Classification of coal mine subsidence in Iowa
1987, Association of Engineering Geologists, Symposium Series,
Number 4, pp. 83-94
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.