Distributed strain monitoring of overburden delamination propagation at a deep longwall mine

Abstract

Clear understanding of overburden failure propagation during underground mining is critical in managing safety and environmental issues such as rock burst hazards, groundwater inrush and longwall convergency. This paper presents an in-situ monitoring study of overburden strain dynamics at a kilometre-deep longwall mine using distributed fibre optic sensing technology (DFOS). A deep borehole was drilled from the ground surface to near the mining seam and installed with an optical fibre cable to detect rock deformation in a contiguous fashion. In addition, a Global Navigation Satellite System (GNSS) device was installed at the borehole collar to monitor surface subsidence. The results suggested that strata delamination contiguously extended to 347 m, about 34 times the mining thickness. The remaining 600 m thick strata above this height behaved as the intact zone. Such an overburden deformation pattern resulted in minor surface subsidence of only 0.6 m or 6% of the mining thickness. A step-wise, upward breakage of the optical fibre cable (due to excessive strain) was observed and found to correlate with other ground behaviours, including an increase in the total microseismic energy and periodic weighting on the longwall supports. These observations corroborate that the dynamic deformation of overburden is primarily governed by competent stratum units, i.e., the key strata. The monitoring results have served to develop a reliable mine-scale 3DEC model for predicting stress redistributions in the subsequent longwall panels for preventing dynamic hazards. The study also demonstrates that DFOS-based strain monitoring can be a valuable tool to investigate overburden responses to mining and has significant potential for applications in various mining methods, such as block caving, sub-level caving and open stopping.