Investigation of Source of Seeps beneath Earthen Dam, Central Massachusetts

Along with design and construction issues related to earthen dams, seeps have been implicated in numerous dam breaches. Geophysics is one technique that can be used to identify areas of possible existing “water seeps” and other high permeability zones within or beneath existing dams. HGI performed such an investigation ad a dam in central Massachusetts using a combination of seismic and GPR methods. Close to 7300 linear feet of GPR and 700 linear feet of MASW data were collected during the survey. In order to provide the greatest resolution and to ensure sufficient signal penetration depth, a multiple frequency GPR program was implemented using 200-, 100-, 70-, and 40-MHz antenna systems. The MASW seismic method was used to obtain data for analyzing the Vs anomalies of soil layers and to provide stratigraphic information to constrain the GPR interpretation, including that for bedrock. The geophysical survey identified areas of possible shallow and deep seepage both within and beneath the dam. These areas were further investigated during a borehole phase of the project. A stratigraphic analysis was also completed to assist in the dam remediation analysis. Till, bedrock surface trends, and other horizons were mapped along the length of the dam. Anomalous shallow seepage zones were detected at a number of locations along the survey traverses, as well as deep seepage potential at and above the bedrock surface. Additional anomalies of interest included a possible former stream channel beneath the dam and an area with a high concentration of boulders.