Bedrock Seepage Required Surgery - Boney Falls Dam

摘要

Following original construction, the right embankment dam at the Boney Falls Hydroelectric Dam failed 9 years later due to high seepage flows. The reconstructed embankment included a concrete cutoff wall within the embankment to control seepage, but substantial foundation seepage persisted. A breach of the embankment due to piping of embankment material into the fractured rock was identified as a potential failure mode (PFM) and in 2008, the owner elected to partially lower the reservoir and consider treatment options. Several traditional treatment options were considered, including a foundation grout cutoff wall and a secant-pile cutoff wall constructed through the existing embankment. These options were found to be costly with many risks related to magnitude of bedrock fractures and effectiveness in reducing seepage. A unique design approach was chosen to remediate the at-risk portion of the right embankment. The design included plugging inlet features upstream of the core wall and safely collecting seepage downstream of the core wall that may persist. The risks associated with the design were evaluated during a mini-potential failure modes analysis (PFMA) session. Following Federal Energy Regulatory Commission (FERC) acceptance, construction started with a temporary cofferdam and plugging of the inlet features (bedrock cracks) upstream of the core wall. A graded filter and toe drain system was constructed downstream of the cutoff wall to collect seepage flow, while preventing migration of fine soils. The reservoir has been successfully returned to normal pool elevation and normal hydro generation, and the seepage flow was effectively eliminated.