Using Geophysics to Evaluate an Embankment Dam Sinkhole
U.S. Society on Dams, 29th Annual USSD Conference, Nashville, Tennessee, April 20-24, 2009
Michael McCaffrey, PB Power, Boston, Massachusetts;
Phil Sirles, Zonge International, Denver, Colorado;
William Black, Norcal Geophysical, Inc., Cotati, California;
Paul Shiers, PB Power, Boston, Massachusetts;
Ray Barham, Alcoa Power Generation Inc., Knoxville, Tennessee
Publication – [pdf] DAMS_Sirles 2009 using geophysics to evaluated embankment dams USSD conference
Located near Knoxville, Tennessee, Chilhowee Dam is 80 feet high and 1,500 feet long with two embankment sections, a concrete gated spillway, and two concrete nonoverflow sections. A six-foot deep sinkhole formed near the left abutment on the upstream slope of the Chilhowee Dam’s embankment in February 2000. Geotechnical investigations were performed to evaluate the sinkhole, including borings, test pits, instruments, and geophysics. The dam posed two distinct challenges to a geophysical investigation: 1) a very complex geometry of the embankment with upstream sloping clay core, many filters each side of the clay, rockfill shells, and a steeply sloping rock foundation contact; and, 2) restrictions from the hydro power generation at the dam. The focus of this presentation is the multiple surface geophysical methods used for subsurface evaluation to help determine dam remediation.
Two geophysics methods were used: 1) Self-potential (SP) survey to evaluate dam seepage, and 2) three dimensional (3D) seismic refraction survey to evaluate the extent of soft clay found in previous borings. The seismic investigation used an innovative 3D refraction technique to evaluate the internal embankment materials, and represents to our knowledge the first refraction data set ever collected, processed, and presented in full 3D format at an existing dam. SP results indicated two distinct preferential flow paths through the embankment. One of these flow paths crossed the sinkhole, the other was adjacent and near parallel. Geophysics results and conclusions were used together with results of geotechnical investi-gations, embankment design and as-built information to make engineering evaluations of dam safety, the impact of the sinkhole, and extent of remediation.