents: a guide casing, a portable probe, a portablereadout unit, and a graduated electrical cable. Guidecasings, made of ABS (acrylonitrile/butadiene/styrene),are provided by the inclinometer manufacturer and areavailable in various sizes, are suitable for most applica-tions, and are available with telescoping couplings forinstallations where significant vertical compression isexpected. Aluminum alloy casing has been used, butseveral instances of total corrosion within a few monthshave been reported, and its use is not recommended.

(2) After installation of the casing and surveying ofits tip location, the probe is lowered to the bottom and aninclination reading is made. Additional readings are madeas the probe is raised incrementally to the top of the cas-ing, providing data for determination of initial casingalignment. The differences between these initial readingsand a subsequent set define any change in alignment.Provided that one end of the casing is fixed from transla-tion or that surface translation is measured by separatemeans, these differences allow calculation of absolutehorizontal deformation at any point along the casing.

(3) Inclinometer casing can also be installed horizon-tally in the downstream shells of embankment dams,providing data for determining vertical deformation.

(4) In-place inclinometers operate in the same guidecasing as conventional probe-type inclinometers, but aseries of gravity sensing transducers are left in place inthe casing. When compared with conventional inclinome-ters, advantages include more rapid reading, an option forcontinuous automatic reading, and an option for connec-tion to a console for transmission of data to remote loca-tions or for triggering an alarm if deformation exceeds apredetermined amount. Disadvantages include greatercomplexity and expense of the hardware, environmental(water) damage protection for the electronics, and theinability to confirm data quality by using the inclinom-eters “check-sum” procedure. In-place inclinometers are

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Figure 4-22. Principle of inclinometer operation (Dunnicliff 1988)

most advantageous when placed so that they span a previ-ously defined shear zone.

g. Liquid level gages. Liquid level gagesaredefined in this manual as instruments that incorporate aliquid-filled tube or pipe for determination of relativevertical deformation. Relative elevation is determinedeither from the equivalence of liquid level in a manometeror from the pressure transmitted by the liquid.

(1) The primary application for liquid level gages ismonitoring settlements within the foundation or fill ofembankment dams and levees. In general, they are alter-natives to vertical probe extensometers, settlement plat-forms, and subsurface settlement points, allowinginstallation to be made without frequent interruption tonormal fill placement and compaction, and minimizing thepotential for instrument damage.

(2) In general, liquid level gages are sensitive toliquid density changes caused by temperature variation, tosurface tension effects, and to any discontinuity of liquidin the liquid-filled tube. Precision claimed by the manu-facturers of these instruments is sometimes unrealistic,and the various sources of error must be minimized.Gages can be categorized assingle-point gagesand full-profile gages.

(3) The simplest single-point gage is shown in Fig-ure 4-23. The gage is normally read by adding liquid tothe liquid-filled tube at the readout station, causing over-flow in the cell such that the visible level at the readoutstation stabilizes at the same elevation as the overflowpoint. The vent tube is essential to maintain equal pres-sure on both surfaces of liquid, and the drain tube isneeded to allow overflowed liquid to drain out of the cell.

Figure 4-23. Schematic of overflow liquid level gagewith both ends at same elevation (Dunnicliff 1988)

(4) Other types of single-point liquid level gages arearranged such that the readout station can be either at a

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higher or lower elevation than the cell. Figure 4-24shows an example of the first configuration. The uppersurface of the liquid column is at a known elevation at thereadout location; therefore, relative elevation of the trans-ducer and reservoir can be determined from the pressuremeasurement and liquid density. Experience has shownthat, despite using de-aired liquid in these instruments,continuity of liquid can rarely be maintained, causingreading errors. Improved versions are available, in whichthe entire liquid-filled part is backpressured with gas. Byrecording the measured pressure and backpressure as thebackpressure is increased, the point at which any discon-tinuities are forced into solution can be determined, andthis source of error is eliminated.

Figure 4-24. Schematic of liquid level gage with pres-sure transducer in cell, with readout unit higher thancell (Dunnicliff 1988)

(5) Most full-profile gages consist of a near-horizontal plastic pipe and an instrument that can bepulled along the pipe. Readings are made at points withinthe pipe, and the entire vertical profile can be determined.Differences in vertical profile with time provide data fordetermination of vertical deformation. The only applica-tion for this type of gage is in the downstream shells ofembankment dams. The most frequent use has been aspart of ahorizontal plate gage, requiring installation of anear-horizontal telescoping pipe in the downstream shell.Horizontal deformation measurements are made by pass-ing an induction coil probe extensometer (paragraph 4-4b)along the pipe, and vertical deformation measurements are

made with a full-profile gage, based on the principleshown in Figure 4-23.

(6) Another version of full-profile gage, thedoublefluid settlement gage, has been successfully used to moni-tor vertical deformation along a continuous loop of nylontubing installed throughout various zones in embankmentdams, including the core, transition zones, and both shells.The gage is shown in Figure 4-25.

Figure 4-25. Schematic of double fluid settlement gage(Dunnicliff 1988)

h. Recommended instruments for measuring defor-mation of embankment dams and levees.Table 4-3 sum-marizes suitable instruments. The choice among thevarious possibilities depends on site-specific consider-ations and on a comparison of cost of the total monitor-ing program.

4-5. Measurement of Total Stress

Total stress measurements in soil fall into two basic cate-gories: measurements within a soil mass and measure-ments at the face of a structural element. In this manualthe terms embedment earth pressure cellsand contact

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earth pressure cellsare used for the two basic categories,respectively.

a. Applications. Embedment earth pressure cells areinstalled within fill, for example, to determine the distri-bution, magnitude, and direction of total stress within anembankment dam. Applications for contact earth pressurecells include measurement of total stress against retainingwalls and culverts. The primary reasons for use of earthpressure cells are to confirm design assumptions and toprovide information for the improvement of futuredesigns; they are usually inappropriate for constructioncontrol or other reasons.

b. Embedment earth pressure cells.Attempts tomeasure total stress within a soil mass are plagued byerrors, because both the presence of the cell and theinstallation method generally create significant changes inthe free-field stress. It is difficult and expensive to matchthe elastic modulus of the earth pressure cell to that of an

individual soil. It is also very hard to place the cell underfield conditions so that the material around the cell hasthe same modulus and density as the surrounding soil orrockfill, and with both faces of the cell in intimate contactwith the material. It is also very difficult and costly toperform a truly representative calibration in the laboratoryto determine the cell response or calibration factor.Therefore, it is usually impossible to measure total stresswith great accuracy.

(1) There are two basic types of embedment earthpressure cells:diaphragm cells and hydraulic cells.Examples of the two types are shown in Figure 4-26.

(2) Numerous factors affect measurements, includingthe ratio of cell thickness to diameter (aspect ratio), theratio of soil stiffness to cell stiffness, cell size, and fieldplacement effects. The accepted field installation pro-cedure involves compacting fill with heavy equipment,installing the cells in an excavated trench, and backfilling

Table 4-3Suitable Instruments for Measuring Deformation of Embankment Dams and Levees

Measurement Suitable InstrumentsAdditional Instruments forSpecial Cases

Vertical deformation ofembankment surface, andground surface at and beyondtoe of embankment

Optical levelingTrigonometric levelingSatellite-based systemBenchmarks

Horizontal deformation ofembankment surface, andground surface at and beyondtoe of embankment

Electronic distance measurementsTriangulationSatellite-based systemHorizontal control stations

Vertical deformationbelow surface

Single-point and full-profile liquid level gages

Double fluid settlement gagesHorizontal plate gagesHorizontal inclinometersBenchmarks

Settlement platformsa

Buried platesSubsurface settlement

pointsa

Probe extensometerswith induction coil ormagnet/reed switchtransducers, installedverticallya

Horizontal deformationbelow surface

Horizontal plate gagesProbe inclinometersa

Horizontal controlstations

Fixed embankmentextensometers withvibrating wire lineardisplacementtransducers

In-place inclinometersa

a If carried up through fill, there will be significant interruption to construction and the probability of inferior compaction. If inclinometercasing is used in an embankment dam, it should be installed within the filter zone, not within the core.

Source: Dunnicliff (1988)

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Figure 4-26. Basic types of earth pressurecell: (a) diaphragm cells and (b) hydraulic cells(Dunnicliff 1988)

around and over them by hand tamping or light machine.The probability is high that cells are therefore surroundedby a zone of soil with greater compressibility than theremainder of the fill, that imposed stresses are thereforeredistributed by arching, and that substantialunder-registration occurs.

(3) Experience has shown that hydraulic cells are lesssubject to error than diaphragm cells. Diaphragm cellsare designed and calibrated for a uniformly distributedload on the active faces, and point loads, stress nonunifor-mities, or arching will cause significant errors. Hydrauliccells are also subject to errors from these causes but to alesser extent than diaphragm cells. The best choiceappears to be a hydraulic cell with aspect ratio less than1/15, thick active faces, grooves in the faces as shown inFigure 4-26(b), and a thin layer of liquid. The grooves

help to minimize the effect of nonuniform loading on thefaces of the cell and lateral stress effects.

c. Contact earth pressure cells.

(1) Measurements of total stress against a structureare not plagued by so many of the errors associated withmeasurements within a soil mass, and it is possible tomeasure total stress at the face of a structural elementwith greater accuracy than within a soil mass. However,cell stiffness and the influence of temperature are oftencritical. The primary needs are to install the sensitiveface of the cell exactly flush with the surface of the struc-tural element, and to ensure that the cell does not create a“soft spot.” Again, a grooved hydraulic cell with a thickactive face and a thin layer of liquid appears to be thebest choice.

(2) Contact cells may be subjected to a conditionthat does not exist for embedment cells. As concrete inthe structural element cures, its temperature increases, andthe temperature of the cell is therefore increased. Thepossibility therefore exists for the cell to expand, pushingthe weak concrete away from the cell and, when the con-crete takes its initial set and cools, for the cell to cool,contract, and uncouple itself from the surrounding con-crete. In this condition the cell would not be responsiveto subsequent stress changes. It is therefore preferable toconstruct the cell with one steel plate approximately0.5 in. (13 mm) thick and to place that plate against theconcrete, thereby ensuring that any expansion occursoutward.

4-6. Measurement of Temperature

Applications for measurement of temperature fall into twogeneral categories. For example, during attempts to detectseepage and to monitor changes in the seepage pattern inan embankment dam, temperatures are monitored inter-nally within the embankment. Second, when a transduceritself is sensitive to temperature change, then a thermalcorrection to measured data may be required.

a. Types of instruments.Three types of instrumentsare most frequently used for remote measurement oftemperature: thermistors, thermocouples, and resistancetemperature devices (RTDs). The namethermistor isderived fromthermally sensitiveresistor. A thermistor iscomposed of semiconductor material that changes itsresistance very markedly with temperature. Athermo-couple is composed of two wires of dissimilar metals,

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with one end of each wire joined together to form a meas-uring junction. At any temperature above absolute zero, asmall voltage is generated between the wires at the otherend. This voltage is proportional to the difference intemperature of the measuring junction and the temperatureof the cold junction. Resistance temperature devicesdepend on the principle that change in electrical resistanceof a wire is proportional to temperature change. The wireis usually mounted on a postage-stamp-sized backing orwound on a small-diameter coil.

b. Recommended instruments for measuring temper-ature. All three types of instruments are suitable forremote measurement of temperature, the choice dependingon the application. Comparisons are given in Table 4-4.All three types have wide temperature range and rapidresponse to temperature changes.

