21st U.S. Symposium on Rock Mechanics, The University of Missouri, Rolla, May 1980
Authors: J. F. T. Agapito, S. J. Mitchell and M. P. Hardy (AAI), J. R. Aggson (USBM), and W. N. Hoskins (Tosco Corp.)
The concept of vertical stress as being equal to the weight of the overburden is intuitive and relatively accurate in most cases. In contrast, the magnitude of the horizontal in situ stress field generally cannot be predicted without measurements. Brown and Hoek (1978) found that the ratio of the average of the two horizontal stresses to the vertical stress varies widely, from 0.5 to greater than 3.5 near the surface and from 0.3 to 1.0 at depths below 10,000 feet. Many underground mines experience ground control problems that are due in part to high in situ horizontal stresses. One group of mines with such problems operates in the Beckley coalbed. The Beckley coalbed underlies an area of approximately 600 square miles in portions of Fayette, Raleigh, and Wyoming Counties in south central West Virginia. More than 2 billion tons of mineable metallurgical-grade, low-volatile bituminous coal, ranging from 3 to 10 feet in thickness, make the Beckley coalbed an economically significant unit. However, ground control problems in the Beckley coalbed have been encountered throughout most of the underground mines in the area, resulting in a significant potential loss of exploitable resources. In recent work by the United States Bureau of Mines, it was determined that observed ground control problems were due in part to high horizontal stresses (Aggson 1978b). This conclusion was reached after conducting strain relief overcoring in the Beckley #1 coal mine of Ranger Fuels, Inc. As a result of these findings, the Bureau initiated a program to determine the horizontal stresses throughout the Beckley coalbed and to evaluate the effectiveness of changes in mining geometry in alleviating the stress-induced roof control problems. This study, conducted under Bureau of Mines contract J0285020, developed in two stages: (1) the determination of horizontal stresses in the roof by overcoring to verify the regional existence of high stresses, and (2) numerical analyses based on these stress determinations to investigate alternate mine geometries that might reduce ground control problems.