Bridge/Road Widening and 3D ERI Across a Creek

Late in the summer of 2003 Subsurface Evaluations was consulted to evaluate the possibility of conducting an electrical resistivity imaging (ERI) survey as part of a proposed widening of an existing bridge.  The existing bridge spanned a drainage canal and consisted of two lanes with minimal shoulder area.  The client was concerned about the geologic conditions located beneath the canal where the support pilings were to be driven for the new bridge construction.  The site in Hillsborough County, Florida, is located within an area characterized by a karst terrain, where sinkholes are known to develop.

Despite the obvious complications associated with conducting resistivity surveys in water as much as 10 feet in depth, SEI was able to design a survey and develop specific field equipment that produced results that correlated well with previously conducted soil test borings.  Analysis of these data revealed what appeared to be a cone-shaped body of low resistivity (clayey) material that penetrated into the surrounding high resistivity (sandy) soils.  These conditions were interpreted to be a buried depression within the limestone bedrock that had been infilled with silt and organic muck.  Subsequent geotechnical soil boring drilled into the features mapped by SEI appeared to confirm this interpretation.