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GEOTECHNICAL INSTRUMENTATION

David Marks AdvDipLab(Civil), OMIEAust, CEngO

Abstract Recent above average .rainfall intensities have had an adverse impact on the stability of slopes and embankments across Queensland's state controlled road network. Both during times of exceptionally wet weather and during normal weather conditions, data derived from geotechnical instrumentation and tools have been invaluable in enabling Queensland Department of Transport and Main Roads (TMR) technical staff in the areas of site investigation, design verification, construction specification, quality control and legal protection. A further benefit has been the confidence it gives geotechnical engineers in providing safety for the state's road users.

This article gives a brief insight into the range of instruments used to gather geotechnical data to aid technical decision-making as part of normal construction practice or before undertaking road and embankment rehabilitation. It concludes with a case study of their use on TMR's South West Transport Corridor project.

Background Events worldwide have led to the conclusion that the world's climate is changing. It is believed that this will lead to higher average temperatures and increased extreme weather conditions. From a slope and embankment stability aspect, increased rainfall causes surface erosion, rock slides and increased infiltration of water into the underlying strata. The effects of this water infiltration are increased bulk density and a reduction of the shear strength properties of the soil or rock. If adverse weather conditions continue, it is likely there will be increased emphasis on geotechnical instrumentation to monitor the safety of the road network.

Geotechnical instrumentation Instrumentation is an integral part of geotechnical engineering, providing an essential tool for monitoring safety and performance during the construction and maintenance phases of many civil engineering projects. It supplies data on a variety of engineering parameters such as rainfall, pore water pressures, horizontal and vertical deformations, temperature, and stresses and strains induced in soils, rock and structural steel members. This information is often applied to infrastructure projects involving embankments on soft ground, earthen embankment dams, excavated and natural slopes, underground excavations and tunnelling applications.

The effective use offield instrumentation requires a thorough understanding of geotechnical principles and careful planning of instrumentation projects. Each step (equipment installation, monitoring, data analysis and reporting) is critical to the success or failure of the overall project and depends heavily on the combined capabilities of the instruments and their operators. Structural design engineers and geotechnical engineers use data derived from geotechnical instrumentation to obtain a factor of safety to enable a reliable estimate 'of performance. The technological advances made in instrumentation equipment are mirrored by stakeholders' increased levels of confidence in civil engineering construction activities. Not only do projects benefit in terms of cost and time, so too does the general community.

QUEENSLAND ROADS Edition No 11 September 2011 25

TMR's Geotechnical Services Unit is a fully NATA endorsed geotechnical laboratory within Engineering and Technology Division. It is part of the Geotechnical Branch, it is comprised of geotechnical engineers, engineering geologists, geologists, technologists, draft persons, field and laboratory staff and administration staff. The unit provides geotechnical instrumentation services to districts and external clients throughout the state.

Which instruments do what? The following describes the range of instrumentation, techniques and tools used by TMR's Geotechnical Services Unit to collect data about road and embankment performance.

• Instrumentation for monitoring pore water pressures - This includes observation wells, hydraulic standpipes, pneumatic and vibrating wire piezometers (VWP) and time domain refiectometry (TDR) moisture probes (Figures 1-7). These can provide data for the determination of safe rates of fill placement, slope stability, lateral earth pressures, uplift pressures and monitor the effectiveness of drainage schemes.

o Figure 1. Vibrating wire piezometers

Figure 2. VWP attached to inclinometer casing to record pore pressures - South West Transit Corridor (SWTC) project

Figure 3. Drilling and installation of sta",dpipe piezometer casing - Toowoomba pilot tunnel

26 QUEENSLAND ROADS Edition No [1 September 201 [

Figure 4. Data loggers for VWP - Tu~!y Bypass project

Figure 5. Setting up remote sensol" logging of VWPs - Nundah Tunnel

Figure 6. TDR moisture probe monotor & multiplexer unit - Gatton Bypass

Figure 7. TOR moisture probe installationsGatton Bypass

QUEENSLAND ROADS Edition No 11 Seplel11ber 2011 27

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Measurement of vertical deformation - There a number of instruments used for measuring vertical deformation, including settlement cells, plates, gauges (hydraulic and vibrating wire), horizontal profile gauges, magnetic extensometers, spider magnets and Sondex settlement systems. Data from these instruments can confirm consolidation predictions, final adjustments to grade/ embankment heights, and the performance of engineered foundations. These will be explored in more detail in" the following sections.