4-7. Measurement of Seismic Events

The outdated concept that seismic instrumentation ofembankment dams and reservoir sites is only a researchtool has given way to the modern concept that seismicinstrumentation is necessary for moderate-to-high hazarddams in seismic areas. It is also desirable in traditionallynonseismic areas. With the advent of digital seismic

equipment, it can now be an integral part of a dam safetyprogram in each CE district. The digital earthquake datacan be gathered by site personnel and district personnelby the use of a portable computer. When the digitalinstrument is installed with a modem and telephone linethen remote access from several offices is available.Servicing parameters such as battery voltage, memoryavailable, functional test of accelerometers and the num-ber of events recorded are available to check the integrityof the instrument from a remote location. Software fortime history display and for full analysis makes datareporting quick and easy. Digital accelerographs can bepart of other data acquisition systems: for example, atrigger device for piezometer recordings or for a slopestability study during an earthquake, a telephone callingsystem to report an earthquake event, or a multirecorderinstallation to study the response of an intake tower.

a. Seismic instrumentation.Three types of record-ing devices are popular today. They are (1) accelero-graph, (2) seismic acceleration alarm device (SAD), and(3) nonelectronic peak accelerograph recorder. Typicallythree accelerographs and one peak accelerograph recorderare installed per site. The SAD is installed where a dis-play of peak acceleration is required immediately

Table 4-4Comparison of Instruments for Remote Measurement of Temperature

Feature Thermistor ThermocoupleResistance Temperature Device(RTD)

Readout Digital ohmmeter ormultimeter

Thermocouple reader Wheatstone bridge with millivoltscale

Sensitivity Very high Low Moderate

Linearity Very poor Fair Fair

Accuracy High Moderate Very high (but may bereduced by lead wireeffects)

Stability Excellent Good Excellent

Type of lead wire Two-conductor Special (bi-metal) Three-conductor

Repairability of lead wire Straightforward Less straightforward (cancause errors)

Straightforward

Applicability for instrumenttemperature corrections

Preferred Possible Possible

Suitability for automatic dataacquisition

Fair Excellent Good

Source: Dunnicliff (1988)

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following an earthquake. For all USACE facilities, theSAD is supplied by the USACE Waterways ExperimentStation (WES).

b. Accelerograph.An accelerograph measuresacceleration triaxially at the location the instrument isanchored to the structure. These accelerations aremutually perpendicular and are called vertical, longitudi-nal, and transverse. All instruments at the structure arealigned in the same direction usually with the longitudinalaxis parallel with the long axis of the dam. Accelero-graphs are available in analog and digital models. It isrecommended that digital instruments be installed andconnected to a modem and telephone line. Typically thethree accelerographs are located on the center of the crest,on one of the rock abutments and on downstream rock, aminimum distance of “three heights of the dam” from thetoe of the structure. A short, lightweight shelter anchoredto a thin concrete pad is recommended for housing theaccelerographs (Figure 4-27). The instrument is poweredby 12-volt batteries, and it is necessary to have AC powerand a trickle charger to maintain peak battery capacity.

Figure 4-27. 41-2 shelter on a concrete pad for hous-ing accelerograph

(1) Analog accelerograph. The analog instrumentrecords data on 70-mm film which must be recoveredunder low light conditions and chemically processed todevelop the film. A component diagram of an analogaccelerograph is shown in Figure 4-28. The film roll is15.24 m (50 ft) in length and can normally record25 earthquake events. Full-scale recorder range is 1.0 g.Recording scales larger than 1.0 g are available. The

analog earthquake data are optically digitized, and thecomputer- generated tape is processed with mainframecomputer software at WES for data analysis andreporting.

(2) Digital accelerographs. The digital instrumentcontinuously digitizes the three internal force balanceaccelerometers and stores data in solid state memory. Acomponent diagram of a digital accelerograph is shown inFigure 4-29. When an earthquake event triggers theinstrument, sensed by the three internal accelerometers,the pre-event data are saved plus the data occurring dur-ing the earthquake and post-event data following cessationof the motion. Digital data are saved with a file numberand header information. The digital instrument will returnto digitizing accelerometer data until another earthquakeevent occurs and another data file is saved. Full-scalerecorder range is 1.0 g. Depending upon memory size,512K bytes is standard, approximately 20 minutes ofrecording time is available. The data files are down-loaded locally by an RS-232 connection to a portablecomputer or to a remote computer by modem and tele-phone. Personal computer software is provided with eachinstrument to display the data for each internal forcebalance accelerometer in time history plot format and toread the associated header information as shown in Fig-ure 4-30. Computer software is also available for moredetailed analysis processing, for example, single anddouble integration and spectrum analysis.

c. Seismic acceleration alarm device (SAD). TheSAD stores and displays a vertical acceleration peak levelin a wall-mounted unit easily read by site personnel. Thedisplay increments are 0.05 g and it has a range from0.05 g to 0.5 g. The unit can be connected, by a relayclosure, to a remote alarm, annunciator panel, telephonedialer, or recorder. The SAD is AC powered with abattery backup.

d. Peak accelerograph recorder (nonelectronic).The peak accelerograph recorder senses and records accel-eration peaks triaxially. These accelerations are mutuallyperpendicular and are called vertical, longitudinal, andtransverse. It is a self-contained passive device requiringno external power or control connections with a sensi-tivity as low as 0.01 g. The records are scratched perma-nently on metal plates or magnetized strips of recordertape. The triaxial plates or tapes are read with an opticalloupe and a calibration factor is applied to obtain peakacceleration levels. Full-scale recorder range is 2.5 g.Typically, one peak accelerograph recorder is co-locatedwith an accelerograph at the crest station for redundancyand events greater than 1.0 g.

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Figure 4-28. Analog/accelerograph component diagram (courtesy of Kinemetrics, Inc., Pasadena, CA)

Figure 4-29. Digital accelerograph component diagram

Figure 4-30. Digital accelerograph data plot

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e. Strong-motion instrumentation program (SMIP).In 1973 the issuance of ER 1110-2-103 essentiallyrequired instrumentation of all USACE dams that existwithin seismic risk zones 2, 3, and 4 (ER 1110-2-1806).ER 1110-2-103 also established the WES Strong-MotionInstrumentation Program (SMIP) and its responsibilities.WES provides assistance to each CE district and divisionby reviewing each structure’s installation plan to assureconformance with HQUSACE policy, by maintainingrecords of instrument locations and servicing, by provid-ing instrumentation services personnel for installation andmaintenance of CE instruments, by gathering, processing,and analyzing earthquake records obtained, and by fur-nishing copies of records and reports to the district con-cerned. WES reviews and evaluates new instruments andassists districts in the selection and purchasing of neededequipment. (Information about SMIP is provided in anannual engineer circular for Corps-wide distribution.)

4-8. Measurement of Seepage EmergingDownstream

a. Introduction. Seepage measuring devices areused in embankments to measure amounts of seepagethrough, around, and under the embankment. Monitoringthe seepage that emerges downstream is essential toassessing the behavior of a dam during first filling. Thefirst indication of a potential problem is often given by anobserved change of seepage rate. Also monitoring thesolids content in the seepage water can provide importantinformation. Observations of seepage rate can be corre-lated with measurements of piezometric pressure, and beused to examine the effectiveness of drains, relief wells,and cutoffs. Seepage measurements are often made dur-ing the operating life of a dam to monitor long-term per-formance. Relief wells, drainage outlets, channels, andditches are common measuring points of seepage. Someseepage-measuring methods commonly used are discussedin this chapter. These devices and others are described indetail in U.S. Bureau of Reclamation publications (1984,1987).

b. Weirs. Seepage flows are often measured withweirs that have regular shaped overflow openings, such asV-notch, rectangular, trapezoidal, etc. The seepage ratesare determined by measuring the vertical distance fromthe crest of the overflow opening to the water surface inthe pool upstream from the crest. The measurement isthen used to compute the flow rate, by reference to tablesfor the size and shape of the weir. The critical parts ofweirs are easily inspected and cleaned, and any improperoperations can easily be detected and corrected quickly.Weir flows are often measured by monitoring the water

level in the weir stilling basin. Methods of water levelmonitoring include a staff gage, gas purge pressure trans-ducers, submerged pressure transducers, chart recorders,shaft-encoders, sonic transducers (paragraph 4-2e), andforce transducers. Figure 4-31 shows a useful version ofa weir with force transducers, manufactured by Geonor,Norway. Criteria for design of weir stilling basinsinclude adequate length, prevention of false readings byblockage, and protection from freezing.

Figure 4-31. Remote-reading weir for monitoring leak-age quantities (courtesy of GEONOR, Inc.)

c. Parshall flumes.Parshall flumes are speciallyshaped open-channel-flow sections that can be installed inchannels or ditches to measure flow rate. Theconstricting throat of a flume produces a differential headthat can be related to flow rate. Methods of water levelmonitoring are the same as those for weirs. Criteria fordesign of flume approaches include adequate length,alignment, prevention of false readings by blockage, andprotection from freezing.

d. Calibrated catch containers.When occasionalmeasurements of low flows are required, the flow can bediverted into a container of known volume, and the fillingtime measured.

e. Velocity meters.Several different types ofvelocity meters are available commercially, from WES,and from the Missouri River Division Laboratory. Their

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methods of operation vary. Some use the pitot tube prin-ciple, others include propeller-type devices, acoustic flow-meters, and electromagnetic current indicators. Most ofthese devices can be used to measure the flow in pipes orin open channels. Also available is a portable velocitymeter which operates on the principle of electromagneticvelocity determination. This device can be used formeasuring water velocity in pipes or in open channels,which is then converted to flow.

f. Thermotic surveys/thermal monitoring.Flowinggroundwater, of even minor magnitude, influences near-surface soil temperatures to a measurable extent.Thermotic techniques are particularly useful in identifyingzones or paths of high permeability and piezometric flowconcentrations within fractured rock and surficial deposits.Although these techniques do not replace borings or con-ventional instrumentation, they can make other monitoringactivities more accurate and economical by directing thelocation of more quantitative investigation methods suchas boreholes and pump tests.

g. Precipitation gages.Precipitation can directlyaffect seepage measurements and should be measured andevaluated with any seepage measurement data. There arenumerous precipitation gages available commercially andtheir selection should be based on the specific siteconditions.

h. Water level gages.The difference in headbetween the reservoir and tailwater levels of a dam, or theriverward water level and the landward side of a levee,provides the potential for seepage through, around, orunder the embankment. Therefore, these water levelsshould be measured and evaluated with any seepagemeasurement data. There are numerous chart recorders,encoders, manometers, and pressure transducers availablecommercially and their selection should be based on thespecific site conditions. Pressure transducers that can beused to measure these water levels parallel techniquesused to measure piezometric pressure and are discussed inparagraph 4-3.

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Chapter 5Automation Considerations

5-1. Introduction

Automation of instrumentation can assist in the assess-ment of the safety of dams and levees. This is particu-larly true for monitoring that requires rapid and frequentdata collection or for instruments that are inaccessible. Inrecent years, the technology of devices for measuringseepage, stresses, and movements in dams and levees hasimproved significantly with respect to accuracy, reliabil-ity, and economics. Although the initial installation of anautomated data acquisition system (ADAS) may appear tobe more expensive than traditional instrumentation sys-tems, the overall long-term cost, in many cases, is noweconomically competitive. Automation should receiveconsideration for all systems that are to be installed dur-ing new dam construction, major rehabilitations, structuralmodifications, or any major effort that would support amajor instrumentation system. Instrument upgrades andreplacements could be justified on a case-by-case basis.The trends of decreasing manpower and resources that areallocated for dam safety efforts within the Corps of Engi-neers suggest that engineers consider the automation tech-nology to accomplish the tasks required. The instrumentautomation concept generally includes an instrument ortransducer that is linked to a datalogger or computer withcommunication capability that allows data retrieval locallyor from a remote location. Figure 5-1 shows a generalschematic of a typical ADAS. This chapter addressesareas that are characteristically unique for automation.

5-2. Applications

The monitoring of any project or feature can be auto-mated depending on instrument type and feasibility.Automation is not limited to new installations related tonew construction, research, or specific investigation.Existing systems can be retrofitted. However, it is notappropriate to automate in all cases. Sufficient engineer-ing justification is necessary. The following situations areexamples of justifiable applications for automatinginstrumentation.

a. Historical movement or pressure trends exceedthe established thresholds, or trends are raising concernsthat warrant frequent measurement.

b. Formal studies or investigations have identifiedspecific or potential problem (stability, performance) areaswarranting frequent measurement.

c. High hazard potential in the event of uncon-trolled releases and timeliness of response to performancechanges is critical to public safety. (Consequences offailure or partial failure)

d. Structural or foundation features are required toperform in a manner that was not anticipated in the origi-nal design.

e. Complexity of design and/or complexity of con-struction requires that design considerations be verified.

f. Abnormal extent or frequency of remedial meas-ures has been or is required.

g. Instruments or systems have met their usefullife, failed to perform satisfactorily, or have degraded withtime and are required for performance assessment.

h. Timely response to unusual developments isadversely affected by remoteness of geographic location,inaccessibility, or lack of qualified onsite personnel.

i. Increased reliability and/or high frequency ofmonitoring is critical to the assessment of conditions.

