FORMATION OF REGOLITH-COLLAPSE SINKHOLES IN SOUTHERN ILLINOIS - INTERPRETATION AND IDENTIFICATION OF ASSOCIATED BURIED CAVITIES

Three regolith-collapse sinkholes formed near the Dongola Unit School and the Pentecostal Church in the southern Illinois village of Dongola (Union County) during the spring of 1993. The sinkholes appeared over a three-month period that coincided with development of a new municip al well. The new well was drilled through clay-rich, valley-fill sedim ent into karstified limestone bedrock. The piezometric surface of the limestone aquifer is above land surface, indicating the presence of an upward hydraulic gradient in the valley and that the valley fill is a cting as a confining unit. Pumping during development of the well lowe red the piezometric surface of the limestone aquifer to an elevation b elow the base of the valley fill. It is hypothesized that drainage of water from the sediments, the resulting loss of hydrostatic pressure a nd buoyant force in overlying sediments, increased intergranular press ure, and the initiation of groundwater flow toward the well resulted i n rapid sediment transport, subsurface erosion, and collapse of the va lley-fill sediment. The sinkholes follow an approximately east west al ignment, which is consistent with one of the two dominant alignments o f passages of nearby joint-controlled caves. A constant electrode-sepa ration resistivity survey of the school playground was conducted to lo cate areas that might contain incipient sinkholes. The survey revealed a positive resistivity anomaly trending N75E in the southern part of the study area. The anomaly is linear, between 5 and 10 m wide. and it s trend either intersects or is immediately adjacent to the three sink holes. The anomaly is interpreted to be a series of pumping-induced ca vities in the valley-fill sediments that formed over a preexisting cre vice in the karstified bedrock limestone.