Magnetic extens~meters and Sondex settlement systems - These systems are used to monitor settlement and heave in embankments at a number of discrete points. The data indicates the depths at which settlement occurs, as well as total settlement.

Entry point for readout probe

Base datum magnet

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Two types of magnetic extensometer are used (Figure 8). The first has individual spider magnets at set depths referenced to the base datum magnet. The spider magnets are coupled to the surrounding soil and move up or down on the outside of the plastic tube as heave or settlement occurs. The second is the Sondex system which has magnets attached to concertinaing outer tubing which can compress or expand with the surrounding soil.

Both systems are read by drawing a readout probe between the bottom and the top of the casing. The readout probe indicates the relative depth of each magnet as it is passed and, when referenced against the location of the base magnet, provides data on incremental and total settlement.

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Spider magnets attached to Inclinometer casing

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Figure 8. Magnetic extensometer

QUEENSLAND ROADS Edition No 11 September 2011

• Horizontal Profile Gauge - The horizontal profile gauge (HPG) or horizontal inclinometer is used to obtain a high resolution profile of settlement or heave typically associated with embankments and landfill areas. The casing is installed in a horizontal trench, with the far end attached to an inaccessible dead end pulley unit. The probe is pulled along the full length of the casing and readings are recorded every 0.5m interval (Figure 9).

The system consists of inclinometer casing, a horizontal probe, a control cable, a pull-through cable and readout unit. The initial survey establishes the position of the casing and subsequent surveys reveal the displacement of the casing if ground movement has occurred. This is a low maintenance system since there are no hydraulic lines or pressure sources and the results ofthe surveys can provide a complete profile of differential settlements along the given alignment.

Datum located away from embankment

• Settlement plates and survey markers -Settlement plates indicate the rate and extent of settlement or heave usually within an embankment structure when the natural material is surcharged. The settlement plate consists of a base plate and extension pipes which protrude through the embankment (Figures 10,11). Extension pipes can easily be added as the embankment is constructed. Surveyors take levels on the pipe and from that data produce a settlement history plot. The pipes are robust and low cost but, as they protrude through the embankment, are prone to damage by construction traffic. They are also only temporary units as they are cut off and covered once the roadway is opened.

The survey markers are a simple device which can be read by a surveying team to provide a history of reduced levels and coordinates over a period of time. The markers are generally small star pickets or similar that can be easily placed in areas of interest. Typically they are installed in areas not prone to traffic damage. They can provide information on movements similar to the settlement plates. Both the settlement plates and survey markers require an accurate survey bench mark which is used as the datum point for all subsequent readings.

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. 4

Figure 9. Installed horizontal profile gauge (HPG)


Extension pipes

Survey markers ~----'L-~~-~-~--~

Figure 10. Settlement plate installed in an embankment

Figure 11 . Settlement plates during construction - Granard Road Interchange

30 QUEENSLAN D ROADS Edition No 11 September 20 11

• Settlement gauges - Settlement gauges indicate the rate and extent of settlement within an embankment structure when the natural material is surcharged. The overburden pressure created by the embankment material can cause the natural material to settle and consolidate.

A very simple settlement gauge consists of a water-filled tube connected to a water settlement indicator pipe buried within the embankment and interconnected with a gauge readout box located far enough away from the influence of any embankment settlement (Figure 12).

The readout box hflS a graduated tape measure inside which shows the level of the water within the indicator (essentially a 'U' tube manometer). The indicator has additional tubing connected to it which allows the unit to be open to atmospheric pressure, and also a drain pipe which removes excess water during the actual reading phase. The benefit of this type of unit is that once the indicator has been effectively buried within the embankment, the settlements can be reliably monitored on a long-term basis well after the road has been opened to traffic.