5-3. Advantages and Limitations of Automation

The advantages and limitations of an automated systemfollow. The limitations can be minimized with appropri-ate attention to planning and use of the system.

a. Advantages.

• Increased accuracy-reduced human error.

• Increased frequency-more data, less system error.

• Increased data reliability and consistency.

• Replaces lost manpower.

• Timeliness of information-obtain data wheneverneeded.

• Data and system validity checks enhance dataquality.

• Alarms for exceeding data thresholds and systemhealth.

• Remote diagnostics, calibrations, andprogramming.

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Figure 5-1. Automatic data acquisition system geotechnical instrumentation

b. Limitations.

• Produces large volumes of data; overtaxes storagemedium.

• Installation could be expensive.

• Removes personal attention from the field.

• Lightning; variable voltage potential isdestructive.

• Excessive downtime with overly complexintegrations.

• Sufficient computer and electronics expertise notavailable.

• Potentially higher maintenance costs.

• Requires a constant electrical power source.

• Requires use of electronic transducers whichhave least long-term reliability of any type used.

5-4. Description of Automated Systems

Automation is the integration of the electronics, computer,and communications technologies that is applied to geo-technical and structural engineering concerns of waterresources projects. It performs a variety of tasks thatcontribute to efficient and effective performance analyses.EM 1110-2-2300, Appendix D, very generally describesthe features and capabilities of an automated system. Theappendix contains examples of networks and

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communications alternatives. It also recommends anapproach to choosing the features of a structure to bemonitored and selecting the sensors that are appropriate toautomate.

a. Configurations. Configurations of automated sys-tems can be categorized as one of three basic types asfollows:

(1) Datalogger. An electronic component collectsdata from attached instruments upon command from apersonal operator. The system can include modems forremote communication, but generally requires that theintelligence and operation be external to the system.Figure 5-2 shows a stand-alone datalogger configuration.

(2) Scada. Scada (supervisory control and data acqui-sition) is a host computer that controls remote monitoringunits (RMUs) which are intelligent dataloggers. RMUsacquire data and report those data upon command fromthe host computer. The remote unit can carry out theacquisition of data and store the information untilcommunication with the host computer is established.The host computer is the system intelligence. It is pro-grammed for frequency and scheduling of data acquisi-tion. Personal intervention is not required for this systemto operate. Figure 5-3 shows a Scada configuration.

(3) Distributed intelligence. Computers are located ateach remote monitoring unit in the network. All arelinked to a central computer; central network monitor(CNM) and communications can be established with thecentral computer as well as with each other. The remote

units are fully responsible for the frequency and schedul-ing of data acquisition as well as initiating communica-tions. Raw data can be reduced and certain otherdecisions can be made at these remote points withoutpersonal or central computer intervention. Figure 5-4shows a distributed intelligence configuration.

b. Communications.Communication of informa-tion occurs at three different locations: sensor todatalogger, remote dataloggers to central computer, andcentral computer to remote office. The most commonmodes of communicating from the sensor to thedatalogger are electrical transmission through cables andsound transmission (acoustics) through the air. The mostcommon local communication, linking the remote units tothe host or central computer, is electrical cabling or radiotransmissions. Remote communication from the projectsite to the district office is commonly by telephone,microwave, radio, or satellite. All modes of communica-tion are not appropriate for any one application. Consid-eration must be given to lightning, presence of water, andother conditions which would affect the selection of amode of communication. Design criteria will assist inmaking that determination.

c. Power supply.Automated systems can be ener-gized by alternating current, battery, or solar panels andare normally supplemented by uninterruptible power sup-plies or emergency generators. Each of these alternativeshave positive and negative characteristics for variousapplications. The sizing and maintenance of the primarysupplies and backups should be addressed early in theplanning process.

Figure 5-2. Stand-alone datalogger configuration

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Figure 5-3. Scada configuration

5-5. Planning a System

All of the Corps of Engineers guidance and documenta-tion that have been published emphasize that the properapproach to planning a system that is reliable and cost-effective include: obtaining the appropriate information,applying good engineering judgment, using Corps

expertise when available, and learning from theexperience that has been gained by others. The specificguidance is described below:

a. Detailed planning.Experience has shown thatdam safety engineers must have some knowledge of proj-ect management and procurement issues to be successfulwith their automation endeavors. Experience has alsoshown that there are certain characteristics inherent inautomation that must be understood to enhance the suc-cess of the project. This planning tool addresses thefollowing major issues: assessing in-house resources,determining contracting mechanism, preparing CommerceBusiness Daily announcements and bid documents, techni-cal design considerations, estimating, and others. Refer toUSCOLD 1993 for more detail in each of these areas. Itis important that the planner follow the suggested order ofthe tasks in the procedure to avoid complications at laterstages of the planning process.

Figure 5-4. Distributed intelligence configuration

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b. Automation experience.USCOLD 1993, Appen-dix D, is a summary of automation experience in Federaland private sectors. It lists dam projects and variousfeatures of dams that have been automated, the equipmentused, the sensors installed, and persons that can be con-tacted for each of the projects listed. This document isvaluable to designers that are interested in learning aboutthe performance of systems that they are considering.Contacts with those persons listed should be an early stepin the planning process to identify the feasibility of pro-ceeding with the project and the appropriate direction.

c. Technical expertise.The majority of automationdesign, operation, and routine maintenance can be accom-plished by conscientious geotechnical engineers, and mod-erate levels of computer and electronic expertise, most ofwhich is available within the Corps installation. Newautomation developments and complex applications willoften require a higher level of expertise which may onlybe available through contractor support. Lack of in-housecomputer and electronic expertise may also require adependency on consultants and manufacturers to design,install, troubleshoot malfunctions, and implement correc-tions. Because of this contractor dependency it is highlydesirable to specify a number of performance tests of allfeatures of the system. Simultaneous manual andelectronic data acquisition should also be accomplishedduring system performance testing. An increased level ofcost will be proportional to the degree of dependency onoutside expertise. This should be considered when plan-ning and designing automated systems. Several Corps ofEngineers Districts as well as WES have a significantamount of expertise in planning, design, and system oper-ation and maintenance. Emphasis is placed on the use ofin-house expertise as a priority with outside assistance(consultants, manufacturers, etc.) used as necessary.

d. Selection of instruments.If there is sufficientjustification for automation (see paragraph 5-2), and pre-liminary assessments indicate that automation is feasible,judgment must be applied to the selection of instrumentsto be automated. Table 5-1 lists the methods and relativeease of automating various dam safety instruments. Thefollowing criteria should be considered when choosinginstruments to be automated:

(1) Performance. Instrument performance is accept-able. Data are consistent and meaningful. Maintenance/calibration has not been excessive or ineffective.

(2) Longevity. The instrument is expected to con-tinue to function for at least 5 years.

(3) Location. The physical location of the sensor(including the tip elevation and material type) is appropri-ate to monitor the feature/condition of interest.

(4) Ability to be automated. The instrument is ableto be automated. Sensors are available and compatiblewith the current installation such that manual readingcapability can be retained as backup. The new sensor willsatisfactorily perform in an automated environment(response frequency, electronic conversions, communica-tions media, etc.).

5-6. Designing an Automation System

CWGS 16740 is technical guidance for the instrumentsystem designer. It addresses important issues that arecharacteristic of automated systems that demand attention,i.e., grounding techniques for lightning protection, the useof shielded cable, proper power supplies, etc. The guidespecification is not a specific recipe but design and instal-lation considerations for engineering judgment.

5-7. Implementation

Obtaining funding support for automation is often a con-cern. The best opportunities for implementing automationis for projects that are under construction (new projects,dam safety remedial efforts) and approved structural mod-ifications, investigations, and research. Smaller projectsmay be accomplished with routine allocations ofoperations and maintenance (O&M) funds which arenormally limited.

5-8. Data Acquisition, Management, Analysis,and Reporting

These issues are discussed in Chapter 7. That informationis applicable to automated systems. In addition to thatdiscussed, automated systems can provide large quantitiesof data at no additional cost. Discrete management ofthis volume of information is necessary to keep it fromexceeding storage, to reduce time involved with communi-cations, and to prevent it from becoming cumbersomewhen processing, analyzing, and reporting.

5-9. Maintenance

Utilizing data to determine the need for maintenance isdiscussed in Chapter 7. That approach is also applicableto automated systems. Automated systems can produce alarge volume of data that are accessible on a very fre-quent basis. This provides greater opportunity to assess

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Table 5-1Methods and Ease of Automating Various Instruments (Adapted from USCOLD 1993) (Continued)

Category

Instrument1 1 2 3 4 Automation Method Manual Backup

Piezometers

Open standpipe • Pressure transducer down standpipe Usually straightforward by measuring towater surface

Twin-tube hydraulic • Pressure transducers connected to lines Straightforward, by existing pressuregages

Pneumatic • Pressure actuating and meas. system Straightforward, with manually operatedreadout

Vibrating wire • Frequency counter Ditto

Electrical res. strain gage • Strain gage, completion circuitry Ditto

4-20 mA pressure transducer • • Current readout Ditto

Sonic • Time readout Ditto

Deformation Gages

Tiltmeter2 • • See footnote 2 Ditto

Probe extensometer • N/A N/A

Embankment and boreholeextensometer2

• • See footnote 2 Straightforward, with manually operatedreadout

Inclinometer, probe type • N/A N/A

Inclinometer, in-place type2 • See footnote 2 Straightforward, with manually operatedreadout

Plumb line and invertedpendulum2

• • Infrared or light sensor line positiontransducers

Ditto

Liquid level gage2 • • Pressure transducer Ditto

Earth Pressure Cells

Pneumatic • Pressure actuating and meas. system Ditto

Electrical res. strain gage • Strain gage completion circuitry Ditto

Load Cells and Strain Gages

Electrical res. strain gage • Strain gage completion circuitry Ditto

Vibrating wire • Frequency counter Ditto

Hydraulic load cell • Pressure transducer Ditto

Temperature

Thermistor • • Resistance readout Ditto

Thermocouple • Low level voltage readout Ditto

Resistance temperature device • • Resistance readout Ditto

There are four categories in order of increasing difficulty, specialization, and cost of automation:

CATEGORY 1: Straightforward. Category 1 can be done by numerous manufacturers of laboratory and industrial ADAS.

CATEGORY 2: More Specialized Requirements. Thermally unconditioned environments, inaccessible power, nonstandard signal condi-tioning, low level voltage measurement, wide common mode voltage range.

CATEGORY 3: Most Specialized Requirements. Hostile outdoor environment, hard-wire communication unavailable - radio network re-quired, difficult sensors to automate like plumb lines and pneumatic piezometers, sensors which communicate via serial or parallel inter-face.

CATEGORY 4: Usually Not Practicable to Automate. Technical complexity or cost outweighs benefits, significant reliability problems inhostile environment, customized automation hardware required, impracticable to automate.

1 For a definition of instrument terms, see this manual and Dunnicliff (1988).2 Depends on type of transducer.

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Table 5-1 (Concluded)

Category

Instrument 1 2 3 4 Automation Method Manual Backup

Miscellaneous

Reservoir and tailwater elevation,rainfall gage2

• • See footnote 2 Ditto

Weir for measuring water flow2 • • See footnote 2 Ditto

Seismograph analog • N/A Seismographic chart record

Seismograph digital • N/A Seismographic chart record

instrument performance easily and often. Additionaldiscussion regarding calibration is in Chapter 8 and it isalso applicable to automated systems. Some componentscan be calibrated with expertise already existing at manyCorps installations; other calibrations can be accomplishedwith minimal training and additional equipment. Calibra-tion of custom-developed components could require theattention of outside expertise.

a. Planning. Automation experience has indicatedthat maintenance can be minimized with proper planningand forethought. The following is suggested:

(1) Determine the person(s) responsible and themeans of accomplishing the maintenance as early as theplanning and design phases of the project.

(2) Obtain clear, simple documentation, O&M man-uals, as-installed drawings, and electronic schematics fromthe contractor and/or manufacturer.

(3) Involve the operation and maintenance personnelin the planning and design phases as well as during theinstallation and acceptance phases.

(4) Use the component concept to facilitate changingmalfunctioning devices with spares and/or replacementparts as necessary. Use equipment that has standardconnectors, parts, and operating procedures to assurecompatibility with a variety of manufacturers of similarcomponents.

