Geotechnical aspects of the Graham Lake Dam remedial measures project

Graham Lake Dam is a 550-foot long earthen embankment dam with a maximum height of 45 feet, an 80-foot long gated spillway and a high hazard rating. The dam is located in Ellsworth, Maine and is part of the Ellsworth Project which is owned and operated by Bangor Hydro-Electric Company. The dam was constructed between 1922 and 1924 by dumping earth from a trestle without controlled compaction, with lower portions apparently being dumped through standing water. In addition to its loose character as a result of construction methodology, the dam is founded on a sensitive glacio­marine silty clay of soft to medium consistency.
Stability assessments conducted as part of the FERC Part 12 Inspection process determined that portions of the embankment were susceptible to seismically induced liquefaction and the downstream slope had a lower than standard factor of safety against rotational slope failure because of a high phreatic surface and soft foundation soils. This high phreatic surface resulted in a small seepage zone along a bench at the bottom third of the downstream face that tended to worsen with increasing reservoir stage. A temporary toe drain was installed to control locally quick soil conditions that were observed in some areas of the seep. Besides the stability and seepage problems, hydraulic assessments determined that the spillway was inadequate to pass the predicted Inflow Design Flood (the Probable Maximum Flood) without overtopping the earth dam. CONT.