GEOPHYSICAL SURVEYS ON CARROLL COUNTY DAM

Carroll County Dam (North Mississippi) is an earthen dam approximately 9m high and 229m long, built in 1965 to obstruct an existing little creek and creating a reservoir of approximately 0.12Km. In 2016, during a routine visual inspection, sand boils were observed downstream of the dam. The dam usually retains very little water; however, exceptional flood events could significantly increase pore water pressure and potentially lead to dam failure. For this reason, during the spring of 2017, the National Center for Physical Acoustics in collaboration with the Mississippi Department of Environmental Quality Dam Safety Division has been conducting extensive geophysical measurements to investigate the dam. Several seismic refraction and electric resistivity surveys were conducted at the dam. The as-built plan, historic topographic maps and available borehole information were used to aid the interpretation of the geophysical data. Preliminary results from the study indicate two possible conditions leading to seepage. The location of geophysical anomalies being close to pre impoundment channel support water seeping through an old pre impoundment stream channel. The second possible condition is water seepage through a small lens of silty sand (higher porosity) imbedded within a clay layer that was not properly sealed during preparation of the base of the dam.