2011 MADCS Conference

FMA is a tool to help identify: Priorities for rehabilitation Improvements for operation, maintenance

and data collection

Uses a team of people to brainstorm possible modes of failure Facilitator, Dam owner, engineer and dam

safety person

Looks at all the potential ways a dam could fail and their likelihood of failure

Not just “another study”

Not a result of analysis by only one engineer or one company - all information is reviewed by a team with different perspectives

Not just a group of outside professionals – the owner is involved for the whole process

EVERY DAM OWNER WANTS TO AVOID THIS…

Many eyes looking at data may uncover unforeseen problems

Obvious concerns may be of less significance than previously thought

Enhancements in monitoring and inspections

Data gaps exposed Broader range of individuals become

educated and aware of potential problems

WHY DO A FAILURE MODE ANALYSIS?

Same concept as a FERC FMA, but requiring far less time and money

More applicable to typical Montana earthen dams

Reasonable cost and effort for private owners

Four basic components1) identify facilitator and assemble core team2) collect data for team review3) conduct FMA (morning dam inspection, afternoon brainstorming

session)4) summarize results in a report

Identify a Team Facilitator Identify a Team Organizer Identify a Core Team Identify Participants

Collect Data Construction drawings &

reports Monitoring data Inspection reports &

photographs Studies O&M plans

Core Team Review of Data Thorough review Ask, “How could the dam fail?” Have failure modes been identified and

is proper monitoring being done? Review should be accompanied by

Questionnaire sent by Facilitator to “think failure modes”

Things to Do Before FMA (Facilitator & Organizer)

Meeting time and location Visual aids Conference call before meeting

Morning Inspection Before FMA All participants involved Look over dam/appurtenances “Think and see” potential failure

modes Interview dam tender or operation

personnel

Afternoon Brainstorm FMA Session All participants involved Identify and discuss potential failure

modes Organize into failure conditions:

Normal operationSeismic loadingHydrologic loading

Categorize failure modes

Failure Categories

I – Highlighted Failure Modes (High repair priority; high failure risk; high failure consequences)

II – Failure Modes Considered but not Highlighted (A concern, but lower repair priority; lower failure consequences)

III – More Info Needed in Order to Classify (Information lacking; need more data)

IV – Failure Mode Unlikely but Not Ruled Out (Unlikely but not impossible; low priority)

V – Failure Mode Ruled Out (Not realistic; no concern)

Failure Mode Considerations Factors making it less

likely or more likely Consequences Risk Reduction Measures Future Data Needs

Major Findings and Understandings Probably the most important

component All participants state their own MFU’s Summation of FMA Recorded for the

results report

Documentation of Major Findings and Understandings

Summary of FMA Actions recommended Statement on adequacy of

documentation Appendices of data Core team should review before

finalizing

Get “free” participants toreduce cost but don’t sacrifice quality

Clearly organize data foreasier review

Make all visuals large and easy to read Clearly document everything Stay on task – facilitator needs to be

efficient Write up results immediately to maintain

correctness Get Dam Safety to help

Guidance for future operationsDetermine rehabilitation decisions Identify investigations neededPrioritize maintenance and repairs

Example – Eureka Dam

Eureka Dam located in Teton County, northwest of Choteau

FMA for Eureka Dam was conducted in 2008

Relatively informal – not organized in standard fashion

If Eureka Dam failed, Choteau and the highway would be flooded

Issues:

1. Old outlet pipe showing corrosion and settlement

2. Seepage exiting at toe and collected in toe drain trench

Example Category I ModeCategory I - Corrosion and failure where gate tower connects to outlet conduit, causing

piping along outlet, progressive erosion of embankment

Factors that make this more likely

• Settlement at connection, previous gap

sealed recently

• Age of pipe

• Similar failure mechanism in many

Montana dams

• Uncertainty with bituminous coating

in this area;

• Difficult to inspect

Factors that make this less likely

• Bituminous coating appears to

protect pipe well

Example Category I ModeCategory I - Corrosion and failure where gate tower connects to outlet conduit,

causing piping along outlet, progressive erosion of embankment

Consequences of Failure

• Failure of dam

• Loss of reservoir contents

Risk Reduction Measures

• Increase inspections to an annual basis

• Add diaphragm filter around outlet pipe

Data / Information Needs

• Can embankment soils withstand erosion?

• Need sample of embankment soils

Example Category III ModeCategory III - Piping of embankment materials into foundation

Factors that make this more likely

• Some unexplained depressions on

upstream face could be related to piping

of embankment materials into foundation

• No filters or drains in embankment

• No knowledge about foundation preparation

• Settlement of outlet pipe after construction

suggests foundation problems and possible

internal cracking of embankment

Example Category III ModeCategory III - Piping of embankment materials into foundation

Factors that make this less likely

• Embankment constructed under supervision of State Water Conservation Board; foundation prep likely

• Beck piezometers show slight upward gradient from foundation

• Dry embankment and wet foundation support idea that upward gradient is present (water is moving directly from reservoir through bedrock foundation)

• Low percentage of piping failures in dams are from this failure mode

Example Category III ModeCategory III - Piping of embankment materials into foundation

Consequences of Failure

• Failure of Dam

Data / Information Needs

• Is embankment made of material

that is subject to piping?

• Need sample of embankment soils

• What is phreatic gradient within dam

and foundation?

• Nested piezometers in embankment

and toe are needed

Example FMA Results

Recommendations from Simplified Failure Mode Analysis

1. Implement risk reduction measures for Category I failure modes

• Increase outlet inspection frequency

• Diaphragm filter around outlet pipe

2. Collect missing information on Category I and III issues

• Install nested piezometers in embankment and foundation

• Collect samples of embankment and foundation materials

3. Make plans to rehabilitate outlet works

Simplified FMA is cheap and not time consuming

Good for when repair $ are tightGood when unsure where to focus

investigationsHelps get more people involvedCan open eyes to things not thought

of

Gary Fischer, P.E.Hydrometrics, Inc./Carroll Collegegfischer@hydrometrics.com406-443-4150