EARLY DETECTION OF ROCK MOVEMENT WITH TIME-DOMAIN REFLECTOMETRY

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.