10.4.2 Time–domain reflectometry
Time–domain reflectometry (TDR) is another technique of locating a sliding surface, which can also monitor the rate of movement (Kane and Beck, 1996). This method involves grouting into a borehole, a co-axial cable comprising inner and outer metallic conductors separated by an insulating material. When a voltage pulse waveform is sent down the cable, it is reflected at a point where there is a change in the distance between the conductors. The reflection occurs because the change in distance alters the characteristic impedance of the cable. Movement of a sliding plane that causes a crimp or kink in the cable is sufficient to change the impedance, enabling the instrument to detect the location of the movement (Figure 3 &4). 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.
The primary advantage of this technique is that the cable is inexpensive so it can be sacrificed in a rapidly moving slide mass. Also, the readings can be obtained in a few minutes from remote location either by extending the cable to a safe location off the slide, or by telemetry. The ability to make remote readings can achieve significant savings compared to inclinometers because of the reduced travel time. The readout box directly shows the movement without the need to download and plot the results.
When combined with in-place tiltmeters and a datalogger, TDR can be used to determine the depth and direction of movement. Biaxial tiltmeters provide direction, while the TDR cable locates the depth at which movement is expected. The datalogger can be programmed to turn on the TDR cable tester and read the coaxial cable and the tiltmeters. A base station can be programmed to access the data through telemetry.
Figure 3: Working mechanism of TDR system
Figure 4: Computer aided data acquisition form TDR system.