Delineation of sinkhole boundary using Dutch cone soundings

摘要

The foundation design for expanding acogeneration power plant in central Florida, renowned for itssinkhole-prone geology, provided a challenge for geotechnicalengineers. Because of limited available land, the expanded powerplant was built on an adjacent 5-acre tract, where a 50-ft-diameter by 5-ft-deep depression had been identified. Dutch conesoundings, a relatively underutilized method, were used tocharacterize the site and reduce uncertainty. For this application,Dutch cone soundings provided higher quality data at a lowercost than the more common standard penetration test (SPT) method.To investigate the subsurface conditions of the 5-acre tract,ground penetrating radar (GPR) and four exploratory SPTborings were conducted. The SPT borings confirmed that thedepression was caused by an ancient sinkhole. Because of arelatively high groundwater table and a layer of very denseorganic hardpan near the surface, the penetration depth of GPRwas limited to 5-10 ft below ground surface and thus could notdefine the sinkhole's boundary.Th delineate the sinkhole's boundary, 38 Dutch conesoundings were made to 20-160 ft deep, conducted in threephases around the depression. The phase 1 soundings were spacedat 100-to 200-ft intervals around the depression to estimate theinfluence zone. The results of Phase 1 soundings were used tophase 2 and 3 soundings at ca 50-and 20-ft intervals, respectively,to define the boundary of the ancient sinkhole.Using the results of the Dutch cone soundings, the area aroundthe sinkhole was delineated into the following zones: sinkhole-collapsing zone where Strata 2 and 3 were not encountered; zoneunder sinkhole influence where Stratum 1 was not encountered;zone beyond sinkhole influence where Strata 1,2 and 3 were encountered.Because the sinkhole-influence zone was ca 200 ft in diameter,heavy facilities were constructed at least 50 ft from the zone.Completed in 1993, the expansion has not had foundationsettlement problems since that time.