Floor heave or deformation of the mine floor into the mine opening is a problem which has plagued coal mines in this country and others. This paper describes the problem as it was manifested in two coal mines located in eastern Illinois, and reports the results of a U.S. Bureau of Mines-sponsored study conducted by the University of Missouri-Rolla. The ultimate objective of the study was to develop a procedure that could be used to define a potential floor heave problem during the exploration phase of mine design--before initiation of production mining.

Floor heave within the mines did not occur with uniform intensity throughout each mine or even within mine panels. Floor deformation was often deep-seated and involved two subfloor lithologies. A wide variation in measured strength for each of the subcoal lithologies was recorded during laboratory testing; underclay triaxial compressive strength best correlated with underclay natural water content. The severely heaved areas were not located in the deepest or shallowest portions of the mines. Severe floor heave occurred in areas of thicker (greater than 6 ft, 2 m) underclay. Severe floor heave occurred at those sites where the natural water contents of the underclay and claystone were highest. The presence of swelling montmorillonite clay did not seem to be a major cause of floor heave Triaxial compressive strengths measured from underclay samples from severely heaved sites were not the lowest values measured. A bearing capacity model developed by Vesic was modified so that a "heave factor" could be calculated using only that information obtainable from exploration core borings; use of the "heave factor" would have predicted floor heave at the study sites where severe heave occurred.

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