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Comprehensive geophysical approaches for dam safety investigations with emphasis on the surface wave method evaluation

Sanuade, Oluseun Adetola

Investigation of dam foundations is critical for measuring and monitoring dam safety and is often conducted by geotechnical and borehole measurements. However, geotechnical and borehole measurements only provide information at discrete points with sparse spatial coverage. Geophysical methods can characterize dam foundations and facilitate early detection of seepage in a non-invasive and intensive spatial coverage fashion. This research tested and evaluated a suite of geophysical methods to select the optimum combination of methods for dam safety investigation and seepage detection. The research integrated multiple geophysical methods to characterize the dam foundation rocks and delineate seepage zones at Pensacola and Kerr Dams in Oklahoma. The foundation rock of Pensacola Dam exhibited seepage at a few spots after recent flooding events and requires geophysical investigation to characterize the foundation rock and seepage pathways. For Kerr Dam, the foundation rock has experienced gradual erosion due to major flooding events, hence geophysical investigation is required to provide evaluation of the foundation rock and the underlying geological units at the dam. To achieve the objectives at the two dams, this study integrated electrical resistivity tomography (ERT), self-potential (SP), multichannel analysis of surface waves (MASW), seismic shear (S)-wave reflection, and seismic P-wave refraction. The results at Pensacola Dam characterized the foundation rock into an unsaturated rock layer overlying a water-saturated layer. The water-saturated layer showed a high-seepage potential probably caused by previous blasting and excavation of the dam foundation, or improper grouting. The chemical analysis of water samples from the lake and the two wells inside Arch-16 indicated that water seeping at the ground surface may be a mixture of lake and groundwater or a lake water interacting with rock and coming to the surface through fractures. The geophysical surveys at Kerr Dam characterized the embankment fill/overburden, Reeds Spring Formation, St. Joe Group and Chattanooga Formation and delineated fractured zones which may serve as conduits for seepages. In addition, considering shear wave velocity (Vs) as a vital parameter for assessing the strength of foundation rock, this study further evaluated the effectiveness of the Vs surface measurements by comparing them with the downhole Vs data. The evaluation results affirmed that MASW provides reliable Vs profiles for geophysical characterization. Integrating the geophysical results at the two dams has significantly improved the understanding of the conditions of the foundation rocks at both dams, delineated seepage/weathered zones, and served as a baseline to develop an effective monitoring system.