The Karst Pages

INTRODUCTION TO KARST ENVIRONMENTAL PROBLEMS Sinkhole Collapse

While some sinkholes form slowly by solution of the underlying carbonate rock, other sinkholes develop as a result of the collapse of surface or near-surface material. There are two basic types of sinkhole-forming collapses: 1) bedrock collapses, and 2) regolith collapses. Bedrock collapses are rare and generally occur due to enlargement of cave passages in limestone. The enlargement causes the roofs above the passages to weaken and eventually collapse to create sinkholes. Regolith collapses are much more common than bedrock collapses and generally result from regolith falling into openings in the underlying limestone.  In areas where the water table is usually above the regolith-bedrock contact, collapses often occur when the water table drops below the regolith-bedrock contact, either during droughts or during high-volume pumping (Figure 10). Physically, the collapses in this case are caused by loss of buoyant support for the regolith arches that span openings in the limestone. Collapses are also caused by spalling of saturated regolith down the opening, enlarging the arch, and eventually causing collapse at the land surface. When the water table fluctuates above and below the regolith-bedrock contact, collapse may result from repeated wetting (swelling) and drying (shrinking) of material supporting regolith arches.  Regolith collapses also may occur in situations where the water table is usually below the regolith-bedrock contact (Figure 11). Construction and land use changes that concentrate surface runoff in drains and impoundments may locally increase the downward movement of water resulting in the piping of saturated regolith into openings in the limestone. Loading of the surface by structures, fill, or ponded water, or vibrating the surface by blasting may also occasionally cause the collapse of regolith arches (Crawford and Whallon, 1985). Plate 2 shows the areas of Kentucky most vulnerable to sinkhole flooding and sinkhole collapse problems (Crawford and Webster, 1986b) .