(5) Obtain manufacturer-recommended spare partswith the initial system. Revise the inventory as consump-tion rate is learned.

(6) Train the maintenance staff and system opera-tors. Retrain them as personnel turnover and/or systemchanges necessitate.

(7) Learn the performance characteristics that areunique to the system and utilize the remote diagnostics.

(8) Use project blanket purchase agreements (bpa’s)or other existing mechanism for components already beingmaintained on site (radios, personal computers,telephones, etc.)

(9) Take advantage of warranties. Check the perfor-mance of all features of the system before the warrantyperiod expires. Remote calibration, diagnostics, repro-gramming, and troubleshooting are examples of systemfeatures that may be rarely or infrequently used. Unsatis-factory performance of such features may escape warrantyattention if not checked.

b. Implementation.Indefinite delivery contractsand maintenance agreements with manufacturers are themost expensive means of maintaining systems and theytend to remove the system users from thoroughly under-standing the system.

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Chapter 6Installation

6-1. Introduction

Installation of geotechnical instrumentation, whetherduring construction of a project or in an existing structure,requires special attention to detail. Faulty installation ofinstruments not only can lead to erroneous and misleadingperformance data, but could also affect the integrity of astructure. Methods of installation are dependent on theparameters to be monitored, site conditions, and selectedinstruments. Therefore, qualified, experienced, and com-petent personnel are essential if an installation is to besuccessful. An installation of an instrumentation systemis not considered routine; therefore this chapter includesguidelines and recommendations rather than step-by-stepprocedures.

6-2. Personnel Issues

The purpose of geotechnical instrumentation is to provideaccurate and timely data so that performance evaluationsof a project can be made by personnel responsible forsafety of an embankment dam or levee. The quality andaccuracy of the evaluations are highly dependent onproper instrumentation installation. In addition, Districtand project personnel will be responsible for the mainte-nance of the instrumentation system (see Chapter 8).Therefore it is highly recommended that in-house person-nel perform as much of the actual installation as possible,and that the design and installation of an instrumentationsystem be the responsibility of the instrumentationprogram manager (see Chapter 3).

6-3. Contracting Issues

Although in-house expertise is preferred, contracting maybe necessary. Work such as trenching, conduit installa-tion, and other site work can be performed by generalconstruction contractors. Drilling contractors require closesupervision by a geotechnical engineer during installation.However, a great deal of the work requires an instrumen-tation specialist (who will usually be the instrumentationprogram manager) who is familiar with the particularinstruments and has a good working knowledge of geo-technical engineering and the local site conditions. If acontractor is to be responsible for the installation of aninstrumentation system, the specifications should includethe following items:

• Purpose of the geotechnical instrumentation program.

• Responsibilities of the contractor.

• Instrumentation system performance criteria.

• Qualifications of contractor’s instrumentationpersonnel.

• Quality assurance.

• Submittals.

• Scheduling work.

• Storage of instruments.

• Materials (provide a detailed description of alltypes of instruments included in the contract).

• Factory calibration requirements.

• Pre-installation acceptance tests.

• Installation instructions (provide a detailedstep-by-step procedure for the installation ofeach type of instrument included in the contract).

• Instructions for changed site conditions.

• Field calibration and maintenance requirements.

• Protection of instruments.

6-4. Instrumentation of New Structures

Various options exist when planning installation of aninstrumentation system in a new structure. Instrumentcasing can be installed upward in the embankment as thefill rises. However, it is generally not advisable to extendthe casing vertically through the core material of anembankment. Horizontal runs of tubes and cables mustnot extend horizontally fully through the core (see Fig-ure 6-1d) and must exit the downstream face of theembankment. Vertical runs of tubes and cables can runupward through any zone of the embankment except thecore. The installation of vertical runs of tubes and wiresthrough the core of an embankment is not acceptable andmust be avoided. Obtaining adequate compaction, due todifficult access around the casing, is very difficult, result-ing in the creation of a poorly compacted zone. An alter-native to installing an instrument which requires casing isto use an instrument which requires tubes or cables. Inaddition, the inclusion of instrumentation in an

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Figure 6-1. Examples of acceptable and unacceptableroutings for instrumentation tubes and cables (afterDunnicliff 1988)

embankment construction will, almost inevitably, causedelays and require a general contractor to pay specialattention to details and provide protection when workingaround an instrument that is being installed. Schedulingof cable installation must be coordinated with other activi-ties to avoid damage to cables by construction equipment.Instruments should be sufficiently protected and locatedwith highly visible survey stakes and flagging.

6-5. Instrumentation of Existing Structures

Surface installations, retrofits, or replacements of nonem-bedded sensors do not generally create installation diffi-culties. However, drilling may be the only optionavailable when the installation or replacement of a subsur-face instrument is necessary in an existing structure.Great care must be taken when selecting a drilling methodto be used in an existing embankment. Water or air pres-sure used during drilling of a borehole can damage theembankment by hydrofracturing. A hydrofracture is acrack, shear plane, seepage path, and/or other detrimentalcondition that can develop during the drilling process.Impervious cores are especially susceptible to damage andextreme care must be used in drilling in these zones. For

this reason, the practice of drilling in embankments orfoundations using air (including air with foam) is pro-hibited per ER 1110-2-1807. A summary of drillingmethods is included in Appendix B.

6-6. Drilling Fluids

There are two purposes for using drilling fluids whenperforming a boring: first, to have a medium throughwhich the drill cuttings are moved from the cutting headat the bottom of the boring to the top; second, to stabilizethe sidewalls of the boring from collapse. The viscosityof the drilling fluid can be thickened as necessary toprevent collapse. The drilling fluid adheres to the side-walls and creates a residual skim coat. The standardbentonite mud is an unacceptable drilling fluid forpiezometers because it seals the soil and inhibits piezo-metric response. Biodegradable mud composed of anorganic polymer is acceptable as a drilling fluid because itreverts back to water through enzyme breakdown, leavingno residue. The selection of a drilling fluid must be madewith the compatibility of the installed instrument in mind.If there is any possibility of collapse of the walls of theboring, drilling fluid should not be used. Casing is oftenthe preferred method for support.

6-7. General Installation Procedures

Instruments are often installed in boreholes located ineither abutments or embankments. Many of the instru-ments installed in boreholes have minimum and maximumdiameter requirements. Drilling specifications shouldclearly spell out required diameters, depths, alignments,drilling and sampling requirements, instrument require-ments, construction details, and special requirements. Theinformation provided below is suggested guidance forinstrumentation installation. Each system to be installedshould be evaluated on a case-by-case basis. It is theresponsibility of the instrumentation program manager orgeotechnical engineer to write detailed procedures for theinstallation of an instrument. When deformations areexpected to occur, special considerations are oftenrequired so that pipes, tubes, and wires are not damagedas deformation occurs.

a. Planning. Prior to the actual installation of theinstrumentation, time should be invested in planning theinstallation. Planning includes preparation of site-specificdetailed step-by-step installation procedures and of a listof required materials and installation tools, preparation ofan installation schedule, and coordination of installationwith other parties. Additionally, the lead time necessaryfor the procurement of the instruments should be

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investigated. Many instruments are manufactured whenordered, and large orders may require significant leadtime before delivery to the site. Installation of instrumen-tation may require support from utility companies (elec-tricity or telephone) which may require long lead timebefore service is available. Possible road closures whilecables cross routes of access may be necessary. Theroutes which will be used for pulling instrument cablesthrough conduits should be decided. Other ongoing oper-ations or construction activities may impact on the plan-ning of an installation. The necessity for automation (seeChapter 5) must have previously been addressed, sincecable runs may change to match hardware limitations andusing radio for communication may be less expensivethan hardwire options.

b. Pre-installation acceptance tests. When theinstruments are received at the site, instrumentation per-sonnel should perform pre-installation acceptance tests toensure that the instruments and readout units are function-ing properly. Where electrical components are irretriev-able and must function below the water table,consideration must be given to pressure-testing of instru-ments for leaks. Pre-installation acceptance tests shouldinclude relevant items from the following list:

• Examine factory calibration data to verify com-pleteness (factory calibration and documentationshould be specified).

• Examine manufacturer’s quality assurance inspec-tion check list to verify completeness (qualityassurance procedures and documentation shouldbe specified).

• Check cable length.

• Check tag numbers on instrument and cable.

• Check, by comparing with procurement docu-ments, that the model, dimensions, materials,product performance criteria, etc. are correct.

• Bend cable back and forth at point of connectionto the instrument while reading the instrument toverify connection integrity.

• Check water pressure or humidity test compo-nents as appropriate for the service entity to iden-tify leaks.

• Verify that instrument reading as required com-pares favorably with factory reading.

• Perform resistance and insulation testing, inaccordance with criteria provided by the instru-ment manufacturer.

• Verify that all components fit together in thecorrect configuration.

• Check all components for signs of damage intransit.

• Check that quantities received correspond toquantities ordered.

Results of the tests should be documented and include thefollowing items:

• Project name.

• Instrument type and number.

• Identification of any testing or readout equip-ment used during testing.

• Personnel responsible for testing.

• Date and time of testing.

• Measurement and observations made during test-ing, as listed above.

• Test results, pass or fail.

c. Installation documentation.In addition torecording pre-installation information, a record of installa-tion should be documented and include the followingitems:

• Project name.

• Contract name and number.

• Instrument type and number, including readoutunit.

• Planned location, orientation, depth, length, andbackfill volume data.

• Personnel responsible for installation.

• Plant and equipment used, including diameterand depth of any drill casings or augers used.

• Method of trenching and backfilling.

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• Date and time of start and completion.

• Measurements or readings required during instal-lation to ensure that all previous steps have beenfollowed correctly.

• A log of subsurface data indicating the elevationsof strata changes encountered in the borehole.

• A determination if the strata at the transducerlocation was or was not that anticipated in theinstrumentation plan.

• Type of backfill used.

• As-built location, orientation, depth, length, andbackfill volume data.

• Three-dimension schematic of utility runs toinstrument, locating all junction boxes (buried andsurface access).

• Results of post-installation acceptance test (seebelow).

• Weather conditions at the time of installation.

• Miscellaneous notes, including problems encoun-tered, delays, unusual features of the installation,and details of any events that may have a bearingon instrument behavior.

d. Post-installation acceptance test.Upon comple-tion of the instrument installation, installation personnelshould demonstrate that the instrument was correctlyinstalled and is functioning properly. The details of thetest will depend on the type of instrument, and all possi-ble quality checks should be made. A series of readingsshould be made (minimum of three) during a short spanof time to demonstrate that the instrument reading can berepeated. The installation may have an effect on theparameter which is to be measured and the instrumentshould be allowed to stabilize and the acceptance testrepeated. The details of the test should be included withthe installation report.

e. Care and handling.To ensure satisfactory per-formance of an instrument, it should be protected fromthe elements prior to installation. The manufacturers willusually describe conditions which are unsuitable for theirdevices. All instruments should be kept free from dirtand dust. Some instruments or their cables degrade indirect sunlight. Some instruments are sensitive to electric

or magnetic fields. Others should be protected fromextreme temperatures, humidity, water, shock, and/orchemical precipitates. All instruments should be handledcarefully. Cables and tubes should be protected fromnicking, bending, and kinking.

6-8. Installation Procedures for Piezometers inBoreholes

Installation of a piezometer in a borehole is not a routineprocess. Many factors must be considered if the piezom-eter is to function properly.

a. Example of detailed issues when planning instal-lation. As an example of the thought process that isnecessary when preparing detailed installation procedures,the following questions should be answered when select-ing a method for installing a piezometer in a borehole insoil: (adapted from Dunnicliff 1988)

(1) What are the soil types?

(2) Is sufficient lead time necessary for procuringspecial piezometers?

(3) Are there artesian conditions?

(4) Are there excess pore water pressures?

(5) What drilling methods are available?

(6) What skill/care/experience is available amongpersonnel who will be responsible for installations?

(7) How much vertical compression will occur in thesoil above the instrument?

(8) Are soil samples required?

(9) How can the borehole be supported?

(10) How much time is available between comple-tion of installation and the need to establish zeroreadings?

(11) What are the requirements of response time?

(12) What borehole diameter will be used?

(13) How will the casing or augers be prepared,cleaned, backfilled, and/or pulled?

(14) Is a sounding hammer required?

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(15) What type of bentonite will be used?

(16) What is the required waiting time for swell?

