Conventional geotechnical investigation techniques like test pits and borings can sometimes impose additional hazards to job sites in situations involving ground stability void detection/delineation, uneven surfaces, difficult terrain or industrial infrastructure. In these cases geophysical methods can provide useful information while mitigating jobsite hazards.
In one such instance, THG was contracted for an emergency job in Northampton County, Pennsylvania, involving open sinkholes adjacent to a major electrical transmission tower. In order to assess and remediate the open holes, each approximately 30 feet in diameter and 25 feet deep, heavy equipment including drill rigs, dump trucks and cement trucks would need to access the site. Uncertainty about the stability of the ground near and surrounding the open sinkholes made conventional geotechnical analysis and remediation efforts unsafe.
Due to the low-impact nature of near-surface geophysical surveying, THG was able to image the subsurface and assess the stability of the surrounding area. THG deployed 2-D electrical resistivity (EI) imaging to image the subsurface for the presence of additional voids and other geologic features of concern. Eleven profiles were collected in various orientations around the site and imaged to a maximum depth of 80 feet below grade. Additionally, MASW was performed at several locations to help constrain the EI data and determine bedrock parameters. Both survey techniques required limited equipment and was performed on foot to limit stresses on the potentially unstable ground.
No additional voids were interpreted from the EI data. The data collected by THG was used to determine the safest truck route to access the sink holes so they could be stabilized and backfilled. Using the proposed access route determined through the geophysical data, the ground remained stable throughout the remediation efforts.