ROUTINE INSTRUMENTED AND VISUAL MONITORING OF DAMS BASED ON POTENTIAL FAILURE MODES ANALYSIS

Authors: Jay Statler (US Bureau of Reclamation) and Manoshree Sundaram (FERC-CRO)

Presented By: Mike Carpenter, GEI Consultants, Inc.

USSD Committee on Monitoring of Dams and Their Foundations White Paper (Draft)

U.S. Society on DamsVisionTo be the nation's leading organization of professionals dedicated to advancing

the role of dams for the benefit of society.

Mission — USSD is dedicated to: Advancing the knowledge of dam engineering, construction, planning, operation,

performance, rehabilitation, decommissioning, maintenance, security and safety;

Fostering dam technology for socially, environmentally and financially sustainable water resources systems;

Providing public awareness of the role of dams in the management of the nation's water resources;

Enhancing practices to meet current and future challenges on dams; and

Representing the United States as an active member of the International Commission on Large Dams (ICOLD).

White Papers by the USSD Monitoring of Dams and Their Foundations Committee:

1. “Why Include Instrumentation in Dam Monitoring Programs (Nov 2008, Barry Myers and Jay Stateler – published Nov 2008)

2. “Development of an Instrumentation Program” (Jim Hamby, Lead Author, with Pierre Choquet and Brad Long as Co-Authors, in progress)  

3. “Operation and Maintenance of an Instrumentation System” (Amanda Sutter, Lead Author, with Pierre Choquet and Brad Long as Co-Authors, in progress)

4. “Instrumentation Data Management and Analysis” (Chris Hill, Lead Author, with Manoshree Sundaram as Co-Author, in progress)

5. “Routine Instrumented and Visual Monitoring of Dams Based on Potential Failure Modes Analysis” (Jay Stateler, Lead Author, with Manoshree Sundaram as Co-Author, in progress)

Why Provide Instrumentation? The purpose of instrumentation and

monitoring is to maintain and improve dam safety by providing information to:

evaluate whether a dam is performing as expected and

warn of changes that could endanger the safety of a dam

Principal Causes of Concrete Dam Failures and Incidents

(FERC Ch. 9, ICOLD 1992, ASCE 1988)

Overtopping

Foundation leakage and piping

Foundation sliding

Condition of lift chains for

spillway gates – corrosion and

pitting can lead to potential

failure during operation leading to

overtopping

Gate Hoist Failure

Piping in pervious foundation strata

Sliding Along Foundation

Principal Causes of Embankment Dam Failures and Incidents

(FERC Ch. 9, ICOLD 1992, ASCE 1988)

Overtopping Erosion of embankments Embankment Leakage and piping Foundation leakage and piping Sliding of embankment slopes Sliding along clay seams in foundations Cracking due to differential settlements Liquefaction

Spillway blockage; inadequate spillway capacity; erosion of aux. spillway

Flow through Conduits;

aging conduits

Piping in pervious foundation strata

Piping in Embankment

Slope Instability & Liquefaction

Purpose of Instrumentation

Provides data to:• Characterize site conditions• Verify assumptions;• Evaluate initial construction• Evaluate performance design features• Observe performance of known

anomalies• Evaluate performance with respect to

PFMs.

FERC Guidelines – Ch. 9 Instrumentation and Monitoring

“Every instrument in a dam should have a specific purpose.

If it does not have a specific purpose, it should not be installed or it should be abandoned.”

FERC Guidelines – Ch. 9 Instrumentation and Monitoring “Installation of instruments or accumulation of

instrument data by itself does not improve dam safety or protect the public. Instruments must be carefully selected, located, and installed. Data must be conscientiously collected, meticulously reduced, tabulated, and plotted, and must be judiciously evaluated with respect to the safety of the dam in a timely manner. A poorly planned program will produce unnecessary data that the dam owner will waste time and money collecting and interpreting, often resulting in disillusionment and abandonment of the program.”

PFMA Performance Parameter Process (BuRec)

Three basic steps:1. Identify the most likely failure modes for

the dam and associated structures.

2. Identify the key instrument monitoring parameters.

3. Define thresholds and actions.

Outcomes of the PMFA relative to surveillance and monitoring

Identification of enhancements to the surveillance and monitoring programs;

Identification of gaps in data (Category III);

Identification of risk reduction opportunities.

