HYPOTHESIZED MECHANISM FOR THE INITIATION OF SOIL CAVITIES AND SUBSEQUENT COVER-COLLAPSE IN KARST TERRAIN

摘要

Cover collapse is the unpredictable collapse of unconsolidated earth material over soluble bedrock. In Kentucky, cover collapse costs an estimated $20 to $60 million annually. The Kentucky Geological Survey began keeping a catalog of case histories in 1997 and now receives about 24 reports a year. A total 354 cover-collapse sites were evaluated for this paper, which reports on efforts to discover a relationship between what can be seen (or other otherwise measured) of cover-collapse at the surface and what is happening in the subsurface. The evaluation of measurements made of the cover collapses found 45 % are underlain by Ordovician age carbonates, 41% are on Mississippian rocks, and 13% are underlain by Silurian-Devonian stratigraphy. Diameter, elongation (asymmetry ratio), and Riley Sphericity (circularity) were calculated from the length and width of the collapse opening. Distribution of the data is log normal. The diameter of the collapses average 2.4 m. Student's T-tests resulted in a significant difference in depth between collapses underlain by Mississippian versus Silurian-Devonian carbonates. Collapses on Mississippian rocks have large differences in the variance for asymmetry, circularity, and diameter when compared to the variance for Ordovician and Silurian-Devonian cover-collapses. Only 7% of the cover-collapse sites occur within larger, pre-existing sinkholes. During evaluation of these correlations, it was observed that the cumulative count of cover-collapse per calendar month follows an annual cycle. Collapses are at a minimum in February, but increase to a maximum in July. The count begins to decrease in August and continues to decline through December into January. It is suggested that the air temperature is an important component of the cover-collapse process, possibly accelerating drying of the soil cover, resulting in subsequent collapse.