Downhole Overcoring Stress Measurement at a Western Underground Coal Mine

Knowledge of the magnitude and direction of the horizontal secondary principal stresses is a critical factor in designing the layout and mining sequence of underground openings. Typically, horizontal stress measurement has been accomplished using overcoring or hydraulic fracturing. Hydraulic fracturing can be accomplished from surface boreholes prior to development, but the technique relies on numerous assumptions regarding the rock mass and the stress field, and interpretation of results is somewhat subjective. Traditional overcoring provides a more precise measurement, but requires underground access. To allow for precise measurement of the horizontal stress field from surface boreholes, the In-situ Stress Tool (1ST) was developed by SIGRA Pty of Brisbane, Australia. Used in conjunction with HQ wireline coring, the 1ST contains a battery-powered data logger that records tool orientation and monitors six diametral deformations as the tool is overcored. The IST has been used successfully for several years in Australia, but has only recently been introduced in the United States This paper discusses how the IST was used to measure the pre¬mining horizontal stresses at a western U.S. coal mine. Seven successful measurements were obtained in two holes at depths ranging from 250 to 800 ft. Results were consistent, showing moderately biaxial stresses greater than those expected from gravity loading, but relatively low compared to rock strength. In addition to describing the overcoring equipment and procedures, various problems encountered during testing and the accuracy of the results will be discussed.