(17) Is a swelling retardant needed? If yes, whatmethod will be used?

(18) Where will the seals be located?

(19) What will be the required depths of the seals?

(20) Will centralizers be required?

(21) Which method of installation is mostappropriate?

b. Selection and placement of sand filters.The filtermaterial to be placed around an open standpipe piezom-eter must be designed to meet filter criteria with respectto the screen and to the gradation of the soil layer inwhich it is being installed. Filter material for all types ofpiezometers should be generally clean from fines and notrestrict the response of the instrument. Filter material canbe placed by pouring the material in the top of the boringor tremied into place. If the material is poured from thetop, it should be saturated with water and poured slowlyto avoid bridging in the annular cavity of the boring. Ifthe tremie method is used, water should be used to flushthe material through the tremie pipe. The tremie pipe isgradually raised as the material builds up in the hole. Asounding hammer (Dunnicliff 1988) should be used toverify depths of placement.

c. Selection and placement of seals.Seals areneeded above (and below if necessary) filter zones toensure that representative readings are collected fromsingle soil stratum. The seal should be constructed ofbentonite. A method of compressing slightly moist ben-tonite into pellets was developed in the Sixties. However,the pellets bridge easily within the casing if they arepoured through fluid. Bentonite now exists in the form ofangular gravel. This is usually a pit-run graded material,which does not start to hydrate and swell as quickly ascompressed bentonite pellets, and if poured slowly, fallsto the bottom of the borehole without bridging. If a soilstratum is to be sealed off below the location of apiezometer, great care must be taken with the bentonite toavoid making contact against the sidewalls at the locationof the sand filter by using casing. Installation of thebentonite in powdered or slurry form should not beallowed as these methods will coat the casing on the waydown and possibly foul the filter below.

d. Multiple-stage piezometers.Multiple-stagepiezometers (more than one piezometer in a borehole) aregenerally not recommended due to the possibility of verti-cal flow of water between sealed zones. If multiple-stagepiezometers are necessary, extreme care must be takenwhen installing the bentonite seals and the number ofstages should be limited to two.

e. Lengths of screens in embankments.Screens arenormally minimal in length and situated in aquifers inorder to measure the actual pore pressure. In an embank-ment, the magnitude of pore pressure may be less impor-tant than noticing that changes are occurring over thelong-term life of a project. If these conditions exist, itmay be necessary to install screens of much greaterlengths, as indicators of changing conditions.

6-9. Installation Procedures for OtherInstruments

Many of the same or similar questions must be answered(see paragraph 6-8a) when developing installation proce-dures for other types of instruments. Manufacturers ofinclinometers and borehole extensometers normally supplygood instructions for the installation of the downholecomponents. However, care must always be taken sincevariable site conditions might alter these procedures.Additional information can be found in Dunnicliff (1988).

6-10. Backfilling Boreholes

Various materials may be used to backfill a borehole.Grout, granular fills such as sand and pea gravel, andbentonite gravel may be used. Grout has the advantagethat it readily fills all the void spaces. However, groutshould not be used if it will bleed off into the surroundingground, and in this situation a pregrouting step may beappropriate. Grouting guidance is available in EM 1110-2-3506 and Driscoll (1986). Grout mixes should be testedto determine appropriate ratios of water, cement, benton-ite, and sand (rarely used) that will ensure good perfor-mance between the instrument and the surrounding soil orrock.

6-11. Protective Housings

Each installed instrument must be protected with a protec-tive housing that is provided with a vented locking cap.Protective housings should be grouted into place not onlyto secure the cap but also to prevent surface water fromflowing into the instrument. Unfortunately, instrument

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housings protruding above the ground are often a targetfor vandals. In extreme cases or in roadways, an instru-ment may be flush-mounted with the surface, and sur-rounded with a gate box similar to those used by utilitycompanies. The drawbacks of this method are that theboxes are hard to find with a snow covering and drainagemust be considered so that surface water does not flowdown the instrument.

6-12. Documentation

After completion of the installation of all instruments, thefollowing documents should be bound and placed in areport:

• Description of instruments, readout units, andautomation equipment.

• Plans and sections sufficient to show instrumentnumbers and locations.

• Appropriate surface and subsurface stratigraphicand geotechnical data.

• Instrument calibration and maintenanceprocedures.

• Instrument installation procedures.

• Instrumentation and/or automation documentationfrom manufacturers, including calibration dataand warranty information.

• Pre-installation acceptance test documents.

• Post-installation acceptance tests.

• Instrument installation reports.

• A list of suggested spare parts needed for repairor maintenance.

• Procedures for data collection and processing.

• Names, addresses, and phone numbers ofmaintenance/repair sources.

It is recommended that at least two copies of the docu-mentation be prepared. One should be maintained on fileat the project site. The other should be filed in the Dis-trict office.

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Chapter 7Data Management, Analysis, andReporting

7-1. Introduction

Data management and analysis are crucial to understand-ing the behavior of a monitored structure, for detectingunsafe developments, and for determining the perfor-mance of the instrument systems. The major aspects ofthe instrumentation planning process are data manage-ment, engineering analysis, and formal reporting. A planfor the management and analysis of data should be inplace before the instruments are installed. The planshould indicate the frequency of data collection, the extentand timeliness of processing, the level of analysis, thereporting requirements for in-house engineers and review-ing authorities, and the people responsible for accomplish-ing each of these tasks.

7-2. Data Management

The management of data consists of data collection,reduction and processing, and presentation.

a. Data collection. Data collection should beginwith a well-defined established schedule. The schedule isdependent on project-specific requirements. The require-ments are dependent upon instrument characteristics, siteconditions, construction activity, or the occurrence ofunusual events. The schedule should be updated when-ever these conditions or instrument readings indicate theneed. Data collection procedures should adhere to thefollowing guidelines:

(1) Personnel consistency. Data will be most consis-tent if collected by the same person. If this is not possi-ble the designated instrument reader should have a backupinstrument reader who is also familiar with the instru-ments. Data collection personnel should read the instru-ments in the same manner every time.

(2) Instrument consistency. Using the same readoutunit to read an instrument every time will give the mostconsistent readings. The readout instrument should beconnected or aligned the same way for every reading.Readout units should not be interchanged, because read-ings can be dependent on readout unit and transducercombinations.

(3) Multiple readings. Multiple readings should betaken to the most representative value.

(4) Coordination of instrument readings. Instrumen-tation systems should be designed to have different typesof instruments show changes under the same conditionsand in the same time frame. Therefore, the instrumentwhose values, which need to be compared, should be readas close to the same time as possible.

(5) Data records. Instrumentation data shouldinclude the instrument reading and also any informationthat identifies the project, instrument, readout unit, reader,date, visual observations, climate, remarks, and any siteconditions that might affect the value of the reading. Allcalibration checks, averaging, or median values should beshown. Data from field books or field data sheets shouldbe transferred to data calculation sheets or data computerfiles and the originals filed.

(6) Data entry. Recording readings in field booksallows for comparison of current readings with previousreadings at the time the readings are collected. This earlyor initial comparison of the data aids in assuring the cor-rectness of the readings and also allows for early detec-tion of problems with instruments so that corrective actioncan be implemented without delay. The readings in fieldbooks should be transferred to data sheets or computerfiles as soon as possible after being obtained, to avoidlosing data if the field book is lost or destroyed. Thetransfer should be checked for transposition errors. Fielddata sheets used to record field readings directly shouldbe forms with spaces for recording all the factors neces-sary. Field data sheets should include previous readingsfor immediate comparison with current readings, or acopy of previous readings should be available for currentreading comparison, as a first step in data management.Portable data acquisition recording devices are helpful intransmitting data. These devices may save time and mini-mize errors.

(7) Communication. Communication among person-nel responsible for data collection, data processing, datareviewing, and data analysis is vital to the data acquisitionprocess. The data collection personnel should communi-cate to the data review and analysis personnel all condi-tions that could affect the readings. The data review andanalysis personnel should communicate the results of theirwork to the data collection personnel to indicate whetherthe instruments are being read correctly, if the instrumentis operating correctly, if the reading schedule is adequate,etc.

(8) Warning of unusual conditions. Readings thatexceed established threshold levels should be reported

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immediately. Appropriate personnel should be notifiedwhen an instrumentation reading, changed condition,activity, or visual observation reveals that a problem ordangerous situation has occurred, or is occurring, andimplement the steps listed in the Project Operation andMaintenance Manual, Emergency Contingency Plan.Communication and cooperation among all partiesinvolved in data acquisition is essential, especially whenproblems arise.

(9) Special considerations of automated data collec-tion. Automated data collection allows for the adjustmentof the data collection frequency to capture the behaviorduring a significant event. For example, if an event at aproject causes a rapidly fluctuating reservoir elevation, thedata collection frequency should be altered to record theeffect of the event. Automatically recorded readings thatare beyond threshold values should be checked. Thisearly or initial check of the data aids in ensuring theaccuracy or correctness of the readings and also allowsfor early detection of problems with instruments so thatcorrective action can be implemented without delay.

b. Data reduction and processing.Data processingand reduction consists of converting the raw field datainto meaningful engineering values necessary for graphi-cal presentation, analysis, and interpretation. Severalcalibration constants may be needed to convert the fieldreadings to engineering values. In the past, these conver-sions were performed by hand, but currently can andshould be performed by computers to eliminate conver-sion errors. Where possible, initial and preliminary reduc-tion and processing of instrumentation data should beaccomplished in the field, where anomalous readings,errors, or malfunctioning instruments can be readily iden-tified and/or corrected.

(1) Timeliness of data reduction and processing.Expeditious transfer of instrumentation readings from theproject site to the reducing/processing/reviewing office isessential in timely data management. Facsimile machines,local computer networks, or express mail services shouldbe used to expedite data transfer. All instrumentation datashould be reduced and processed as soon as possible,preferably while the conditions under which the data wereobtained still exist (e.g., reservoir stages, high velocityflows). Backlogging or stockpiling of instrumentationdata for later reduction and processing prior to preparationfor annual data submittal or scheduled inspections isunacceptable.

(2) Error checking. Checking for errors in instru-mentation data should be accomplished at each level of

collection and processing (from reading of instruments inthe field to final interpretation of the instrumentationdata). Checking should commence with proofreading ofdata values to ensure that readings have been properlyrecorded on field data sheets or notebooks and transcribedcorrectly from field data sheets or notebooks to reportingforms or summary tabulations. Instrument readingsshould be compared with ranges specified by the review-ing office and with previous readings under similar condi-tions. Conformance with previously established trendsshould be determined. Anomalous readings should beidentified and checked as necessary. Data sheets shouldreflect the anomalous readings and possible causes ofsuch readings.

(3) Reduction and processing methods. Simplecomputer programs should be used to expedite data reduc-tion. Careful proofreading of input values is recom-mended, and a standardized input format should be used.Programs which are used to reduce instrumentation datashould be carefully tested and verified by the user toensure that they are operating correctly throughout theexpected range of instrument values. These computerprograms range from simple programs which performonly data reduction calculations to complex databasesdesigned for data reduction of many different types ofinstruments. The more complex databases can be usedfor data storage on electronic media, production of plotteddata, some preliminary analyses, and maintaining historyfiles of instrument readings. It is recommended thatcomputer programs be used for data reduction, data stor-age, and graphical plots. Regardless of the computerprogram used, periodic manual checking of data calcula-tions and results by knowledgeable instrumentation per-sonnel is essential. As with any computer system, aprocedure or time frame for backing-up the data is anessential step in data processing. A relational database,such as the Instrumentation Database Package developedby the Corps of Engineers, is helpful in all phases of datamanagement.

c. Data presentation.Numerically tabulated dataare not conducive to detecting trends, evaluating unantici-pated behavior, and comparison with design values. Plotsof the data are needed to provide visual comparisonsbetween actual and predicted behavior, a visual means todetect data acquisition errors, to determine trends orcyclic effects, to compare behavior with other instruments,to predict future behavior, and to determine instrumenta-tion maintenance requirement needs. Plotting enables datato be compared readily with events that cause changes inthe data, such as construction activities and environmentalchanges. Plotting also provides a visual means to

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evaluate unanticipated behavior, and to determine theeffectiveness of remedial correction.

(1) Types of plots. Several types of plots can aid inevaluating various project conditions. Some examples aregiven below.