Distance varies

Settlement gauge readout box Water level

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Figure 12. Settlement gauge based on 'U' tube manometer principle

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• Measurement of lateral deformation -Measurement of lateral deformation can be undertaken in a number of ways: load and strain monitoring, including vertical inclinometers, rod extensometers, electro level beam sensors, crack meters, strain gauges and load bolts. These can evaluate stability of slopes and embankments and verify performance and safety of structures and embankments (Figures 13-17).

Figure 13. Inclinometer casing prior to installation - David Low Way

Figure 14. Installation of an inclinometer probe with VWP attached - SWTC project

Figure 15. Monitoring of strain gauges - Barneys Point Bridge NSW

Figure 16. Electro level beam sensors - SWTC Bridge

Figure 17. Inclinometer probe and data logger

32 QUEENSLAND ROADS Edition No 11 September 2011

• Measurement of total stress in soils - Earth pressure cells measure stresses normal to the cell's face and have either single or double-sided sensing faces (Figure 18). Embedded earth pressure cells are typically installed in fill and embankment materials to determine the distribution, magnitude and direction of total stresses.

Contact earth pressure cells are generally installed at the interfaces between soils and retaining wall type structures, culverts, and piles etc.

The cells are typically formed from two circular stainless steel plates. The edges of the two discs are welded together to form a sealed cavity which is filled with hydraulic-type fluid. The pressure transducer, either vibrating wire or pneumatic, is attached to the cell via a connection pipe. The cells are embedded in fill in a variety of different configurations or mounted on structures with direct burial cable trenched to a remotely located readout unit.

Small "pockets" are excavated in the main fill area and the cells are positioned for maximum performance. The pockets are carefully backfilled to the surrounding material density, thereby minimising any possible bridging effects which can prevent total stresses from being exerted onto the cell face.

Figure 18. Various types of earth pressure cells

• Rainfall gauges - Rainfall gauges monitor local rainfall, particularly where the impact of rainfall is critical to slope stability. Gauges can range in complexity from a simple manual rain gauge to a fully automatic gauge with internet remote sensing (Figure 19).

Figure 19. Rain gauge set up for remote sensing


• Aerial photo studies - Stereo aerial photography can be used.

• Drilling/Geological mapping/Field testing -Soil and rock core samples can be taken from a series of borehole drill and test pits. A variety of insitu field tests such as cone penetrometer and field shear vane can be utilised to determine insitu material properties. Analytical predictions can be confirmed by combining the data from field samples analysed in the laboratory and data obtained from the insitu field tests.

• Surface level monitoring - Surface surveying techniques together with a number of surface monuments can be used to establish surface movement. Terrestrial laser scanning and radar surveying can also be used to monitor small surface movements.

• Geophysical surveys - A ground magnetic survey may be used to investigate the stratigraphy of sub-surface layers, and could include ground penetrating radar and seismic surveys.

Which parameter to measure? Each project is unique and identifying which parameters are critical for design and evaluating performance will impact on the selection of the instruments to be used. The keys to success depend on adequate planning and execution of installations, combined with the capabilities of the instruments and the capabilities of our people.

Soil and rock mass are often complex and more than one parameter may require investigation; it is, therefore, useful to measure a number of parameters that can be correlated from various measurements. For example, inclinometer data showing an increased rate of movement may be correlated with piezometer data showing elevated pore pressures.

Ground conditions can also affect the choice of instrumentation. Soils of high permeability with large volume changes in groundwater can be adequately captured by the use of a standpipe piezometer, whereas soils with low permeability will require a diaphragm type (pneumatic, vibrating wire) piezometer, which offers faster response times due to increased sensitivity.

As resource costs increase, the development of remote onsite sensing and data logging is becoming a necessity. Many companies now specialise in providing remote access to data in near real time via the internet. On some construction projects the department has systems in place to provide SMS messaging when certain critical parameter alarm levels are reached. This contribution to an overall site management plan ensures safety processes are implemented quickly and efficiently.

Instrumentation used on some larger projects The following is a snapshot of the range of projects that have been instrumented by the department.

• Barneys Point Bridge, RTA, NSW - Bridge abutment instrumentation for construction and long-term monitoring. Inclinometers, horizontal profile gauges, use of strain gauge and load bolts on geogrids, settlement gauges and pneumatic piezometers.

