Geo-Measurements with Metallic TDR Cable Technology for Infrastructure Surveillance

This paper reviews the new field of geo-measurement with metallic cable time domain reflectometry (TDR) for surveillance of transportation facilities. TDR is radar in a coaxial cable, and is especially advantageous for remote monitoring because of its inherently digital nature. Advances can be separated into those that leverage use of long cables as transducers“along” technology and those that employ probes at the end of cables“point” technology. “Along” applications have the unique advantage of monitoring all along long cables – sometimes as long as 1000 ft – for disturbances whose location cannot be predicted in advance. This application has been found particularly advantageous for remote monitoring of highways for mining induced subsidence, development of sinkholes, and landslides. It has also been found useful for monitoring of bridges for scour induced foundation movement and earthquake induced cracking of piers in inaccessible areas. Point applications leverage the “up-hole” or out of the ground location of the electronics to measure at the end of many cables from one instrument. This application has been found uniquely advantageous for measuring water content of subbase materials beneath pavements, porewater pressure in landslides and dams. Simultaneous measurement of water content and density is also under development. A special focus is placed on the use of “along” technology for measurement of deformation. Two case histories are given that compare response of slope inclinometer and TDR cable for monitoring slope instability in rock and soil. Also included are guidelines for cable installation.