Article · January 2000 citations 16 reads 1,575 1

Tilt Measurement. The coaxial cable used in TDR must be damaged by shearing, tension, or a combination of both to show up on a cable signature. For this reason, TDR cannot be used where shear zones do not occur and monitoring tilt is necessary. For example, inclinometers should be used behind retaining walls and other structures to monitor tilt.

2. 
   Magnitude of Movement. Some attempts have been made to correlate the amount of slope movement with a corresponding change in size in TDR spike (Francke et al., 1994). While some correlation between spike size and movement as recorded in a laboratory experiment have been determined, few correlations have been made in the field.
   

3. 
   Direction of Movement. TDR can only locate the shear plane. It cannot provide the direction of movement.
   

TDR CABLES

Types
Virtually any coaxial cable can be used for TDR measurements. Experience with many varieties of cable have led to the recommendation by this author that certain cables be used. RG59/U cable manufactured by Belden (www.belden.com) and other manufacturers is inexpensive and easyto use. However, it suffers from signal attenuation and should not being used in deep holes where fine signal resolution is desired.

For best results jacketed, foam-filled cable is recommended. Cablewave FLC12-50J cable (www.cablewave.com) has excellent properties and is easy to install. Unjacketed

cables suchas 7/8-in CommScope P3-875-CA (www.commscope.com) must be coated, for example with paint, prior to installation to prevent adverse interaction with grout. This is very time-intensive. The sensitivity of unjacketed cable is not good enough to warrant its use. Large diameter cables, such as the CommScope cable, greater than ½-in diameter are not recommended for most work because they are bulky and difficult to manipulate.


Preparation
Down-hole cable ends can be prepared by cutting them square, sealing the ends with liquid electrical “tape,” and slipping a tight-fitting rubber or plastic boot over the end. The connection should be wrapped securely with electrical tape to prevent water infiltration. Care should be taken not to allow the inner and outer conductors of small diameter flexible cables to contact each other and cause a short circuit.


Crimping cables as a method to precisely locate movement zones is not recommended. Crimps cause the loss of energy and confusing cable spikes, masking actual deformations. In addition crimping cables is time intensive. The benefits are marginal at best and, more likely, detrimental.


Cables can be installed by weighting the end of the cable and lowering the cable end to the bottom of the hole. This may not always be necessary, and the cable may be pushed downhole, especially when installing in hollow stem augur or casing. Cables installed in this manner in dense grouts may float out of the hole and may need to be restrained until the grout sets.


Grout
Cables work best in shear. Grouts must be stiff enough to cause cable deformations. Grout consist of a 10% bentonite, 90% cement slurry works well, but 100% cement also can be used. Installing cables in ungrouted holes, or with sand backfill will likely result in poor performance. In some cases it may be quicker and easier to attach the TDR cable to the plastic grout pipe and leave the pipe after grouting.


Attaching Cables to the Outside of the Inclinometer Casing
As mentioned previously, TDR cables, especially small diameter flexible cables, can be attached to the outside of inclinometer casing. This is commonly done, and adds virtually nothing to the cost of the hole. However, the TDR cable will perform much better by itself and careful consideration should be given as to whether the inclinometer is needed at all. The bending of the casing strongly influences the behavior of the cable. This usually delays the onset of signature spike development.


If it is desired to attach cables to an inclinometer, plastic casing should be used. The use of square steel pipes should be avoided since the steel will not shear easily, if at all, and thus will result in inaccurate cable readings, if any can be recorded at all.


Connectors
When ½” foam-filled cable is used, N-Female connectors work well. Installation should be
as per connector specifications and the connection made water-tight with shrink-fit tubing. A N-male-to-BNC-female adapter shall be placed on the of the N-female connector and protected with a small rubber or plastic boot.


When RG59/U cable is used, a BNC female connector should be installed on the cable end. Connector may be of the twist-on or crimped variety. Connections should be made water-tight with shrink-fit tubing.


After connections have been made they should be tested immediately with a cable tester to verify that they are installed properly. Cables can be made secure by protecting them in a “Christy”-type box, suitable for the traffic rating at the location.