(a) Time history plot. Time history plots display timeversus the change in parameter (see Figure 7-1). Parame-ters such as water level, seepage, pore water pressure,deformation, and temperature can be plotted against time.Dual Y-axis time history plots allow the plotting of asecond parameter, such as pool, tailwater, or rainfall, withthe first parameter.

(b) Positional plot. Positional plots show a changein parameter (water level, temperature, deflection, etc.)versus the position of an instrument (see Figure 7-2).These positions can be shown as cross section, X-Y coor-dinate, station, offset or depth. An example of a posi-tional plot is an inclinometer plot as shown in Figure 7-2.This type of inclinometer plot shows horizontal deforma-tion versus depth and changes with time.

(c) Other plots. Multiple plots and plotting of multi-ple parameters is possible. An example is Figure 7-3.Plotting multiple parameters can be useful in examiningquestionable conditions.

Figure 7-1. Example of a time history plot

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Figure 7-2. Example of a positional plot

(2) Guidelines for plotting. Some guidelines forplotting follow:

(a) Appropriate scale should be chosen for the analy-sis. Determination of minute changes requires a scale ofsmall increments. Scales with increments so large thatthey would not show the data trends should not be used.Exaggerated scales that would magnify minor changes tomake them appear alarmingly large should not be used.

(b) Standardize the graph formats and scales for allthe projects or features as much as possible to minimizeconfusion and effort of interpretation.

(c) Location and cross-sectional sketches should beincluded on the graph to orient the reader to the subjectarea.

(d) Multiple graphs should be used to explain asituation by showing related conditions.

(e) If appropriate, the predicted behavior and/orlimits of safety values should be shown along with theactual monitored behavior.

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Figure 7-3. Example of a multiple parameter plot

(f) Significant influences of the measurements shouldbe noted (i.e., construction activities, reservoir level onp i e z o m e t e r l e v e l p l o t s , t e m p e r a t u r e o nexpansion/contraction plots).

7-3. Engineering Analysis

Data analysis is the interpretation and evaluation of thedata as affected by various conditions. It is a continuousprocess from data collection through reporting. At everystep of the analysis, the evaluator should be conscious ofthe potential for invalid data and the improper use of thecalculations, so that incorrect interpretations are not made.Proper analysis will address two basic aspects of damsafety monitoring: the performance of the instrumentsystem, and the performance of the structure or featurethat is being monitored.

a. Timeliness of data.The field reading should becompared to the previous data set as it is recorded in thefield. Data should be entered into the computer by elec-tronic transfer or immediately upon returning to theoffice. The computer should have an automatic check todetermine the significance of value variations upon entry.Questionable results of either of these two proceduresshould be brought to the immediate attention of the instru-mentation program manager. Reduced data in plot formatshould be immediately reviewed upon completion ofprocessing. In-depth analyses should be accomplishedcommensurate with the degree of concern associated withthe monitored feature. Reduced data should be providedto other involved offices (hydraulics, structures, opera-tions, etc.) as appropriate. Under normal circumstanceswith conscientious attention by qualified personnel, signif-icant dam safety concerns can be detected within hours;

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valid and meaningful information and preliminaryanalyses can be made available within one day, two daysmaximum. The severity of the situation can acceleratethe assessment.

b. Analysis techniques.An analytical technique canbe considered the viewing of the current information inthe context of past experience. It should also consider thepredicted behavior of the monitored feature. The reviewand analysis personnel should consider the followingtechniques when analyzing the data.

(1) Compare current data with the most recent dataset to detect anomalies, discernible short-term behavioralchanges, and instrument malfunctions.

(2) Compare the current data point with historicalperformance over a significant period of time to ascertainconsistency of instrument performance for the monitoredfeature. This can also indicate compliance of the newinformation with an established trend.

(3) Compare current data point with the initial read-ing for that point to determine the magnitude of changeover time. This can indicate instrument drift or funda-mental characteristic behavior of the structure.

(4) Compare trends of behavior over time with trendspredicted during design, with values relating to calculatedfactors of safety, and/or with any other predicted behav-ior. Note that the historic behavior of a structurebecomes the base for comparison of future behavior andthe performance predicted during design becomes lessrelevant.

(5) Compare the results of one instrument systemwith those of complementary systems to confirm or denyan implied physical change (e.g., consolidation settlementswith dissipation of pore water pressure, pore water pres-sures with functioning of drains).

(6) Use statistical analyses to assess the performanceof instruments. Automated systems can acquire a largequantity of data which is conducive to calculating stan-dard deviations and variance of instrument response. Thisis also helpful in determining calibration frequency.

c. Outcome of data analysis.There are many out-comes of data analysis. The appropriate personnel shouldconsider the following:

(1) Determine when to test, calibrate, or abandoninstruments.

(2) Determine if the schedule of observation shouldbe altered.

(3) Reevaluate which areas of the project requirepriority attention.

(4) Determine the need for further study (slope sta-bility, seepage, and other structural performance analysis).

(5) Confirm or refute previous studies.

(6) Prepare the processed data for formal presenta-tion and develop the engineering position that will bereported.

d. Pitfalls to avoid. The following are some datapitfalls to avoid:

(1) Lack of data comparison in the field resulting ininvalid data.

(2) Delaying data entry, analysis of processed data,and the dissemination of information to involved offices.

(3) Assuming data are valid and calculations wereexecuted properly. Software calculation and calibrationfactors should be periodically checked.

(4) Assuming that change in data is reason for con-cern. Instrument may be appropriately responding to acondition.

(5) Assuming no change is satisfactory. Instrumentmay not be operating.

(6) Failing to recognize or incorporate all factorsthat influence the data (e.g., seasonal temperature changesaffecting structural movement, temperature, rainfall, reser-voir level values).

(7) Assuming contour plots are accurate. Thoseplots developed by automation or computer softwareshould not be used without a careful review by an experi-enced geotechnical engineer or geologist who should bethoroughly familiar with the software.

7-4. Formal Reporting and Documentation

Formal presentation of data may be quite different thanthe plotting prepared for data analysis. Formal presenta-tions summarize and present data to show trends, enablecomparison of predicted design behavior with actual

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behavior, document key aspects of the instrumentationmonitoring program, and identify necessary remedialmeasures. Reporting requirements are contained in avariety of regulations and formal reporting guidance. Asthe reporter complies with these regulations, considerationshould be given to the following:

a. Choose the aspect of performance that is to beportrayed.

b. Identify all conditions and information that signi-ficantly enhance the portrayal.

c. Group instruments that are pertinent (e.g., crosssection of foundation piezometers only) to relate to cutoffeffectiveness.

d. Refrain from using formal reports to serve as adepository for permanent filing of all data ever acquired.The focus should be on reporting the condition that ismonitored.

e. Scales for the data presentation used for analysis(see 7-2c(2)) may show detailed changes over a short

period of time. Detailed changes over a short period oftime may be desirable for analytical purposes. However,formally reported assessment of the same information mayshow an acceptable trend in the long-term which wouldrequire a different scale. Scales for analysis may bechosen for seeing, but the final reported assessment of thebehavior may be insignificant change and in compliancewith an accepted behavioral trend in the long term.

f. Focus on clarity and significance of informationon plots.

g. The text of the report should discuss changes,and identify trends and rate of change with time. Speci-fic values should be stated in units that are meaningfuland understandable. A specific statement should be madewith regard to the engineering judgment of the situation,the acceptability of the condition, and the intentions forfollowup.

h. Guidelines for plotting in paragraph 7-2c arealso relevant for formal reporting.

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Chapter 8Instrumentation Maintenance

8-1. Introduction

Proper design and installation of instruments provides areliable and functional system. With usage and the pas-sage of time the instrumentation system must be main-tained to continue its reliable and useful function. Thischapter deals with maintenance including recalibration ofinstrumentation systems during service life. Maintenanceand recalibration of general instrument types and mostcommonly used instruments are discussed. The basicmaintenance and recalibration concepts discussed for thesetypes of instruments can be applied to all other types ofinstruments.

8-2. Importance of Maintenance andRecalibration

Careful attention to factory calibration, pre-installationacceptance tests, installation, and post-installation accep-tance tests should ensure that instrumentation systems areoperationally satisfactory to monitor the performance ofthe dam or levee. However, to ensure that the systemsremain in a satisfactory operating condition during theirservice life, regular maintenance and recalibration are nec-essary. Lack of regular maintenance and recalibrationwill result in invalid data which can lead to incorrectconclusions. Lack of maintenance will also jeopardize thefunctioning of the system, and risk the loss of individualinstruments, which in many cases cannot be replaced.

a. General concepts.Maintenance is the responsi-bility of the owning agency. The agency’s maintenancerequirements and procedures should be planned during thedesign of the instrumentation system and outlined in theproject Operation and Maintenance (O&M) Manual.Planning for maintenance and recalibration is discussed inChapter 3. Although it may not be practicable to developa project-specific maintenance manual because of cost, theO&M Manual should include as many guidelines as possi-ble, and should include the instrument manufacturer’sinstruction manuals. Maintenance procedures should bebased on the manufacturer’s instruction manual and on theproject-specific site conditions, and include such needs aspreventative maintenance program schedules, trouble-shooting, cleaning, drying, lubricating, battery servicing,disassembly, and repair and replacement instructions foreach type of instrument.

b. Maintenance contracts.Maintenance and recali-bration are best accomplished by the owning agency’spersonnel, and the first option should be to use Districtpersonnel before contracting out services. However,instrumentation systems and components usually havelimited warranty periods, and a maintenance contract withthe manufacturer could be a wise investment. Govern-ment contracting procedures, specifications, and guidanceshould be followed for any contractual agreements. Anycontractual agreements for regular maintenance and recali-bration should define scope of work, requirements, proce-dures, schedules, contractor qualifications, experience, andpersonnel qualifications and experience.

c. Personnel responsible for maintenance andrecalibration. Personnel should be reliable, dedicated,and motivated people who will pay attention to detail.They should have a background in the fundamentals ofgeotechnical engineering and have mechanical and electri-cal ability. They should be able to understand how thedam or levee functions and how the instruments function.The maintenance and recalibration checks of an instru-mentation system during service life are best supervisedby or accomplished by the data collection personnel, whoare in the position to notice potential problems and toobserve malfunctions, deterioration, or damage. Qualifi-cations for data collection personnel are discussed inChapter 3. They should be trained to perform scheduledpreventative maintenance and to initiate corrective action,repair, or replacement without delay. Their trainingshould also include all personal safety training requiredfor conditions, equipment, and materials related to theperformance of their duties. If maintenance, recalibrationchecks, or corrective action are not routine or are beyondthe capability of data collection personnel, the data collec-tion personnel should communicate this to the officeresponsible for inspection and analysis, so that arrange-ments can be made for action to be taken by the instru-mentation program manager.

d. Service history.A service history record ofmaintenance, recalibration, repair, and replacement of thecomponents of the system should be kept and communi-cated to the personnel responsible for analysis. Therecord should include dates, observations, what the prob-lems were, what was done, how it was accomplished, whowas involved, and anything else that will help to under-stand or interpret the instrumentation data. A servicehistory establishes the general behavior that is characteris-tic of the system and suggests what frequency of

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maintenance is needed. The service history also facili-tates the transfer of responsibility due to turnover inpersonnel.

e. Spare parts.Appropriate spare parts and inter-changeable components should be available for replacingfailed or questionable components without interruptingsystem operation during service life. An inventory ofspare parts and instruments should be available andrevised as necessary. Spare readout units should also beavailable in case of malfunction of primary readout units.

