Clear Lake Dam was constructed in the 1920’s and a water filtration facility was added in the 1950’s. The earthen embankment dam is approximately 30 to 40 feet high by 650 feet long with a concrete spillway. The New York State Office of General Services has been overseeing the inspection and assessment of the dam for New York State Department of Corrections and Community Supervision. McMahon & Mann was retained to provide geotechnical engineering services for the project.

We developed a subsurface exploration program to evaluate the conditions of the earthen embankment. Seven borings were advanced along the crest and toe of the embankment. Groundwater monitoring wells were installed in each of the borings to collect groundwater elevations for use in the stability analysis. We completed rising and falling head tests and packer tests to estimate the permeability of the embankment soils and underlying rock.

The data collected during the subsurface explorations was used to complete an assessment of the earthen embankment slope. The assessment included developing a model of the subsurface soil, rock, and groundwater conditions, completing calculations to estimate the factor of safety for various conditions, comparing the results with the criteria outlined in the New York State Guidelines for Design of Dams and providing recommendations to bring the dam into compliance.

We completed design plans showing the necessary modifications to the upstream and downstream embankment slope and the concrete spillway. Modifications to the dam embankment include installation of a downstream drainage blanket to collect seepage, flattening of the downstream slope, and installing additional erosion protection on the upstream slope. Improvements to the concrete spillway included the installation of 28 double corrosion protection rock anchors which will act to stabilize the spillway and adjacent chute walls.