Boney Falls Hydroelectric Project, PMF and Seepage Improvements

Built in 1921, the Boney Falls Dam suffered its first failure only 9 years after construction. Even following the 1930 reconstruction, significant seepage persisted. After recognition of the risk the seepage presented to the embankments, GEI conducted a multi-year evaluation, and oversaw construction of a two-phase improvement project to remediate the risks related to uncontrolled seepage and associated ground loss.

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Key Challenges

GEI conducted investigations, characterization and remediation design of seepage routes beneath the embankment dams, which were built on jointed limestone. In 2010, the investigation to evaluate the seepage conditions included standard penetration tests, bedrock coring, packer testing, dye testing, colloidal velocity measurements, and piezometer construction. Seepage flows were confirmed to be passing through the upper 10 feet of the bedrock foundation through a network of seams and joints, which jeopardized only localized areas of the embankment. To treat the localized areas of risk, GEI evaluated various rehabilitation options, assisted the owner in selection of the best option, and prepared a Phase I and II design documents to remediate the seepage flows.

For the left embankment, the remedial design included involved increasing the crest width of the embankment to minimize the risk a sinkhole could compromise the entire crest width. This risk reduction approach was accepted by the Federal Energy Regulatory Commission (FERC) with the understanding that this area of the embankment did not retain significant amount of water.  The left embankment was also raised to accommodate probable maximum flood reservoir levels.

The right embankment was at higher risk of ground loss failure mode due to it retaining 16 feet of water, and the historic failure in this area. The owner elected to excavate the embankment and treat the foundation, in lieu of constructing a bedrock cut-off wall.

The construction on the right embankment was conducted between 2012 and 2013 as Phase I and II Improvement Projects. GEI oversaw the construction as the Engineer of Record and the Quality Control Inspection Plan (QCIP) Manager. Upon completion of the construction, the reservoir was refilled to the normal licensed pool, with little to no seepage flows emanating from the right embankment.

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