ABSTRACT

ABSTRACT

Gob pressure measurements were made in a Western U.S. coal mine as part of a long-term program to evaluate cave progress and to determine the influence of geological discontinuities on caving conditions, load transfer, and resulting instabilities. Gloetzl cells were selected for the measurements due to their simple, robust construction and a history of being able to monitor pressures in a broken medium. A methodology was developed for the successful installation and protection of the cells and hydraulic lines in the gob as the face retreated. The measurements indicated a cave progress controlled by the frequency of major faults. The pressure-mining progress profile was compared to those observed in other parts of the world. It was concluded that the significant differences in profiles was caused by the thick-bedded strata, the existence of high lateral stresses, and the spacing between faults. Recommendations are given for future applications of Gloetzl cells for gob pressure measurements.

INTRODUCTION

Background

When the overburden is undercut through removal of the coal seam, a redistribution of stresses takes place, transferring the weight of the undermined rocks to the panel boundaries. As the undercut dimensions increase, bed separation and roof falls develop. The falls (or the cave) progress up into the roof until the mined space is filled with broken rock. At this time, compaction of the caved rocks occurs, permitting a partial transfer of overburden load to the floor. The amount of load transferred through the gob and the distance beyond the face where full recompaction of caved rocks occurs are of great importance for determination of the stress redistribution to panel boundaries. The load transfer through the gob is a function of large scale geology (i.e., existence of major faults, lateral stress fields, and roof rock formations), as well as small scale structures (i.e., fractures) which affect the size distribution and stiffness of the caved rocks. Thus, the nature and degree of load transfer could vary from site to site. Evaluation of cave progress by in-panel measurements (in the gob, under the panel, or over the gob) is believed to be essential for evaluation of caving conditions and load transfer mechanisms. Such measurements, however, have been very limited due to the difficulties and costs of obtaining in-panel measurements on a large scale. Alternatively, researchers have preferred to perform measurements at panel boundaries, and then speculate as to what is happening in the gob. Difficulties with the latter approach are that the measurements should be obtained on a very large scale since the caving methods influence the strata much beyond the panel boundaries. Problems exist with the reliability of most instruments and the meaning of measurements because of the nonlinear behavior of coal at the face.

Review of In-Panel Measurements

In-panel measurements have been performed for evaluation of cave progress. Generally, they can be grouped into measurements obtained above the gob in the roof, in the gob, and under the panel. A brief description of these measurements follows. Measurements above the gob generally have been made in boreholes drilled from the surface or other openings located above the panel.

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