One of the most difficult and ever present problems of underground coal mining is that of strata control. Strata control is necessary, not only for the safety of the miners and to retain access to the mining face, but also to facilitate transportation of the mined coal, to prevent entrapment of valuable equipment, to maintain remedial roadway repair work at minimum levels, to fully ventilate all working areas of the mine, and to prevent the loss of access to future minable coal reserves.
Locally, within a mine, three components the coal mine opening must be stabilized before control of the strata can be achieved. These components are: the roof, the coal pillars, and the floor. An alteration of even one of these components affects the stability of the other two. It is the mine floor, though, which actually supports the roof structure through the intermediary action of the pillars.
In the past, much research has been spent investigating roof and pillar stability problems. Quantative research dealing with floor problems is virtually non-existent and Only recently have investigators shown much interest in this element of mine stability The characteristics of the floor Strata and how they react to external factors (such as increases in water content and applied stress)usually are not include in the mine design process.
Floor heave has long been a nagging if not critical problem-- not only in coal mines, but also in uranium mines, tunnels and other large underground openings. In a coal mine using a typical room and pillar mining scheme, floor heave is the process which occurs when the stress applied to the floor material by a coal pillar exceeds the bearing capacity of the floor strata. When the floor strata beneath the coal pillar fail, the pillar moves downward and displaces the material under it. This material, in turn, moves outward and upward into the mine opening.
As the sub-coal material fails, the pillar moves downward and the load it supports is transferred to adjacent pillars. If these are already marginally stable, they then may overload their supporting material until it also fails. Thus, floor heave can spread through a large portion of the mine in a progressive manner. As the pillars push into the floor, flexure of the roof strata may cause it to weaken and fail. As a result subsidence of the ground surface above the mine may occur. Cribbing or supplementary support does not slow floor heave once it begins.
In 1975 the University of Missouri-Rolla was awarded a research grant by the U.S. Bureau of Mines to conduct a three year study of the phenomenon of floor failure in coal mines. The objective of this study was to provide mine operators/engineers with information for predicting areas of a mine where possibilities for excessive floor heave exist. These objectives were achieved by first conducting a broadly based survey to determine the engineering properties and controlling parameters of the