8-3. Recalibration During Service Life

After instruments are in service, they will require regularrecalibration to maintain proper operational characteristics.However, some instruments such as an embedded trans-ducer cannot be recalibrated. As a general rule recali-bration should be performed on a well-defined schedule.Formal schedules and reporting requirements for routinerecalibration are necessary to ensure that the readingsobtained are reliable. Routine recalibration of instrumen-tation system components is best performed by personnelresponsible for data collection, and coordinated with thedata analyses personnel. Instrumentation data will have tobe interpreted for indications that recalibration is needed.The data should be watched for abrupt reading changesfor no apparent reason. If the data are showing a long-term change, it should be verified whether or not thechange is actual or is due to a malfunctioning instrument,which may need to be recalibrated. Procedures for recali-bration during service life should be included in the O&MManual. Components of most instrumentation systemscan be classified into three groups: portable readout units,retrievable components at field terminals, and embeddedcomponents. Maintenance of these three components arediscussed below. Other instruments that do not fit intothe three-component classification are discussed later inthis chapter.

a. Portable readout units.Portable readout unitsgenerally are vulnerable to mishandling and subject tocalibration changes. Some readout units can be checkedand recalibrated by following procedures provided by themanufacturers. If they cannot be checked in this way,they should be sent to the manufacturer for calibration,adjustment, or repair. Local commercial calibration com-panies that use equipment traceable to the National Insti-tute of Standards and Technology are also capable ofcalibrating many types of portable readout units. Frequentfield calibration checks can be accomplished on someportable readout units; for example, inclinometer probes

can be checked by examining repeatability at the bottomimmovable length of an installed inclinometer casing.

b. Retrievable components.Retrievable compo-nents are components that are mounted to a field terminalthat can be removed for recalibration or replacement. Forthis reason, components should be mounted in the fieldterminal with shutoff valves or disconnect plugs in such away as to allow for easy removal and replacement. Somecomponents can be recalibrated without removal by usingstandard recalibration equipment located in or brought tothe field terminal.

c. Embedded components.Embedded componentsare those that are buried or installed so that they are nor-mally inaccessible. Many of these embedded componentscannot be recalibrated because of their inaccessibility.For those that can be recalibrated, standard proceduresshould be developed for recalibrating the instrument typesin question. To develop standard procedures, contact themanufacturers or other personnel with experience (seeUSCOLD 1993). Embedded components that can berecalibrated should be included in a routine recalibrationschedule.

d. Calibration equipment.Calibration test equip-ment should be traceable to the National Institute of Stan-dards and Technology. Test equipment used to recalibratecomponents of an instrumentation system must be main-tained in good working order so that recalibration of thecomponents can be made with accuracy.

e. Recalibration frequency.The recalibration fre-quency should depend on the type of instrument, theapplication of the instrument, and the operating environ-ment. Some instruments require recalibration checksfrequently during each day of readings, others requirerecalibration checks daily, weekly, monthly, or yearly. Asa general rule, recalibrations should be made on a fre-quent schedule rather than an infrequent schedule. Amore frequent schedule will allow data collection person-nel to find any changes in calibration sooner, so that aminimum amount of erroneous data will be collected,processed, and reviewed before the instrument is recali-brated. A more frequent schedule could also aid in find-ing a malfunctioning instrument before it is damaged.

8-4. Maintenance During Service Life

a. Portable readout units.Portable readout unitsshould be protected from mishandling, and be kept clean

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and dry. Attention should be given to all connectionports, O-rings, seals, accessory wires, and plug-in connec-tors. Protective plugs and caps should be in place wheninstruments are not being read, to protect the connectorsand to keep them clean and dry. If protective plugs andcaps are not provided by the manufacturer, they should beadded to the instrument. Preferably, the plugs and capsshould be attached to the units to avoid loss, and to makethem easy for data collection personnel to use. All mov-ing parts should be checked for tightness, damage orwear; worn or damaged parts should be replaced. Cablesshould be checked for damage to the outer coating andany cable markings. If there is moisture present in thereadout unit, it should be removed by drying. Many unitsare supplied with desiccant that changes color when mois-ture is present. Batteries should be replaced as needed.Nickel-cadmium (nicad) batteries and lead-acid batteriesare rechargeable batteries that require two differentrecharging procedures. The manufacturers’ instructionsfor checking, maintaining, and recharging batteries shouldbe followed.

b. Retrievable components.Retrievable componentssuch as in-place inclinometers, electrical transducers, andthermistors can be installed as retrievable instruments sothat they can be removed for maintenance and recalibra-tion, and reinstalled or replaced. Protective barricadesand protective enclosures should be maintained in goodcondition. Retrievable components, any wires, tubes,cables, and their enclosures should be kept clean, dry, andprotected from rodents, vandals, and transient voltagesurges. All plugs, caps, and covers should be maintainedin good order and used properly. Maintenance inspectionsof the retrievable components and their protective struc-tures should include inspections of wires, cables, tubes,etc. where they exit from the ground or structure. Theseareas are vulnerable to damage by accidents, weather,vandals, etc. When retrievable components are to beremoved, observations of the existing installation andphysical conditions should be noted. The instrumentshould be read before removal and after reinstallation.Any maintenance, recalibration, or replacement should bedocumented and communicated to the data management,reporting, and plotting personnel. Recommendedfollow-up checks and readings should be made to verifysuccess of maintenance and to validate readings.

c. Embedded components.Embedded componentsare normally inaccessible and maintenance is not possible.However, there are various embedded components thatrequire maintenance. Inclinometer casings, open stand-pipes, and relief wells can be inspected by downholevideo cameras to determine if maintenance is required.

Chemical analysis of water samples taken from thesetypes of embedded components can be used to determinewhat maintenance treatment procedures are required forbacterial or chemical contamination.

d. Maintenance frequency.Maintenance should bepreformed on a regular schedule and in accordance withprocedures in the project O&M Manual or the manufac-turer’s instructions. Maintenance frequency shoulddepend on the type of instrument, the application of theinstrument, and the operating environment. A more fre-quent maintenance schedule may allow data collectionpersonnel to find a malfunctioning instrument before it isdamaged.

8-5. Instruments Requiring Specific Maintenance

Instruments that require more than the basic maintenancefor portable readout units, retrievable components, orembedded components are discussed below.

a. Twin-tube hydraulic piezometers.

(1) To determine if maintenance is required for twin-tube hydraulic piezometers, the data collection personnelshould compare current readings with previous readings toobserve trends. The two gages can be compared to see ifthe pressures are equal. Under normal conditions thegages will indicate similar pressures. If they do not,flushing then rehabilitation is needed. The data collectionpersonnel should observe the tubes and gages for bubblesand water clarity. Sediment or mineral deposits derivedfrom the piezometer liquid may also be present. Whenflushing is performed, the data collection personnel shouldcheck to see if the discharged water contains gas, sedi-ment, or mineral deposits.

(2) Maintenance tasks required for twin-tube hydrau-lic piezometers include flushing with de-aired liquid toclean the lines of any mineral deposits, and to removeaccumulated gas bubbles from the tubing and manifoldsystem (Dunnicliff 1988). When de-airing a piezometer,water under pressure may pass through the filter into thesurrounding soil, causing excess pore water pressure inthe soil that requires time to dissipate. In order to mini-mize or eliminate the buildup of pore water pressure inthe soil when de-airing piezometers, a vacuum should beapplied to one tube of the piezometer while de-aired waterunder a minimum pressure (calculated as shown byDunnicliff 1988) sufficient to dislodge and move the airbubbles is applied to the other tube. Flushing proceduresand pressure calculations are described in Dunnicliff(1988). The system should be checked frequently for

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blockage and/or presence of gas. Sediment and mineraldeposits derived from the liquid in the system must beflushed from the system if they are present. If depositsare allowed to accumulate in the system they will eventu-ally plug the system and the instruments will be lost.Pressure gages should be checked periodically by compar-ing them with a master gage. Master gages andindividual gages should be maintained and recalibratedand inaccurate or damaged gages should be replaced. Fortwin-tube hydraulic piezometers located in terminal struc-tures above the frost line, protection from freezing isrequired, preferably by heating the structure. Maintenanceof the heating equipment and control of moisture in thestructure will be necessary.

b. Pneumatic piezometers.Data collection personnelshould observe the instruments and readings to detectsymptoms that may indicate that maintenance is needed.Unusual readings, sticking flow meters, evidence of mois-ture, leaking gas sounds, and valve malfunctioning aretypical symptoms indicating that maintenance is needed.Pressure gages in pneumatic piezometer readout unitsrequire occasional calibration checks. Flowmeters willoccasionally have to be cleaned to remove moisture orresidue. The tubing may occasionally have to be flushedwith purified, dry nitrogen gas to evaporate moisture fromthe tubing. O-rings and valves may have to be replaced ifleaks are detected in the system.

c. Open standpipe piezometers.The hydrostatictime lag, frequently called response time, of each openstandpipe piezometer should be established by performinga rising or falling head test at the time of installation.Test data may be converted to hydrostatic time lag for thepiezometer by the method described in Chapter 2. Thehydrostatic time lag for the piezometers should be reestab-lished periodically for a comparison with the initiallyestablished hydrostatic time lag. Such comparisons givean indication of the condition of the piezometer. Open-standpipe-type piezometers require periodic response teststo determine if the piezometer is effective or if mainte-nance is needed. In soils of low permeability, piezometerequalization can be obtained by adding or removing waterin increments and observing the rate of rise or fall. Thepiezometers should be sounded periodically to determineif the tips contain sediments. If sediments are found, thepiezometer should be flushed to remove sediment depositsfrom the bottom of the piezometer. Periodic disinfectingtreatments may be required in piezometers that are locatedin environments that allow bacterial growth in thepiezometers. Care should be taken when obtaining meas-urements to avoid introducing contaminants into thepiezometer from the readout probe.

(1) In open standpipe piezometers where the waterlevel rises above the frost line a nontoxic low-freezing-point fluid should be used to protect the piezometer fromfreezing. Maintenance may be required to replace thefluid so that it does not freeze and break the standpipe.Regardless of type of fluid used, the difference in speci-fic gravities of the low-freezing-point fluid and the insitu pore water must be taken into account in adjustingthe hydrostatic pressure shown by the piezometer.

(2) Observation wells are similar to open standpipepiezometers except they have no subsurface seals, there-fore strata are vertically connected. Observation wellshave limited use; however, their maintenance is basicallythe same as that for open standpipe piezometers.

d. Relief wells. Relief wells are outside the scopeof this manual. Information on relief wells can be foundin EM 1110-2-1914 and Driscoll (1986).

e. Probe extensometers.Probe extensometer read-out devices must be kept clean and free of grit. Measur-ing tapes and cables should be inspected for kinks andkept dry. The riser pipes may require flushing to removesilt, corrosion, or plugging material.

f. Fixed borehole extensometers.The extensometerreference head must be kept clean of dust, grit, and mois-ture. The reference head may have to be reset if move-ment has occurred by following the manufacturer’sinstructions. Protection from direct sunlight or physicaldamage is required.

g. Inclinometers.The inclinometer probe, cable,and readout unit should be sent to the manufacturer forfactory calibration and/or repair when either the checksums, the internal diagnostics, a malfunction, or unusualreadings indicate that there is a problem with the instru-ment. Inclinometer casings may require occasional flush-ing to remove debris or sediment deposits from thebottom of the casing. Mechanical brushing can be used toremove biological or chemical residue or incrustationfrom the casing grooves. Inclinometer casings that havecorroded, deteriorated, or deflected to such a state thatreadings can no longer be obtained, can sometimes bere-lined with a smaller inclinometer casing so that theinclinometer can still be used. Other types of instrumentscan also be installed in deteriorated inclinometer casingssuch as a slope extensometer or possibly a shear strip.These methods of instrumenting and re-using an existinginclinometer casing are normally much less expensivethan drilling a new hole and installing a new inclinometercasing.

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h. Seismic (strong motion) instruments.Mainte-nance of seismic (strong motion) instruments should beperformed on a regular basis. Strong motion instrumenta-tion is covered in ER 1110-2-103. A technician trainedby the United States Geological Survey (USGS) or WESshould check strong motion instruments on a periodicbasis. The technician should check to see if the instru-ment has been tripped, check the operation of the instru-ment, and make sure the instrument has an ample amountof film. If it has been tripped, the exposed portion of thefilm should be removed for development and analysis.

i. Seepage measurement sevices.Routine mainte-nance is basically similar for most open-channel seepagemeasuring devices such as weirs, flumes, or seepagepipes. The channel and channel banks, or pool and poolbanks near the device should be cleaned of weeds anddebris. The sides, crest, outlets, and any measuringdevices such as scales or staff gages should be cleaned ofdirt, scum, mineral deposits, and bacterial or vegetationgrowth. The channel or pool should be cleaned of siltand sand deposits as they accumulate. If the cleaning hasto be accomplished before the flow reading is taken, theflow reading should not be taken until the flow returns toits normal state.

(1) Weirs and flumes should be checked to make surethat they remain level and are at the same elevation as the

zero reading on the staff gage. Weir notches or crestsshould be checked for nicks or dents that reduce theiraccuracy. Nicks or dents should be dressed or repaired ifit can be done without changing the shape of the weiropening. If it cannot be repaired, the weir notch or crestshould be replaced.

