Chapter 2. Laboratory Testing
In prior research for Caltrans, Kane and Beck (1994) used an RG59/U cable attached to an inclinometer casing and backfilled with coarse sand. When installed without grout, RG cables appear most likely to fail in tension as they are stretched between opposite sides of a shear plane, FIGURE 2-1. By relating the cables signatures to tensile strain, and then to failure, it was believed that a characteristic signature for RG cables under tension could be determined. In addition, knowing the tensile strength and failure strain of the cables would allow an estimate of relative movement across the failure surface.
A series of tests were run on RG59/U coaxial cable. The purpose for testing was to determine the ultimate tensile strength of the cable, and to identify the correlation between the reflection coefficient (measured in m ) of a TDR signature and cable displacement.
In the past, both shear and tensile tests have been conducted on coaxial cables. Dowding et al. (1988) determined that when a cable was sheared, the TDR signature appeared as a localized spike that is directly proportional to the shear deformation of the cable. FIGURE 2-2a shows a TDR signature during a shear test. When the cable was extended, necking occurred. The signature spike was less localized and included an increase in cable length until failure. FIGURE 2-2b shows a signature during an extension test.
Two configurations of the RG59/U coaxial cable were used for the tests: jacketed and unjacketed. The jacket is a plastic shield used for external protection. TABLE 2-1 describes the properties of the RG59/U coaxial cable.
The equipment used for the strength test included a United (Model FM-20) universal testing machine, to apply a tensile load on the cable; a United (Model 2000 X-Y) plotter, for real-time load-displacement curves; and a United (Model 0.5-2.0) extensometer to measure the extension of the RG59/U coaxial cable. In addition, the TDR readings were collected using a Tektronix 1502B cable tester with SP232 data communication device and stored in computer files (Tektronix, Inc., 1989). The data was then analyzed and graphically displayed using NUTSA software Version 1.02 (Huang et al., 1993) and Microsoft Windows Paintbrush Version 3.1 (Microsoft, 1992). Sections 3.6 and 3.7 describe how to acquire TDR signatures and import them into a report document.
Tensile strength and TDR tests were performed on the RG59/U coaxial cable. Forty specimens (twenty jacketed and twenty unjacketed) were individually tested for strength until failure. Failure was defined as a tensile strength of zero, or approximately zero.
Twenty specimens (ten jacketed and ten unjacketed) were tested for an initial determination of tensile strength. The results were subsequently used as a baseline to estimate strength-deformation behavior for TDR readings. The remaining twenty specimens were tested for both tensile strength and TDR signature simultaneously. Load-displacement curves were also obtained from each test, and used to determine the strain and strength at both the yield and failure points.
The following steps were taken to perform each of the tensile tests. First, each specimen was cut to 0.61 m (2 ft) in length. A 0.61 m (2 ft) specimen was then placed and secured in the testing machine as shown in FIGURE 2-3. A distance of approximately (0.15 to 0.20 m) (6 to 8 in) was maintained between the grips. The extensometer was then clamped to the coaxial cable between the upper and lower grip. FIGURE 2-3 also shows a properly installed extensometer. The 0.15 m - 0.20 m (6 to 8 in)of cable between the grips was then extended using the tensile machine by applying minimal force of less than 10 lbs (45 N). Once the cable was fully extended, the distance between the grips was measured and recorded. Next, the extensometer was adjusted and the plotter turned on. During testing, a load-displacement curve was recorded as shown in FIGURE 2-4. Prior to failure, the extensometer was removed from the cable to avoid possible damage. Immediately after failure, the test was stopped and the final distance between the grips was measured and recorded This procedure was repeated for every specimen tested.
After the tensile tests were completed, the load-displacement curves were used to establish strains at which TDR signatures would be recorded. FIGURE 2-4 indicates points where TDR signatures were recorded.
Of the forty specimens tested, twenty (ten jacketed and ten unjacketed) were used to determine TDR signatures. FIGURE 2-3 shows a complete installation of the RG59/U coaxial cable including the cable tester and the computer. The following test procedure was used to view and store TDR signatures for the twenty specimens tested:
1. The cable tester was turned on and the horizontal and vertical scales were set at 0.06 m (0.2 ft) and 100 m /div, respectively
2. The cable tester was connected to both the coaxial cable and the computer
3. The coaxial cable was placed between the grips and extended to its full length on the testing machine
4. The length between the grips was recorded
5. The test was begun by extending the cable until failure
6. TDR waveforms were acquired and stored at deformations estimated from the first phase of strength testing
7. A final waveform was aquired at failure
TABLES 2-2 and 2-3 present summaries of the test results. Based on these results, the average strength was 610 N (137 lbs) for the jacketed cable and 547 N (123 lbs) for the unjacketed cable. The average yield strain was 0.07 for the jacketed and 0.09 for the unjacketed cable. The average strain at failure was 0.20 for the jacketed and 0.13 for the unjacketed cable.
