Use of Electrical Resistivity Surveying to Evaluate Collapse Potential Related to Road Construction over a Cave

Geophysical surveying using the electrical resistivity method was used to help determine the potential for collapse that could occur resulting from the construction of a road over a known cave in the City of Kirkwood (City), Missouri. The City is installing a new water main which will require construction of an access road through Koestering Park. Underlying the park is historic Watson Cave, which was mapped in 1961. The width and height of the cave varies and appears to be influenced by the presence of vertical fractures that have been widened by solutioning. The depth to the top of the cave is unknown. Exploration by non-intrusive electrical resistivity surveying was performed rather than drilling or test pit exploration because the City desired to limit damage to the cave and densely wooded areas within the park. Electrical resistivity data were collected using a dipole-dipole array along the proposed road alignment. The data exhibited a valuable signature of a high-resistivity anomaly due to the presence of this cave of known dimensions. Based on the survey results, the depth to the top of the cave was estimated to be approximately 30 feet. Also identified, however, were nine other high-resistivity anomalies (possible voids) along the proposed road alignment. The interpretation of these anomalies, in some cases, suggested possible depths to voids of less than seven feet. It was concluded that the limited traffic loads caused by low speed construction-type vehicles would not significantly increase the potential for cave collapse at Watson Cave due to the approximately 30-foot depth of the feature. However, the interpreted shallower features have a comparatively higher potential for collapse. Recommendations included using a Bailey bridge to span large features and using a geogrid-reinforced crushed rock mat to distribute/dissipate heavy wheel loads over narrower features.