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•

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•

Sunshine Motorway and Coombabah trial embankments - Comprehensive installation of monitoring devices including inclinometers, Horizontal Profile Gauges, settlement gauges, pneumatic and vibrating wire piezometers and "geologger" data logging system.

Port of Brisbane Motorway Alliance Group -Comprehensive installation of instrumentation for construction over soft bridge abutment suspected of differential movements.

Construction projects with high earth embankment structures throughout Queensland - Granard Road, David Low Way, Maroochy Interchange, Goodings Corner, John Luscombe Bridge (Mia Mia), Pacific Motorway Oxenford on-ramps. Full suite of construction instrumentation with an emphasis on monitoring lateral movements.

Road construction projects over soft soil areas throughout Queensland - External engineering consultants and alliance groups including Bruce Highway - Cardwell to Tully, Cooroy to Yandina, East West Connector Road, Joint Levee Road and Gateway Arterial. Full suite of construction instrumentation biased to settlement and pore water pressure devices.


• Instrumentation of earth dam structures throughout Queensland - TMR and external consultants including Queensland Alumina Ltd -Red Mud Tailings Dams, Kinchant Dam. A range of devices from inclinometers to remote sensor vibrating wire piezometers were used.

• Instrumentation of slip areas throughout Queensland - TMR and external consultants including Montville Road, Esk - Hampton Road, The Leap - Mackay, Toowoomba Range, South West Transport Corridor and Cunningham's Gap.

Case study - South West Transport Corridor The South West Transport Corridor (SWTC) is a major transport corridor which extends the Centenary Highway to the Cunningham Highway via Ripley. During the advanced stages of earthworks constmction of the Springfield to Yamanto section, large-scale landslide activity was detected in the vicinity of two large cuts (1). These cuts contained a complex geology, comprising trachyte flows extmded on top of sediments and older basalt flows .

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The investigation of such a large-scale landslide was a challenging process. The landslide activity is believed to have resulted from the reactivation of a deep-seated slow movement along shear planes associated with an ancient land surface (Figure 20). This complex movement could not have been detected despite the extensive geotechnical investigations undertaken during the planning and design stages. This moving surface was subsequently detected by a suite of geotechnical instmmentation some of which operated at depths well below the normal drilling depths normally accepted for site investigation.

In order to characterise the landslide mass, the geometry of the failure surface and rate of surface movement, inclinometers, together with a large number of monitoring points, were installed to measure surface movements. Instmmentation also included rain gauges, piezometers and observation wells. Electro Level (EL) sensor beams were also installed to provide data on a bridge abutment which could have been impacted by these differential movements.

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Figure 20. Section showing inferred deep-seated slip surface

QUEENSLAND ROADS Edition No II September 2011 35

The inclinometer is one of the primary instruments used on site to determine subsurface lateral deformations. Data obtained from inclinometers (Figures 21, 22) assists in determination of the magnitude, rate, direction and depth of ground movements. Inclinometer instrumentation equipment is comprised of a biaxial accelerometer probe, graduated control cable, portable data logging system, intemally grooved ABS plastic inclinometer casing and a cable clamping pulley assembly. This set-up enables the operator to survey the borehole from the base to the top in O.5m increments. Subsequent readings determine the change in the shape and position of the casing relative to the initial casing profile.

A total of 54 inclinometers was installed onsite, with depths varying between 25m and 104m. Where possible, 85mm diameter inclinometer casing was used as this diameter has proven longevity in terms of allowing the probe to traverse through the inferred failure zone area (2). As the casing deformation increases, a point is reached where the passage of the probe within the casing becomes blocked. A bore hole with a larger diameter casing has a corresponding greater serviceability life.

Unfortunately, highly fractured zones of trachytes proved difficult during drilling operations and as a result, several smaller diameter boreholes were drilled and only 70mm diameter inclinometer casing could be installed. Although not ideal, these devices provided valuable data during their serviceable lifespan. A total of 36 inclinometers remains on site and is monitored monthly, except in times of high rainfall activity, when the frequency of monitoring may increase to daily for those devices in areas defined as critical.

