Chapter 4. Discussion, Conclusions, and Recommendations
4.1 Discussion and Conclusions
A number of tasks were accomplished with corresponding conclusions. The project began in the Spring of 1995 with a simultaneous review of the literature and installation of cable at Grapevine Landslide. Laboratory testing was performed and RG59/U cable properties determined in Summer 1995. Data acquisition equipment was assembled in the laboratory and installed at the Grapevine site in Summer and Fall 1995. Data analysis and computer programming was carried out in Winter and Spring 1996. Two setbacks occurred in Winter and Spring 1996. Cables leading from TDR 1 and TDR 2 to the data acquisition equipment were vandalized by being cut into pieces. Secondly, rainwater, entering through holes in the battery enclosure, evidently shorted the leads between the solar collector and the battery. This severely burned and melted the wires. Since the extent of damage was unknown, the entire unit was removed and transported back to the University of the Pacific Geotechnical Laboratory. The components subsequently checked out, however, programming of the datalogger was done in the laboratory. Therefore, the entire communications system has not been field-checked.
The current literature on TDR was reviewed and a bibliography on the application of TDR in geotechnical engineering was compiled. The number of uses of TDR in geotechnical/structural engineering is growing. The literature survey suggests that geotechnical applications will continue to increase. However, more research is needed on cable properties and cable behavior in a borehole. Identifying the location of shear planes is relatively straightforward, however, determining the amount of movement along them is not.
As part of the research contract, equipment was purchased and assembled to provide Caltrans a TDR monitoring capability. Two cable testers were acquired: one equipped with rechargeable battery and laptop computer interface, and the other equipped to serve with the components of a remote data acquisition system. Data acquisition components included a datalogger which can also be programmed to acquire other types of data such as rainfall amounts, soil moisture, and extensometer readings.
The various computer programs necessary to use TDR were acquired and used. Essential programming and basic instructions on using the programs are included in this report. Methods were developed for obtaining data manually with a laptop computer and remotely with the data acquisition system. In addition, a methodology is provided for post-processing of TDR signatures for incorporation into reports and other documents.
4.1.4 Grapevine Landslide Monitoring
Three cables were installed in boreholes and monitored at Grapevine Landslide. The cables were read manually during site visits. They indicated that no movement was occurring at the site. However, this may be due to the fact that the cables are attached to the inclinometer casings which prevent them from shearing under small deformations.
4.1.5 Remote Monitoring Capabilities
The remote data acquisition equipment was assembled and tested. It was installed at the Grapevine site, but was not used to collect data. Damage to the system from vandalism and weather prevented the system from being used. A cellular phone contract was purchased and communication with the system was made. However, the system was removed for to check for damage before it could be used remotely.
Laboratory testing of the RG59/U cable was performed to determine tensile behavior and corresponding TDR signatures. The strength/deformation characteristics showed that the cable is relatively weak. Its tensile strength is on the order of 623 N (140 lbs). Unjacketed cables were shown to be more sensitive to movement but undergo more strain at failure than jacketed cables. However, the small difference in material characteristics compared with the magnitude of the earth movements in question suggest that the extra effort in stripping the cables does not provide any advantage.
An empirical method for determining the amount of slip along a plane was also proposed. It was suggested that the amount of movement along a slide plane at the time of cable failure is on the order of 25 mm (1 in).
It is recommended that Caltrans refine its TDR capabilities and methodologies by continuing to use the equipment and comparing results with inclinometer data whenever possible. The following suggestions are made:
1. Do not use TDR cables attached to inclinometer casing. Enough data exists to verify the method. TDR cables are more effective and economical if used in place of inclinometers when possible.
2. Refine the use of the data acquisition equipment and programs. New products are coming on the market at reduced prices, for example, less expensive cellular phones. Campbell Scientific is developing a component that would replace the cable tester (McHugh, 1996). It would be smaller and much less expensive than the Tektronix product on the market.
The current programs available for post-processing signatures, NUTSA, etc., are relatively cumbersome. Streamlining the process and investigating other ways to process the signatures, such as using spreadsheets, should be investigated.
3. Experiment with different types of cable. Many different types of cable are available. RG59/U is very inexpensive, but some cables may be more applicable particular locations or sensitivities desired.
4. Extend the technology beyond its current status. It is possible to link data acquisition equipment through satellite uplinks instead of cellular phones. A State-wide system, monitoring all major landslides, could be implemented and monitored from a single location. Algorithms could be written to trigger early warning devices for motorists.
TDR appears to be able to respond to groundwater levels. Research to allow the monitoring of groundwater level and slope movement should be considered.
McHugh, A. (1996). Personal Communication. Campbell Scientific, Inc.
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