Assessment of Monitoring Needs Based on the PFMA

An instrument must answer a specific question or monitor an identified potential failure mode of the dam or foundation to: Provide an early detection of unusual/

unexpected performance

Provide confirmation of satisfactory performance

Monitoring Consideration for Common PFMs

Seepage-related failure modes for embankment dams (example 1)

Earthquake-related failure modes for embankment dams (example 2)

Failure modes for concrete dams under all loading conditions (example 3)

Flood-related failure modes associated with spillway failure (example 4)

Illustrative Example – PFM No. 1

Seepage-Related failure due to breaching caused by flow through embankment dam that results in piping and transport of embankment material out of the dam or into the toe drain system

Illustrative Example 1 – Monitoring Considerations

Perform regular visual inspections of: D/S slope and toe area

Dam crest

U/S slope

Reservoir water surface

Illustrative Example 1 – Monitoring Considerations

Toe drain and seepage flows

Piezometers and observation wells

Increase monitoring frequency during a flood

Video inspect toe drain system

Key monitoring concepts for seepage-related PFMs for embankment dams

Monitoring water pressures

Data is obtained at discrete points

Regular visual monitoring

Illustrative Example – PFM No. 2

Seepage-related failure of embankment dam in the aftermath of an earthquake due to the formation of a transverse crack in the dam, where seepage flows through the crack and eventually erodes and breaches the dam

Illustrative Example 2 – Monitoring Considerations

Compare baseline data to post-seismic event data

In the aftermath of a significant earthquake, perform an immediate visual inspection of: D/S slope, D/S toe, and areas

Dam crest

U/S slope

Reservoir water surface

Illustrative Example 2 – Monitoring Considerations

In the aftermath of significant shaking, promptly obtain readings

Seismic monitoring equipment with telemetry

Key monitoring concepts for earthquake-related PFMs of embankment dams

PGA criteria

Detection of changed conditions

Automated and/or remote detection capabilities

Baseline data is a must for post-earthquake comparison

Key monitoring concepts for earthquake-related PFMs of embankment dams

Recognized and addressed earthquake issues prior to the event

Installation seismic monitoring equipment

Strong motion data used to validate dynamic models

Illustrative Example – PFM No. 3

Sliding failure at the dam/foundation contact due to poor bonding of the dam’s concrete to the foundation rock and/or insufficient keying under normal, flood or earthquake conditions

Illustrative Example 3 – Monitoring Considerations

Perform regular visual inspections of: D/S face of the dam and gallery walls,

floors, and ceilings Place scribe marks

Structural monitoring survey points

Illustrative Example 3 – Monitoring Considerations

Significant seismic event

Major flood event

Key monitoring concepts for concrete dam failure modes

Failures of concrete dams caused by their foundations

Original construction photographs

Sliding along dis-bonded lift lines

Key monitoring concepts for concrete dam failure modes

Changes with respect to historical performance

Visual monitoring and instrumentation baseline data

Difficult gate operations

Illustrative Example – PFM No. 4

Spillway flow surfaces have flaws such that when subjected to large flows, cavitation results leading to structural damage, headward erosion, and breaching of the reservoir

Illustrative Example 4 – Monitoring Considerations

Perform regular visual inspections of: Flow surfaces

Spillway gallery

Spillway discharges

Post-flood conditions

Key monitoring concepts for flood-related PFMs associated with spillway failure

Pre- and post-flood comparisons

Smaller flood events can identify issues

Key monitoring concepts for flood-related PFMs associated with spillway failure

Issues need to have been recognized

Recognize when failure may be imminent

Closing Remarks The PFM categories discussed are most

common The PFMA team may find that the

available instrumentation is: Sufficient to reach conclusions re. the PFM,

(Category 1 or 2), or Useful, but important issues remain

unresolved and more instrumentation is needed (Category 3), or

That instrumentation can be eliminated or monitoring frequency reduced because the PFM was found to be non-plausible (Category 4)

Closing RemarksThe added value to integrating the PMFA with the dam safety surveillance and monitoring includes: Uncovering data and information

Identifying the most significant PFMs

Identifying risk reduction opportunities

Focusing surveillance, instrumentation, monitoring and inspection programs

Closing Remarks Instrumentation monitoring program established

at one dam may not be appropriate at another dam

Each project be independently evaluated

Structured process that identifies plausible unique PFMs

Develop appropriate monitoring to plausible PFMs

Questions/Comments