Civil Engineering & Design
Graham Lake Dam Rehabilitation
The Graham Lake Dam is part of the Ellsworth Project, which is owned and
operated by Bangor Hydro-Electric Company (BHE). The embankment was constructed
in 1923 as a semi-hydraulic fill. Engineering assessments determined that the
earth embankment contained soils that were susceptible to seismically induced
liquefaction. In addition, the embankment is founded on a sensitive, soft silty
clay which, in conjunction with a high phreatic surface and loose soils,
contributed to a low slope stability factor of safety.
A feasibility study was conducted to determine a cost-effective approach to
remediation. Devine Tarbell & Associates, Inc.’s (DTA) staff designed the
selected remediation and provided Owner’s Representative services on-site during
construction.
DTA’s staff concluded that the most cost-effective approach to remediation was a
concrete gravity flood control structure located at the downstream toe of the
existing dam, integrated with the existing spillway structure. Design of the new
flood control structure was constrained by the limited land available at the
downstream slope, and appeared to require an extensive tieback wall to maintain
the stability of the downstream slope of the dam while digging a 30-foot-deep
excavation to bedrock foundation subgrade, significantly increasing project
costs. As an alternative to the tieback wall, DTA’s staff developed a unique and
cost effective approach using a counterweight berm, constructed in sequential
cells, to replace the soft foundation soils at and below the toe of the dam,
effectively buttressing stability during excavation.
The existing dam consisted of granular soils and would serve as the upstream
cofferdam for the excavation during construction. To control seepage, DTA’s
staff specified a deep well system. After installation of each deep well in the
dam, a pumping test was conducted. The indicated hydraulic response of the dam
to pumping, measured by piezometers, was evaluated using a three-dimensional
groundwater flow model to assess the need for additional wells prior to
excavation. Field instrumentation to monitor slope stability during excavation
and construction included piezometers and slope inclinometers.