COMPARISON OF THREE APPROACHES TO CALCULATING DIAMETER OF A DROPOUT SINKHOLE

The possibility of sinkhole size prediction in the areas, where soluble rocks are covered by impermeable clay layer is considered. Such sinkholes are named by English-speaking authors as “dropout sinkholes” or “cover collapse sinkholes”, because the process of their formation includes collapse (dropping) of clay covering soluble rocks into a karst cavity. Triggers of this effect are: (1) widening of a cavity below the clay due to dissolution of a soluble rock by groundwater; (2) lowering of hydraulic head in confined karst aquifer, primarily caused by groundwater pumping, which results in decreased support of the overlying clay layer; (3) a dynamic load on the clays, which can have either natural or man-made origin. Fresh dropout sinkholes generally have configuration similar to vertical right circular cylinder, but sometimes their shape can be dome-like with very unstable ground “cantilevers”. The appearance of these forms under foundations can lead to deformation and even to destruction of buildings, structures, and infrastructure facilities. Two concepts exist, which can be used as basis for creating models of soil mechanics to calculate diameter of a dropout sinkhole. The first one is based on a viewpoint that clay collapse into a cavity after a trigger action is displayed immediately at the ground surface as a collapse sink. According to the second concept, the preliminary subsurface collapse of clay occurs as fall of ground block resembling a circular paraboloid. As a result, a dome-like cavity appears in the clay layer and above-lying clay fall into it to form a collapse sink. There are no prediction methods based on the two above-mentioned conceptions. The developed approaches to prediction of a dropout sinkhole diameter are adequately comparable, as they use the same input calculation parameters. The prediction results based on these approaches have been compared to an actual diameter of fresh dropout sinkhole. This verification have allowed us to infer that most adequate is the approach based on concept of initial subsurface clay’s collapse preceding the final collapse sink formation.