(2) Regular maintenance of flow or velocity metersincludes keeping all moving parts clean, lubricated, freeof corrosion, and in good working condition. Electrodesof electromagnetic instruments should be cleaned of anyfilm buildup. The calibration of velocity meters should bechecked frequently and recalibrated as needed.

8-6. Automation Equipment

The portable readout units, field terminals, and embeddedcomponents of an automated instrumentation system arebasically the same as any instrumentation system andshould receive the same care and maintenance. Automa-tion equipment is discussed in Chapter 5. Automatedinstrumentation systems, communications systems, elec-tronic equipment, dataloggers, data controllers, and com-puters contain sophisticated electronic components andwill usually require the care of experienced electronicspersonnel.

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Chapter 9Continual Reassessment for Long-TermMonitoring

9-1. Introduction

This chapter addresses factors unique to long-term moni-toring. Historical experience of dam designers, construc-tors, and engineers responsible for dam safety has shownfirst that: the function of a project changes with age,second that deterioration can be progressive with time,and third that problems can develop and occur at any timeduring the history of a structure. Engineers responsiblefor the dam safety of the embankment dams and leveesmust learn and understand the characteristic behavior ofeach embankment and other structure so that this knowl-edge can be applied in any future investigations, recom-mendations, and remedial actions. Ensuring safety is acontinuing process, the responsibility for which must beaccepted for the entire life of a project.

9-2. Long-term vs Construction-RelatedInstruments

The initial design of an instrumentation system shouldconsider not only construction-related instruments, butalso long-term instruments. Long-term instrumentationmay be needed to provide data to validate design assump-tions, to provide information on the continuing behaviorof the foundation, embankment, or abutments, and toobserve the performance of unique features. If deemednecessary, a well-planned system of instruments (seeparagraph 3-6) should be installed to provide data onflows, pore water pressures, and/or deformations at struc-turally significant locations in the foundation, embank-ment, and/or abutments during construction, first filling,and long-term operation. A careful examination of instru-mentation data on a continual basis may identify a newcondition, or may play a role in reassuring the reviewerthat an observed condition does not require immediateremedial measures.

9-3. Primary, Secondary, and TertiaryInstruments

The classification of instruments into primary, secondary,and tertiary categories usually relates to piezometers, butmay apply to all other instruments installed at a project.Figure 9-1 illustrates an example of instrument classifica-tion and sample locations. The primary instrumentsinclude all instruments located along a typical cross

section at or near the maximum structural vertical section.Also included are piezometers installed to monitor struc-turally significant features or suspect areas in the founda-tion, embankment, and/or abutments. Secondaryinstruments are those located in less critical sections orareas. Tertiary instruments include other instruments thathave been installed to verify that other areas of the projectbehave in a similar way to the areas where the primaryand secondary instruments are located. If automation is aconsideration (see Chapter 5), the primary instrumentswould be the first to be included in the automationsystem.

9-4. The Evolving Instrumentation System

One of the early steps in the planning of an instrumenta-tion system (see Chapter 3) is to define the geotechnicalquestions that need to be answered. For a long-termmonitoring system, the questions that need to be answeredare normally associated with developing trends over timeand response under major pool conditions (which for aflood-control project may not occur for many years afterconstruction). Construction-related instruments must notbe maintained unless they can assist in answering long-term questions. Typical questions relating to long-termmonitoring might be: Is the feature responding asassumed in design under high pool conditions? Is theseepage flow increasing with time?

a. Changing conditions. Design criteria or existingconditions can change. For example, filter criteria for adam designed and constructed in the 1930’s are no longeraccepted. Additional piezometers might be warranted toverify that pore water pressures are constant and that corematerial is not being lost. Siltation of the reservoir canoccur with time, affecting the hydraulic gradient of thestructure.

b. Effects of aging.As an embankment dam orlevee ages, new problems or concerns may develop. Itmay be necessary to instrument potential problem areas toverify that conditions are not deteriorating.

c. Reading schedules.Reading schedules may varydepending on locations of instruments and associatedfactors such as reservoir level.

9-5. Steps for Continual Reassessment

Paragraph 3-6 provides a step-by-step procedure for plan-ning a monitoring program. These steps were designedfor a new structure, but they also apply to an existing

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Figure 9-1. Instrument classification and sample locations

structure. The process for planning and executing a mon-itoring program for an existing structure should begin atthe appropriate step. As with a new instrumentationsystem, the steps to follow for modifying or upgrading an

existing system can vary, and are up to the discretion ofthe designer, or a senior-level geotechnical or instrumen-tation engineer. The termsretrofit, rehabilitate, upgrade,and replacehave slightly different meanings with respect

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to instrumentation. The following paragraphs indicatethese differences.

a. Retrofit. The term retrofit signifies changing oradding to an instrument that already exists. An examplewould be the installation of an electronic pressure trans-ducer in an open standpipe piezometer.

b. Rehabilitate. The term rehabilitate is normallyused in conjunction with the repair of an individual instru-ment or system. An example would be re-establishing anew initial for a settlement monument after the necessaryrepairs.

c. Upgrade. The upgrading of an existing instru-ment or system usually refers to the enhancement of afunctioning instrument or system so that more reliable orfrequent data can be obtained. The automation of existinginstrumentation would be considered an upgrade. Theupgrade of an instrumentation system may include theaddition of instruments in order to monitor newlyobserved conditions.

d. Replace.Replacement signifies the installation ofa new instrument or component and the abandonment ofan existing instrument which cannot be repaired. It canalso refer to the removal of a malfunctioning component,which is sent in for repair, replacement, or the installationof a spare part.

9-6. Selection of Parameters to Monitor

Table 9-1 gives, in priority order, measurements whichare applicable for the long-term monitoring of embank-ment dams and levees, together with recommended instru-ments. It must be restated that the selection ofinstruments is dependent upon the unique characteristicsof each structure or associated feature. For example,lateral subsurface deformation within a typical embank-ment dam will be less important than monitoring seepage,but it may be more critical in a potentially unstable abut-ment. Pore water pressures and subsurface settlementmay rank higher if core material was placed and com-pacted too wet.

a. Visual observations.Dr. Ralph Peck has oftenstated that the human eye attached to an intelligent brainis the single most important instrument at a project site.Automation is sometimes criticized because of a concernthat visual observations will not be made. The solution isto schedule regular visual inspections of all areas of aproject, and not to rely solely on the automation equip-ment and instruments for warnings of potential problems.

Moreover, if a major problem were to occur, it is verypossible that the instruments in place would not giveenough advance warning, simply because the instrumentsmay not be located close enough to the problem area todetect the change, or simply because of the delay in dataprocessing and review.

b. Seepage.Regular monitoring of seepage whichappears downstream of an embankment dam or abutmentis essential in assessing the behavior of a structure, notonly during first filling, but also for monitoring long-termtrends (see paragraph 4-8). The long-term monitoringshould also include solids content. Relief wells for damsor levees have specific problems associated with the plug-ging of the filter packs or the accumulation of bacteria orcarbonates (see EM 1110-2-1914). It is often necessaryto install piezometers adjacent to relief wells to carefullymonitor the pressure increases associated with cloggedwells.

c. Pore water pressures within the embankment orabutments.For long-term monitoring, pore water pres-sures are measured in embankments or abutments toverify design assumptions and to monitor changes whichcould be an indication of a deteriorating core or stratum.Paragraph 4-3 includes a discussion of the various typesof piezometers. The most reliable type for long-termmonitoring is the open standpipe, possibly retrofitted witha retrievable transducer.

d. Seismic instrumentation.Seismic instrumenta-tion is often installed at medium to large dams to guidedecisions on remedial actions which may be necessary if astructure has been affected by a seismic event. Also thedata can be a very important research tool, benefitting thedesign of future projects.

e. Vertical deformation.Measurements of surfacesettlement and horizontal movement at various pointsalong the crest and on the slopes should be considered formonitoring long-term deformations. Measurements ofsubsurface vertical deformations can be of importanceespecially for large dams or for structures located onunique or soft foundations. Liquid level gages, horizontalinclinometers, subsurface settlement points, and probeexentsometers can all be used, depending on the circum-stance (see paragraph 4-3).

f. Lateral deformation.Measurements of surfacelateral deformations may be required, but they can oftenbe terminated within a few years after first filling unlessspecial conditions exist. Similarly, subsurface lateral

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Table 9-1Measurements and Instruments for Long-Term Performance Monitoring

Measurement, in Priority Order Recommended Instruments

Condition of entire structure Visual observations

Seepage Seepage weirs or flumesPrecipitation gagePool elevation

Pore water pressure1 Open standpipe piezometersTwin-tube hydraulic piezometersVibrating wire piezometersPneumatic piezometersElectrical resistance piezometers

Seismic events Strong motion accelerographsMicroseismographs

Surface vertical or lateral Surveying techniquesdeformations Global positioning system

Subsurface vertical or Liquid level gageslateral deformations Inclinometers

ExtensometersSubsurface settlement points

Total stress at structure Contact earth pressure cellscontacts

1 Listed in priority order, vibrating wire, pneumatic, and electrical resistance piezometers are only used in special cases.

Source: Dunnicliff (1988).

deformation measurements may be required only if thereare special concerns. Probe extensometers, fixed embank-ment extensometers, and inclinometers are options formonitoring subsurface lateral deformation (seeparagraph 4-4).

g. Additional instrumentation.Thermotic survey/thermal monitoring (see paragraph 4-8f) of seepage watercan be a useful tool for the long-term monitoring of seep-age paths within an embankment or abutment. Tempera-ture measurements are especially suitable for abutmentswith nonhomogeneous soil conditions, allowing the moni-toring of strata where the seepage is occurring. Earthpressure cells can be used but generally are useful onlywhen contact pressures between an embankment and anappurtenant structure are required (see paragraph 4-5).

h. Associated parameters.Various other parametersare often monitored. The purpose for monitoring reser-voir, tailwater, and river levels needs no explanation.Rainfall affects surface runoff, which in turn affects weir

and flume measurements. Air and water temperatures canaffect transducer readings, and these measurements arealso necessary as part of groundwater thermal studies.Barometric pressure can also affect certain transducerreadings. Other conditions should always be noted whenpossible, such as general weather conditions and construc-tion activities in the area. Reviewers are often searchingfor possible causes for unexplainable readings, andanswers can often be found if the miscellaneous data arerecorded.

9-7. Management

a. Data collection schedules.Data collectionschedules must be reviewed within a few years after firstfilling. The number of instruments read at unchanged fre-quencies should generally be reduced to a few primaryinstruments. Readings of other instruments should eithercontinue on a reduced schedule or be terminated. Forexample, the frequency of piezometer readings within adam with a rapidly fluctuating pool should be more

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frequent than the pool cycle. If the pool elevation cancycle dramatically within a few days, a monthly readingschedule for the piezometers and seepage instruments maybe useless. However, depending on the feature beingmonitored and the complexities involved, the originalmonitoring schedule may have to be continued for the lifeof the project for numerous instruments. In summary, thereading schedule should be matched to the conditions.Chapter 7 includes additional information.

b. Maintenance.The maintenance of instrumentsand automation equipment must be scheduled andbudgeted for annually. Additional guidance is included inChapters 5 and 8.

c. Data collection. The manual reading if instru-ments over the long-term has historically been difficult.Available project labor is becoming scarce, and whenlabor is available, temporary or untrained personnel areoften used. Rapid turnover of temporary personnel makestraining difficult. Despite the problems, quality of read-ings and observations must be maintained. The optionsavailable are dependent on the project, personnel, and

required reading frequencies. Options can include theautomation of instrument and/or the permanent hiring ofan instrumentation technician. Paragraph 3-8c includesadditional information.

d. Data processing and presentation.This topic isdiscussed in detail in Chapter 7. For long-term monitor-ing purposes, the data processing and presentation capabil-ities should be available at the project as well as in theDistrict office. This allows personnel to respond to amajor event more effectively, especially if communicationto the District is affected. In addition, a complete recordof instrumentation in plot format should be maintained ateach project office. Historical data must be maintainedfor the life of the project.

e. Data interpretation, reporting, and analysis.Review and analysis schedules are dependent upon theindividual projects and features. Minimum schedules aregiven in ER 1110-2-100 and ER 1110-2-110. Additionalguidance is included in Chapter 7.

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