Classification of coal mine subsidence in Iowa
Paul VanDorpe
1987, Association of Engineering Geologists, Symposium Series,
Number 4, pp. 83-94
ABSTRACT
In Iowa, subsidence above abandoned coal mines is well documented, but not well studied in terms of geologic control. Subsidence poses a significant threat to several communities, most notably, Des Moines, as well as rural areas. Subsurface geology and mining information are usually inadequate to document either causal relationships or physical conditions. Nevertheless, several subsidence incidents can be classified on the basis of surface morphology and geologic control. "Classic" bell-shaped sinkhole development appears to be dictated by geological conditions only grossly similar to those described in Illinois loess terrain. Bell-shaped sinkholes are prone to develop in areas with near-surface granular materials, such as loess, sand, or silt, which have the capability of "flowing," especially when partly saturated. Other sinkholes are noticeably smaller and, in some cases, may have gone unrecognized as subsidence features. Multiple collapse types, such as pits within troughs as described in Wyoming, have not been recognized. However, different types of collapse in the subsurface can occur. Even when geologic conditions are virtually identical, different surface features may occur. Variations of sinkhole development, termed whole-room collapse, have been recognized, although their surface expressions are rather subtle. Likely, there are gradations between single sinkholes, multiple coalescing sinkholes, whole-room collapse, and trough subsidence. In suspected cases of trough subsidence, zones of compression and tension have not been delineated, as documented in examples from Illinois, Pennsylvania and Wyoming; rather, differential surface movement may be the dominant characteristic.