Using time-domain reflectometry for real-time monitoring of subsidence over an inactive mine in Virginia

Closure activities at the United States Gypsum Company facility in Plasterco, Virginia, include realignment of an existing highway outside the predicted limits of long-term subsidence. Concerns about the possibility of subsidence along the existing highway being induced by construction activities, or in the former plant area where excavated rock was being placed, motivated the installation of a real-time monitoring system based on time-domain reflectometry (TDR). This technology involves interrogating solid metallic coaxial cables to determine locations and magnitudes of cable deformation. When the cables are grouted into drill holes and trenches, TDR can be used to determine the location and rate of precursor subsurface movement that is causing cable deformation. Manual monitoring of three cables began in June 2001. Then, during 2002, system capabilities were expanded to automated real-time monitoring of 21 cables with lengths that varied from 10 m (30 ft) to 270 m (886 ft) for a total greater than 2,500 m (greater than 8,200 ft) of cable. The cables were installed in angled holes beneath the existing highway, in trenches along the highway, and in trenches over the former plant area where rock excavated for the new alignment was being placed. The automated monitoring system was controlled by a programmable data logger and incorporated a callback capability. Whenever the difference between the baseline measurement and the current measurement at any location along a cable exceeded a preset alarm threshold, the data logger would initiate a call to responsible personnel.