Time Domain Reflectometry (TDR) can be best described as a "cable radar" which looks for "changes" in the relationship between the two conductors of a coaxial cable. A timed electrical pulse is sent along a cable and, much like running your finger along a smooth object which has a defect, the pulse can detect any changes between the inner and outer conductors. Changes cause a portion of the pulse to be reflected back, and because it is a timed pulse, both the location and the magnitude of the change can be determined. TDR was developed to detect and locate "changes" or damage in communication cables.
Since only a cable and no electronic instrumentation is placed in the ground, TDR becomes a viable way to monitor the long-term stability of a constructed embankment or slope. TDR cables can be placed in a constructed embankment or cut slope and checked against claims of movement 5, 10 or 20 years down the road. Cable ends can be easily secured at the hole location or buried underground for a more secure location against vandals, with no remaining "open holes".
For critical, sleeping or active landslides, TDR can readily be set up in a remote access configuration that can allow for real time monitoring in even the most remote locations. If movement is detected then an alarm can be sounded or the office can be alerted to initiate an investigation.
TDR has proven an economical way to locate shear planes in active slides of both soil and rock masses. Using innovative cable placement, multiple shear planes can be detected. Even tension cracks can be detected from horizontal cable placement.
TDR has proven an invaluable tool in both monitoring for potential ground movement and locating shear planes and tension cracks in active movement zones. The main advantages of TDR are:
TDR cable readings are made in minutes, regardless of depth/length and cable ends can be carried to a safe location if traffic or access is difficult. Remote access can be set up to eliminate the need for costly field excursions.
TDR can readily be installed in any exploratory bore hole. Just drop the cable in and grout in place. No need to excavate a special hole. talk has even been made of "pushing" the cable in place using available push equipment. Cables have successfully been installed in holes ranging from 2 to 6 inch diameters, excavated with hand auger or drill rigs, at depths ranging from 6 to 600 feet and in both soft soil embankments and hard rock. Only the imagination of the engineer limits TDR's potential applications.