From the strain results, it is possible to estimate the shear displacement of a slope when the cable fails. However, the relationship between strain at failure and a corresponding horizontal movement is highly non-linear and is only valid over a thin shear zone. This case is applicable for a grouted cable only. Ungrouted cables, or cables attached to inclinometer or piezometer casings, will not be subjected to such a failure mechanism. Assuming the width of a shear zone of about 12.7 mm (½ in), a reasonable estimate of slope movement can be made. A jacketed cable will fail at approximately 27.7 mm (1.1 in) of horizontal movement while an unjacketed cable will fail at about 25.8 mm (1 in).
Two assumptions were made in the strength testing. First, it was assumed that stopping the test temporarily (10 to 20 sec) did not affect the strength of the cable. Second, the rate of extension was assumed not to have affected the performance of the cable. At this time the effects are unknown. Further investigation is required to determine if these assumptions are valid.
For the TDR tests, five signatures were obtained for each of the twenty specimens tested (ten jacketed and ten unjacketed). The first TDR signature was recorded before the test, three TDR signatures were recorded during the test, and the last TDR signature was recorded immediately after failure. FIGURE 2-5 shows three signatures: the signature at the top was recorded before the test; the signature at the middle was recorded during the test; and the signature at the bottom was recorded immediately after failure. The signature at the top is completely horizontal between points A and B, which represent the upper and lower grips of the tensile machine. The signature in the middle was acquired while a load was being applied. Note that the signature has changed from its original shape. It has become uniformly negative between points A and B. The signature at the bottom of FIGURE 2-5 shows a broken cable. The signature maintained its shape near point A, but shifted up significantly at the failure point.
Overall, the laboratory test results verify similar tests conducted by others such as Dowding et al. (1988). The same signature for tensile strain and failure was obtained under similar test conditions.
Throughout the laboratory testing, a common problem was encountered. Most of the specimens failed at one of the connections. Failure at the connection evidently occurred due to high stress concentrations caused by the grips on the tensile machine. Nevertheless, it is believed that this did not affect the test results significantly.
For best results when comparing TDR signatures, the following is recommended:
1. Increase the sensitivity of the vertical scale on the cable tester (less than 50 m /div, if possible) in order to see small changes in the TDR signature
2. When comparing signatures, use the same vertical and horizontal scale, including the same initial distance
3. Set the cursor at the left-hand side of the screen on the cable tester when acquiring a signature
4. Refer to Sections 3.6 and 3.7 for information on acquiring and manipulating data.
The TDR signatures suggested the following:
1. Visible changes in the reflection coefficient along any particular length of the cable represent deformation at that point
2. When a cable breaks, the reflection coefficient at that point increases to +1
3. Tensile deformation is indicated by a distinct negative trough. FIGURE 2-5 shows the different phases of a typical TDR signature under tension
4. Movement along a shear plane is estimated to be approximately 25.4 mm (1 in) at cable failure (RG59/UBelden 9259 only) for single grouted cables. Ungrouted cables, or cables attached to casings will not provide reliable information
5. The slight difference in response between jacketed and unjacketed cables does not justify the time involved in stripping cable jackets. Jacketed cables are recommended.
Dowding, C. H., Su, M. B., and O'Connor, K. (1988). "Principles of Time Domain Reflectometry Applied to Measurements of Rock Mass Deformation." International Journal of Rock Mechanics, Mining Science, & Geomechanics Abstract, 287-297.
Huang, F. C., O'Connor, K. M., and Yurchak, D. M., and Dowding, C. H. (1993). "NUMOD and NUTSA: Software for Interactive Acquisition and Analysis of Time Domain Reflectometry Measurements." Bureau of Mines Information Circular 9346, 42 p.
Kane, W.F. and Beck, T.J. (1994). &"Development of a Time Domain Reflectometry System to Monitor Landslide Activity." Proceedings, 45th Highway Geology Symposium, Portland, OR, 163173.
Tektronix, Inc. (1989). "SP232 Serial Extended Function Module Operation/Service Manual." Tektronix, Inc., Redmond OR, 103 p.
Microsoft Corporation (1992). Microsoft Windows Version 3.1. User's Guide. Microsoft Corporation, Redmond, WA.
Last modified: 06-18-97
| Table of Contents
| Executive Summary
| Chapter 1. Introduction
| Chapter 2. Laboratory Testing |
| Chapter 3. Installation and Results
| Chapter 4. Discussion
| Appendices
| List of Figures and Tables
