Principles of time domain reflectometometry applied to measurement of rock mass deformation

Time Domain Reflectometry (TDR) is an electrical pulse testing technique originally developed to locate faults in coaxial power transmission cables. Recently, this technique has been adapted for monitoring deformation of cables grouted into rock masses. Rock mass movements deform the grouted cable, which locally changes cable capacitance and thereby the reflected wave form of the voltage pulse. By monitoring changes in these reflection signatures, it is possible to monitor both local extension and local shearing. This paper concentrates on the electromagentic wave theory necessary to quantitatively relate changes in cable geometry to changes in reflected voltage signatures. A finite element model is employed to numerically simulate capacitance changes for deformed geometries produced in the laboratory, and the effect of signal attentuation and resolution of two deformities is assessed on the basis of laboratory test results. Finally, these models are employed to extract heretofore unrealized information from previously from previously collected field data.