A geotechnical engineering company working on a land development project in central Pennsylvania asked GTS to evaluate the subsurface beneath three proposed infiltration basins to determine bedrock depth and find sinkhole activity that could jeopardize the integrity of the basin. The site was a former industrial site that was being redeveloped for non-industrial uses, which required onsite control of stormwater. The engineering company was concerned that sinkhole (karst) activity that the basins could fail if sinkholes developed within their footprint.

GTS used an AGI SuperSting resistivity array to provide detailed subsurface information along three resistivity profiles through the centerlines of the basins. The SuperSting instrument used a 56-electrode array with 1.5-meter electrode separations to sense at least 45 feet into the subsurface. Boreholes and test pits were planned to confirm and refine the geophysical data. The results from one profile are shown on the figure below.

Please click the image to enlarge

As can be seen, bedrock is relatively shallow, which suggests that excavation for the basin will require expensive blasting and chipping. The resistivity data also found at about 100 feet along the profile a zone of deep weathering with more resistive bedrock pinching out and hints of a large probably saturated soil-filled void at least 50 feet below the surface. The pinch out zone probably represents a narrow weathered joint or fracture between the two bedrock masses.

This feature beneath the edge of the bedrock footprint was analyzed as having a strong future sinkhole development potential. Another smaller feature was noted at 185 feet and again at 245 and 300 feet, which is outside the basin footprint. Three confirmatory borings and two test pits were located to investigate the karst features and confirm the shallow bedrock. Soft raveled soil was found at depth in borings, B-1 and B-2, confirming the karst potential. Boring B-3 and test pit TP-1 confirmed the otherwise shallow bedrock. Test pit TP-2 found deeper weathering adjacent to a bedrock ledge, but otherwise did not find raveled soil. Correspondence of the interpreted bedrock horizon from the geophysics and the direct borings and test pits was nearly perfect.

This example demonstrates the value of using shallow geophysics to rapidly gather high-resolution subsurface information in much greater detail than what can be obtained by a few borings and test pits. The geophysics easily found a few high sinkhole potential anomalies that probably would have not been detected by randomly placing borings or test pits as is often done in the industry. The electrical resistivity method found at the first anomaly a significant developing karst feature that almost certainly would have been exacerbated by standing and infiltrating water in the overlying basin probably causing failure of the basin through sinkhole development. The large underlying void may be a significant problem for creating large future sinkholes. As a result of the geophysical investigation, the location, shape and design of the basins is under review.

It is not often easy to convince non-geophysical professionals of the value of this kind of investigation prior to construction, but for a few thousands of dollars, tens or evens hundreds of thousands of dollars can be saved when the basin (or structure) fails. GTS has extensive experience in documenting the cost differential between preventing a problem and fixing a sinkhole problem. An ounce of prevention is sometimes literally worth a pound (or several pounds) of cure!

Electrical Resistivity
Terrain Conductivity
Very Low Frequency
Spontaneous Potential
Seismic Refraction
Ground Penetrating Radar
Vibration Monitoring
Soil Resistance Testing

Bedrock Delineation
Ground Penetrating Radar
Groundwater Availablity
Mining 1
Mining 2
Sinkhole Investigations
Sinkhole Investigations 2
Soil Resistance Testing
Underground Storage Tanks
Vibration Monitoring

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