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Crosshole seismic methods to assess the seismic stability of earthen dams

An in situ geophysical investigation consisting of crosshole and downhole shear wave (S-wave) and surface vibratory tests was performed at Sardis Dam, located on the Tallahatchie River in northwest Mississippi. Tests were conducted in a pile test section at the downstream toe of the dam. The purpose of the investigation was to determine changes in S-wave velocities, which are related to soil strength, in the test section due to pile driving activities. Comparison tests were conducted three times: prior to driving the piles, immediately after, and three months after the piles had been driven. The S-wave crosshole and surface vibratory tests indicated a significant velocity increase, averaging more than 20 percent, due to the driving of piles in the test section. Of the geophysical tests conducted, crosshole testing was the most sensitive in denoting velocity changes. The downhole S-wave test did not detect any significant velocity increases. The scope of this paper is limited to results of crosshole testing.