The boreholes were drilled well below the potential movement zone, which ensured that the inclinometer bases were founded into competent, stable materials. A lay-flat hose was attached to the bottom length of the inclinometer casing and extended in a continuous length from the base of the borehole to the surface. This hose was used to deliver the grouting materials which consisted of a water/cement/bentonite mix. Attention to detail during the grouting phase was critical as poor grouting technique can contribute to voids being created between the inclinometer casing and the sides of the borehole. This in tum can lead to erroneous readings. Samples of each grout mix were collected for laboratory testing. During the inclinometer casing installation, the orientation of the internal inclinometer grooves were aligned using a hand-held compass bearing to ensure that the AO-A180 plane was in the anticipated direction of movement. This was generally in a north-south direction and this orientation of the casing was applied to all installed inclinometers across the site.

The grout was allowed to set for 48 hours before initial readings were taken on the newly installed inclinometer. Again, high attention to detail was shown by using the same inclinometer probe and cable and, where possible, the same operator to take readings on all devices across the site. The readings were analysed and checked for systemic and random errors (3). Each borehole profile was determined to confirm that the installed inclinometer did not exceed 3 a from the vertical. Calculated vector plots provided accurate directions of movement. The presentation and report plots were generated by commercially available inclinometer software.


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INCLINOMETER MOVEMENT (mm) - NORTH TO NORTH WESTERLY DIRECTION

Figure 21. Inclinometer plot - the depth and extent of movement at the failure zone.

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Figure 22. Rainfall activity (blue line) triggers increased movement


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In addition to the inclinometers, a variety of ground water monitoring devices was installed across the site. Observation wells, consisting of slotted 50mm PVC pipe encased in the borehole with a granular sand medium, were installed to monitor ground water levels. The backfill sand extended from the base of the borehole to approximately 1.5m below the surface; this provided an "intake" zone for any sub-surface ground water. The top 1.5m was backfilled with a water/ cement/bentonite grout mix and capped with concrete to prevent the ingress of surface runoff water.

Using conventional drilling techniques, standpipe piezometers (comprising polyethylene water intake elements attached to 25mm PVC riser pipes) were installed to monitor groundwater levels/pressures at specific depths. A granular sand intake zone was created around the piezometer tip and sealed in place with a layer of bentonite pellets above the sand layer.

The remainder of the borehole was backfilled with water/cementibentonite grout mix and capped with concrete at the surface. Vibrating wire piezometers (VWP) were installed to monitor pore water pressures (Figure 23). The VWP consists of a standard body type pressure transducer, signal cable, surface data logger and telemetry system. The pressure transducer was installed in an inverted position with the tip deaired. Where possible, the VWP tip was attached to the outside of the inclinometer casing at designated depths and encased directly in the water/cement/ bentonite grout mix. If the borehole diameter was not of sufficient diameter to allow the VWP to be attached to the outside of the inclinometer casing, the tip was attached to lengths of 20mm sacrificial conduit, which acted as a delivery mechanism to lower the tip to the precise depth location.

BH373 - PLOT OF GROUND WATER LEVEL RL and MAXiMUM DAILY RAINFALL

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Figure 23. Response of ground water levels (maroon line) to rainfall events


The signal cable was terminated at the surface into a secure weatherproof enclosure and connected directly to a data logger. The telemetry/modem system was located centrally on site and the data co llected via a radio link and relayed to a remote server. A total of 70 vibrati ng wire devices was installed and monitored across the site.

Summary Engineers have an obligation 10 design and build sa fe infrastructure and structu res. Geotechnical instflnnenlalion and performance monitoring are an integral part of the plann ing and construction process. They a lso make a significant contribut ion 10 ensuring the long-term safety of the road network.

References I . Starr 0 , Dissanayake A, Clements J, Marks 0 ,

Wijeyakulasuri ya V. SOll lh Wesl Corridor land slide, Queensland Roads Ed ition 8. March 2010

2. Dunnic lifTe J. Geotechnical instrumentatioll/or moniloringjield peiformance, New York: John Wiley. 1988.

3. Mikkelsen P. Advances in Inclinometer Data Allalysis, Symposium on Field Measurements in Geomcchan ics, FMGM, Oslo, Norway. September 2003

4. Guide 10 Geolechnical Inslrumen/alion, Durham Geo Slope Indi cator. 2004

QUEENSLAND ROADS Edition No II Scptembcr2011 39

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