Wastewater Pump House Sinkhole Investigation, Malvern, Pennsylvania

Overview: A large ground collapse and a disappearing nearby stream threatened an adjacent wastewater pump house in Malvern, Pennsylvania. Geotechnical engineers immediately responded by assessing risk and mobilizing equipment for investigation and repair. GeoStructures reviewed available published geological and historical topographic maps, as well as geology and sinkhole information of the site and its surroundings. To study subsurface hydrogeologic conditions, bedrock conditions, and geologic structures, site investigative field exploration included GPR, EM, microgravity surveys, test borings, and rock coring.

The geophysical survey covered all accessible locations and revealed void-like anomalies and a large, linear, low gravity anomaly. The test borings revealed variable depth to rock, ranging from about 14 ft to over 52 ft below existing grades. Such variation is typical in carbonate geology. A deeper slot was encountered locally, and possibly extended into the collapsed area. Highly-fractured rock raveled zones were also encountered by the test borings. The sinkhole activity at the site was considered severe as evidenced by (1) occurrences of several sinkholes in such a small area and (2) the disappearance of streams into the collapsed zone. The sinkhole also appears to have expanded towards the pump house. Considering the size and severity of the sinkhole, and the presence of a deeper slot, compaction grouting was the optimal solution to remediate the ground and protect existing structures.