SH-Wave Seismic Reflection Images of Anomalous Foundation Conditions at the Mississinewa Dam, Indiana

Abstract

Geologic features (i.e., solution cavities, active∕inactive faults, fractures, and filled∕open joints) often go undetected by traditional geotechnical exploration methods, but can have long-term adverse affects on the foundation conditions of large engineered structures. These geologic features can be relatively small and nevertheless have significant consequences for foundation integrity when high hydraulic heads are present. A noninvasive, horizontally polarized shear-wave, seismic reflection survey was performed at a large flood-control facility in northern Indiana, U.S.A., in order to identify a possible geologic cause for irregular settlement along the crest of a compacted earth-filled dam. The objectives included acquisition of a high-resolution bedrock surface image beneath the structure for remedial design considerations, as well as a search for signal anomalies that may be indicative of unfavorable geologic foundation conditions. The resultant SH-wave reflection profile collected coincident with die dam’s centerline successfully imaged a coherent, relatively continuous signal consistent with known points of top-of-rock elevation. Depth to bedrock along the profile varied from [Formula: see text] near the right abutment, to [Formula: see text] near the outlet works at the survey’s western terminus. A distinct, isolated amplitude anomaly in the bedrock reflector directly below the crest settlement area was imaged and interpreted as an enlarged (open) joint or collapsed karst feature. Subsequent geotechnical drilling of the anomaly found a [Formula: see text] discrepancy in the expected top-of-rock elevation, as well as soft foundation fill, thus confirming the geophysical interpretation.