Article · January 2000 citations 16 reads 1,575 1

INTRODUCTION

Time domain reflectometry (TDR) was originally developed by the power and communications industries to locate faults and breaks in cables (Rohrig, 1931)). The discovery that soil moisture content could be determined by TDR led to its extensive use in the agricultural area (Topp and Davis, 1985). In the late 1970's and during the 1980's, the
U.S. Bureau of Mines used TDR extensively to locate coal mine roof failure zones above longwall coal mines (Dowding and Huang, 1994). Other geotechnical applications during this period included monitoring for metal mine roof caving by the Canada Centre for Mineral and Energy Technology (CANMET) (Aston et al., 1994), and attempts at monitoring slopes adjacent to a dragline by Syncrude Canada Ltd. (Lord et al., 1991; O’Connor et al., 1992).


In the early 1990's, the California Department of Transportation (Caltrans) did extensive research to evaluate the application of TDR to landslide monitoring using both remote and locally monitored stations (Kane and Beck, 1999). The results of this study, and the efforts of the U.S. Bureau of Mines, led to the strong interest by the geotechnical and geological engineering communities in using TDR for slope monitoring. This paper presents a review of the principles and equipment necessary for utilizing TDR as a single field installation or as a remotely monitored system that can incorporate other types of sensors. The definitive review of research and information on TDR is available in O’Connor and Dowding (1999).

TIME DOMAIN REFLECTOMETRY

Principle of TDR
In TDR, a cable tester sends a voltage pulse waveform down a cable grouted in a borehole, Figure 1. If the pulse encounters a change in the characteristic impedance of the cable, it is reflected. This can be caused by a crimp, a kink, the presence of water, or a break in the cable. The cable tester compares the returned pulse with the emitted pulse, and determines the reflection coefficient of the cable at that point.


Electrical energy travels at
^{the speed of light in a vacuum,} Figure 1. Cable tester attached to TDR cable undergoing deformation
^{but travels somewhat slower in} due to slope failure.
a cable. This is called the velocity of propagation. When the propagation velocity of a particular cable is known, the distance to any cable reflectioncan be determined by the cable tester.


Coaxial cables are composed of a center metallic conductor surrounded by an insulating material, a metallic outer conductor surrounding the insulation, and a protective jacket. Each cable has a characteristic impedance determined by its material composition and construction. If the cable is deformed, the distance between the inner and outer conductors changes. It is this change that causes a difference in the impedance, and a resulting reflection of the voltage pulse.


Data consists ofa TDR signature,
Figure 2. TDR cable signatures showing development of a shear zone.
Figure 2, composed of the wave reflections for the cable. The length and amplitude of the reflections, a cable signature “spike,” indicate the severity of the damage to the cable. TDR for determining ground movement requires reading the cable signature at regular time intervals. Ground movement, such as slip along a failure zone, will deform the cable and result in a change in cable impedance and a reflection of energy. This change can be used to determine the location of shear movement. The change in impedance with time corresponds qualitatively to the rate of ground movement.


Advantages
For geotechnical applications TDR has many advantages over conventional probe-type, servo-accelerometer inclinometers. These include:

1. 
   Cost. Cable has a significant cost advantage vs. inclinometer casing. Some cables can be purchased for as little as $0.10/foot as opposed to $6 to $10/foot for inclinometer casing. The cost of the electronic equipment is approximately the same: roughly $9000 per unit, but the price of new generation TDR cable testers is much less. The cost of drilling is the same. However, with TDR there is no risk of losing an expensive probe due to a sharp bend in the inclinometer casing, only expendable cable is lost.
   

2. 
   Time. Besides the cost savings in equipment, the rapid reading of TDR cables means that more holes can be read in less time -- an additional savings in man-hours. A single cable, no matter how deep, can be read in less than 5 minutes as opposed to ½ hour to an hours to read an inclinometer, depending on depth. Multiple cables at a site can be run to an easily-accessible central location and all cables read at the same time.
   

3.