Transcript Slide 1
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… WHY DO A FAILURE MODE ANALYSIS? 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 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 components 1) identify facilitator and assemble core team 2) collect data for team review 3) 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 operation Seismic loading Hydrologic 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 to reduce cost but don’t sacrifice quality Clearly organize data for easier 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 operations Determine rehabilitation decisions Identify investigations needed Prioritize maintenance and repairs 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 Category 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 Category 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 Category 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 Category 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 Category 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 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. 3. Collect missing information on Category I and III issues • Install nested piezometers in embankment and foundation • Collect samples of embankment and foundation materials Make plans to rehabilitate outlet works Simplified FMA is cheap and not time consuming Good for when repair $ are tight Good when unsure where to focus investigations Helps get more people involved Can open eyes to things not thought of Gary Fischer, P.E. Hydrometrics, Inc./Carroll College [email protected] 406-443-4150