Early Detection of Rock Movement with Time Domain Reflectometry

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Abstract
Remote operation of a time domain reflectometry (TDR) system demonstrates the effectiveness of telemetric monitoring for early detection of rock movement. During field testing reported here, TDR data were related to shear deformation above a longwall caving operation using a correlation between the signature reflection coefficient and shear deformation established through laboratory measurements. A 175‐m (575 ft) long vertical coaxial TDR cable was grouted into the rock mass above the longwall coal‐mining operation and attached to the aboveground‐pulsing instrument. TDR data were then remotely acquired twice daily by an operator some 500 km away. Comparison of TDR signature changes with longwall‐face location showed that localized changes were detected four days before surface subsidence occurred when mining was still 64 m (210 ft) away. Since the mining was moving toward the cable at a rate of 10 m/day, a warning interval is expected to be larger for premined sites with lower deformation rates.

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