Investigation of Geophysical Methods for Assessing Seepage and Internal Erosion in Embankment Dams: A Study of Through-Dam Seismic Testing at WAC Bennett Dam

Crosshole tomography allows the distribution of seismic velocity between drill holes to be established. The through-dam mode takes advantage of the triangular cross-section of earth embankments, obviating the need for drill holes. Seismic energy, generated on one face of the dam, passes underneath the crest and is detected by sensors arrayed on the opposite face. The sinkholes discovered at WAC Bennett Dam in 1996 provided an opportunity to test the procedure. Using P-wave energy, two series of measurements were conducted: one immediately before remediation of one sinkhole; the second shortly after the sinkhole was repaired. The known defect was successfully imaged by the first round of measurements. Likewise, the results of the second test reflected the positive effects of compaction grouting. At the same time, the dependence of P-wave velocities on saturation was identified as a challenge to interpretation of the results. To investigate these limitations, and to test the viability of using shear wave energy, the CEATI Dam Safety Interest Group sponsored additional research of the through-dam seismic method. As part of this research, three rounds of tests, using both P- and shear wave (S-wave) sources, were carried out at Bennett Dam. The research confirmed the surveillance potential of the through-dam technique and, in particular, its ability to locate nascent sinkholes that have not yet reached the surface. However, if changed conditions are detected, additional independently derived data will be required to fully define the geometry and properties of the potential defect. This volume marks one of a series of reports on the subject of geophysical methods and their use in assessing seepage and internal erosion in embankment dams.