ABSTRACT
Serious floor heave of up to 2.4 m in a 2.4-m high mine entry was eliminated by applying the stress control method of mining, as a last resort, at the No. 5 coal mine of Jim Walter Resources, Inc., in the Black Warrior coal basin near Birmingham, Alabama. Underground observation of the first 3-room entry created using the stress control method is discussed here. The behavior of the test entry, which eliminated the heave problem, is analyzed in relation to similar studies conducted in a salt mine under similar ground conditions by utilizing finite element analysis.
INTRODUCTION
SIGNIFICANCE OF TRIAL APPLICATION OF STRESS CONTROL METHOD
Failures of roof and floor present serious rock mechanics problems in many underground coal mines. These problems have been dealt with mainly by using artificial supports, such as roof bolts, wooden cribs, and concrete linings. These conventional means of ground control become increasingly difficult to use as the ground instability intensifies. Such instability encountered at the No. 5 mine of Jim Walter Resources, Inc., operating in the Black Warrior coal basin in Alabama, was so intense that none of the conventional means of ground control seemed feasible. The stress control method of mining was applied as a last resort. This method in fact proved to be successful in eliminating the intense ground failure problems without requiring any artificial means of ground control. The results of this trial demonstrate that the stress control method is not only a powerful and scientific method but also that it is economical and practical for solving ground failure problems in underground mines. The scientific significance of this development lies in the field verification of the rock mechanics principles of complex ground upon which the stress control method is based, demonstrating that the method is applicable to most underground mining, regardless of the type of ore body to be mined.
HISTORICAL BACKGROUND AND THEORETICAL ISSUES
The stress control method has a long history. The origin of the method may be traced back to coal mining in northern England in the early 1950s, when Charles Holland first introduced the concept of the stress arch and yield pillar (1). But his method did not receive wide acceptance at that time, despite his penetrating vision. In the meantime, the yield pillar was successfully adapted in underground trona mining in Greenville, Wyoming, through the creative work of Bill Fischer of FMC (2). Fischer utilized the yield pillar to sustain temporary stability of failing roof strata in his total extraction retreat mining. The use of yield pillars to provide long-term stability of an advancing new entry system was not developed until the late 1960s in potash, where the stress control method originated. Many potash mines at that time were suffering from extensive floor heave and roof failure problems. The conventional use of yield pillars and stress arches was found to be inadequate to solve these intensive ground failure problems. It was at this time that the stress control method was first introduced, successfully stabilizing the ground and eliminating the saturation roof bolting requirements which had been believed to be essential.
