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, ransing 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.
The study area is located in Raleigh and Wyoming Counties in southern West Virginia near the Virginia Kentucky border, Figure 1. The principal units of economic interest in this area are the Sewell and Beckley coalbeds; however, only the Beckley coalbed is of concern to this study. Stratigraphically, the Beckley coalbed is located within the New River Formation of the Pottsville Group. The New River Formation is characterized by orthoquartzites and shales, is irregular in thickness with a maximum thickness of 1,030 feet, and generally thins to the north. Fold structures consist of gentle anticlines and synclines with their axes striking approximately N 50° E and dipping to the northwest at 1° to 2°. Few faults of significant displacement are found in the study area, although an extensive thrust fault system is located to the south in northern Virginia. The rock sequences immediately above and below the Beckley seam vary in thickness due to their lenticular nature and are absent entirely in some cases.
