Geophysical imaging beneath Lake Chesterfield, Missouri

Lake Chesterfield in Wildwood, Missouri, has been leaking since construction of the earth-fill dam was completed in 1986, despite numerous mitigation efforts. The mitigation efforts, including the injection of grouting and the emplacement of clay liners, has not solve the leakage problem. In the current study, geophysical (subsurface imaging) data was acquired across the drained and dry lake bed and along the base of the earth-fill dam to 1) map variable depth to top of bedrock, 2) determine the variable quality of the bedrock to depths on the order of 80 ft., 3) identify any significant karst features beneath the lake, and 4) identify any probable seepage pathways. These imaging data would assist geotechnical engineering firms in determining the most appropriate mitigation plan. During the geophysical survey, electrical resistivity tomography (ERT), multi-channel analysis of surface waves (MASW), and spontaneous potential (SP) data were acquired across the drained and dry lake bed. Interpretation of the geophysical data indicates the average depth to bedrock beneath the lake bottom is generally between 5 and 10 ft. The quality of the shallow and predominantly limestone bedrock (depths less than 30 ft.) beneath the lake bed is highly variable. In some places it is highly weathered; elsewhere it is relatively intact. The acquired geophysical data indicate the earth-fill dam was constructed, in places, on highly weathered limestone. Three prominent low resistivity zones were mapped along the base of the earth-fill dam. It is likely that these three zones represent drainage pathways beneath the dam --Abstract, page iii