In the Analysis of Retreat Mining Pillar Stability (ARMPS) program, the magnitude of the abutment loading adjacent to a gob area is calculated using an “abutment angle” concept, and the extent of the abutment loading is determined as solely a function of depth from an empirically derived equation. In the recommended calibration method for LaModel [19], it was believed that the empirical equations for calculating the magnitude and extent of the abutment load as used in ARMPS where the best available methods for determining these critical overburden loading values; and therefore, similar calculations were implemented in the LaModel calibration method. However, the latest insitu stress measurements of abutment loading performed in Australia and United States showed that there can be significant deviations in the measured abutment magnitude and extent as compared to the predicted values from the empirical formulas used in ARMPS and LaModel. It seems reasonable that the overburden geology, depth, extraction panel width, and mining height should have a significant effect on the extent and magnitude of the abutment load. However, the current empirical formulas do not incorporate all these significant parameters into determining the abutment load on the pillars. In this paper, stress measurements from six Australian mines and six U.S. mines were back analyzed by using analytical and numerical methods to investigate the effect of the overburden depth and panel width on the abutment loading. The ultimate goal of this research is to improve our ability to design pillar recovery operations by improving abutment loading calculations.
In the 1990s, pillar recovery operations had been associated with more than 25% of all ground fall fatalities [1]. However, more recently, this statistic was improving due to a concentrated effort by the Mine Safety and Health Administration (MSHA) and the National Institute for Occupational Safety and Health (NIOSH) on retreat mine safety, until the Crandall Canyon Mine disaster. One of the important steps promoted by NIOSH and MSHA in this effort was to ensure the global stability of the retreat panel by guiding operators to design panel pillars properly. ARMPS and the LaModel programs have been used successfully in the U.S. for ensuring global stability and designing safe pillar recovery operations for many years. However, the Crandall Canyon incident was a global instability due to improper pillar design, and this mine collapse highlighted the need for better pillar design methods for coal mines under deep cover.
In general, pillar design consists of three steps; prediction of the pillar loads, prediction of the pillar strength, and calculation of the stability factor. Pillar loads themselves can be divided into two categories; development loads and abutment loads.
For predicting development loads, the “tributary area theory” has been used successfully by many researchers for both room and pillar and longwall mines [2, 3, 4, 5, 6, 7]. Previous versions of ARMPS used the tributary area theory to predict development loads. After the Crandall Canyon mine disaster, NIOSH revisited the issue of pillar design for deep cover retreat mining.
