Stress Redistribution in a Longwall Yield Pillar — a Comparison between Active Seismic Tomography and Theory

Abstract

Gate road yield pillars are subject to increases in stress as the longwall face advances through a panel. Safe, efficient underground mining is dependent upon proper pillar sizing. Theoretical models of pillar loading are well accepted, and geotechnical measurements, including convergence monitoring, have provided important information about stress levels and distributions within pillars. Numerical modeling has developed as a method which complements empirical and theoretical pillar behavior concepts, and allows prediction of the behavior of various pillar sizes under changing conditions. Seismic tomography has seen fairly limited application in the mining industry, but has potential to be a valuable method to further improve understanding of coal pillar performance under changing loading conditions. This research presents a case study from a longwall mine in the western USA. A two-entry gate road yield pillar in the headgate of the mine was instrumented with seismometers and convergence monitoring stations, and was monitored over a period of about 6 months as the longwall face approached the pillar. In this paper, the stress distribution imaged using seismic tomography is compared to the stress distribution expected from theoretical models, geotechnical measurements, and numerical models. The tomography results generally agree with those proposed by theory, convergence measurements, and numerical modeling results. The agreement of the methods provides validation for the theorized stress redistribution, and this study provides further evidence that tomography can indicate the redistribution of induced stress within a mined rockmass and is another tool available to ensure that mining is conducted safely and efficiently.