USING GEOPHYSICS TO EVALUATE AN EMBANKMENT DAM SINKHOLE

摘要

Located near Knoxville, Tennessee, Chilhowee Dam is 80 feet high and 1,500 feet longwith two embankment sections, a concrete gated spillway, and two concrete nonoverflowsections. A six-foot deep sinkhole formed near the left abutment on theupstream slope of the Chilhowee Dam’s embankment in February 2000. Geotechnicalinvestigations were performed to evaluate the sinkhole, including borings, test pits,instruments, and geophysics. The dam posed two distinct challenges to a geophysicalinvestigation: 1) a very complex geometry of the embankment with upstream sloping claycore, many filters each side of the clay, rockfill shells, and a steeply sloping rockfoundation contact; and, 2) restrictions from the hydro power generation at the dam. Thefocus of this presentation is the multiple surface geophysical methods used for subsurfaceevaluation to help determine dam remediation.Two geophysics methods were used: 1) Self-potential (SP) survey to evaluate damseepage, and 2) three dimensional (3D) seismic refraction survey to evaluate the extent ofsoft clay found in previous borings. The seismic investigation used an innovative 3Drefraction technique to evaluate the internal embankment materials, and represents to ourknowledge the first refraction data set ever collected, processed, and presented in full 3Dformat at an existing dam.SP results indicated two distinct preferential flow paths through the embankment. One ofthese flow paths crossed the sinkhole, the other was adjacent and near parallel.Geophysics results and conclusions were used together with results of geotechnicalinvestigations, embankment design and as-built information to make engineeringevaluations of dam safety, the impact of the sinkhole, and extent of remediation.