In January 2015, Pyramid Geophysical Services conducted a geophysical investigation at an open sinkhole (ground collapse) in a dry retention pond for a client at their remediation site in Tennessee. The survey was conducted to determine the lateral and vertical extent of the sinkhole feature in the subsurface, and to evaluate the average depth to bedrock in the surrounding area within the pond. The investigation consisted of Multi-Electrode Electrical Resistivity (MER) testing at nine locations across the property, shown in the figure below. Pyramid owns the SuperSting R8 earth resistivity meter manufactured by Advanced Geosciences, Inc.
The resistivity data provided a reliable representation of the geologic conditions within the upper 50 feet of the subsurface at the property. Geologic conditions generally were characterized by a stratum of dry surface clayey soils underlain by saturated clayey soils that were further underlain by limestone bedrock. The survey indicated an average depth to bedrock of approximately 27 feet below the pond bottom. In five of the resistivity profiles surrounding the area of ground collapse, a clear break in the zone of saturated clayey soils was observed. The break is interpreted to represent sandy in-filled soils with increased porosity, resulting in the downward movement of soil and water, ultimately causing the ground collapse. The figure below is a 2D resistivity profile, showing the general conditions encountered around the sinkhole.
A second area of concern was identified to the northwest of the visible ground collapse. The nature of the soils in this area suggests they may also potentially result in a ground collapse in the future. 3D imaging of the two areas of concern provides further evidence of in-filled sandy soils that are “raveling” downward into the underlying rock unit and causing unstable conditions. Raveling is the process by which water transports soil particles downward into cavities in the underlying strata. The figure below is a 3D resistivity rendering, which shows the zones of in-filled, collapsing soils associated with the sinkhole and the second area of concern.
Surveys such as these are invaluable planning tools that help identify and delineate subsurface hazards. Based upon the results of the geophysical data, Pyramid’s client is now able to make informed decisions about the current and future use of the property surrounding their remediation site.