Dam Seepage/ Subsidence Geophysical Survey

An old earthen dam displayed signs of seepage. One state agency wanted to breach and rebuild it immediately, and another agency wanted to repair it. RETTEW was tasked with detecting areas of seepage, soil piping, or erosion of materials beneath the dam.

Site conditions were grassy embankments, a concrete spillway, and suspected sheet piles from former repairs (metallic cultural interference). RETTEW used microgravity to detect erosion (mass-deficient areas) and spontaneous potential (SP) to detect areas of seepage. The panels above show data coverage, SP contours, and microgravity contours. RETTEW detected coincident SP and microgravity anomalies on the south end of the dam (possible active seepage and erosion), and a gravity anomaly on the north end of the dam (possible undermining of spillway). We recommended exploratory borings and possible grouting in identified anomalies. Prior to drilling, a large storm threatened to top the dam. To relieve the water level, the dam was emergency-breached at the location of the southern anomaly. The damage exposed was sufficient to trigger plans for full removal and reconstruction of the dam.

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