A Mooseload of Piezometers: Investigation and Remediation of Seepage at Upper Dam

Two years after the Upper Dam middle embankment construction was completed in 2017, a rising trend in pore pressures was observed in the embankment piezometers. One of the piezometers, installed near the embankment downstream toe, indicated artesian conditions. Seepage and stability analyses with updated piezometer data estimated downstream slope factors of safety below the FERC recommended minimum factors of safety. The high piezometer data also raised concerns about seepage-induced internal erosion. To address the dam safety concerns, the reservoir was lowered, and the design team identified potential failure modes and risk reduction measures; investigated and identified potential seepage paths by reviewing historical documentation, drilling borings, and installing 21 additional piezometers; and designed and constructed a remediation option prior to the 2021 spring runoff to fill the storage reservoir for habitat, summer recreation and downstream hydropower generation.