The investigation of the engineering behavior of subcoal strata (underclay) was undertaken as part of an investigation made into the problem of mine closure as a result of mine floor upheaval. The University of Missouri-Rolla, under a contract with the U.S. Bureau of Mines, is conducting a study into the floor stability of underground coal mines in the Illinois basin. This study includes the investigation of the strength of subcoal strata, the stratigraphy of the area, the floor failure modes and numerical modeling of the behavior using the finite element technique. The floor stability problem can affect the mine operation in many ways. The upheaved floor material can constitute a hazard to transportation along haulage roads requiring floor grading or removal of floor material. The floor failure can slowly reduce ventilation, trap mine equipment and close mine rooms completely. The most serious problem associated with floor heave is the weakening of the coal mine roof caused by differential movement of coal pillars as deformation occurs. These roof problems can cause large roof falls that endanger personnel and equipment. The initial investigations into the floor stability revealed that the problem seemed to be one of large bearing capacity failure similar to that of a footing foundation.
The purpose of this research was to determine in situ strength and elastic-properties of coal underclay for use in analysis of stress strain conditions occurring in room and pillar mining operations. The in situ strengths and deformation properties were to be compared to results from triaxial tests performed on specimen recovered from core drilling operations. The plate bearing tests were not intended as a model of the coal pillars, but as a test method for determining in situ material properties.
