Delineation of sinkhole boundary using Dutch cone soundings

The foundation design for expanding a cogeneration power plant in central Florida, renowned for its sinkhole-prone geology, provided a challenge for geotechnical engineers. Because of limited available land, the expanded power plant was built on an adjacent 5-acre tract, where a 50-ft-diameter by 5-ft-deep depression had been identified. Dutch cone soundings, a relatively underutilized method, were used to characterize the site and reduce uncertainty. For this application, Dutch cone soundings provided higher quality data at a lower cost than the more common standard penetration test (SPT) method. estigate the subsurface conditions of the 5-acre tract, ground penetrating radar (GPR) and four exploratory SPT borings were conducted. The SPT borings confirmed that the depression was caused by an ancient sinkhole. Because of a relatively high groundwater table and a layer of very dense organic hardpan near the surface, the penetration depth of GPR was limited to 5–10 ft below ground surface and thus could not define the sinkholeʹs boundary. ineate the sinkholeʹs boundary, 38 Dutch cone soundings were made to 20–160 ft deep, conducted in three phases around the depression. The Phase 1 soundings were spaced at 100- to 200-ft intervals around the depression to estimate the influence zone. The results of Phase 1 soundings were used to place Phase 2 and 3 soundings at ca 50- and 20-ft intervals, respectively, to define the boundary of the ancient sinkhole. the results of the Dutch cone soundings, the area around the sinkhole was delineated into the following zones: sinkhole-collapsing zone where Strata 2 and 3 were not encountered; zone under sinkhole influence where Stratum 1 was not encountered; zone beyond sinkhole influence where Strata 1, 2 and 3 were encountered. e 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 foundation settlement problems since that time.