Chapter 1. Introduction
This project was conceived based on the first author's experience as a consultant with the U.S. Bureau of Mines Twin Cities Research Center, Minneapolis, Minnesota. The Bureau pioneered the use of time domain reflectometry (TDR) for monitoring rock deformation during strata caving over longwall mines (O'Connor and Wade, 1994; U.S. Bureau of Mines, 1995). Although the expansion of the technology to landslide movement was relatively uncomplicated, the use of TDR for rock slope monitoring is new. At this time, the only other organizations doing research in the field are Neil O. Anderson & Associates, Lodi, California; New Mexico Technological University, Socorro; Northwestern University, Evanston, Illinois; and the Canadian Centre for Mineral and Energy Technology (CANMET), Ottawa, Canada. Their work is described in the appropriate sections.
In December 1993, a prototype experiment began. A single cable strapped to an inclinometer casing was installed in the Last Chance Grade Landslide on U.S. Highway 101 in Del Norte County, California. Inclinometer and TDR data were collected on installation, and later in May 1994. The May data indicated cable deformation in the same region shown by the inclinometer. This work is described in Kane and Beck (1994).
Grapevine Landslide was subsequently selected as a test site for the research described in this report. At the time of its selection the Grapevine slide was moving and appeared to be an ideal location for further testing of the TDR concept and the use of remote data acquisition equipment. Since the initial installation at Last Chance Grade, cables have been installed at a number of sites in California. These locations are shown in FIGURE 1-1 and much of the work is described in Kane et al. (1996).
1.2 Theory of Time Domain Reflectometry
Radar is an early form of TDR. In radar, a radio transmitter sends out a short pulse of energy and measures the time for a reflection, or echo, of the energy from some object. TDR works in much the same way. Coaxial TDR is essentially "closed circuit radar" (Andrews, 1994). An electrical pulse is sent along a coaxial cable and an oscilloscope is used to determine the time it takes for the echos to return. According to Andrews (1994), this technique was mentioned as early as 1931 by Rohrig (1931) to find faults in telephone cables.
In TDR, a pulse waveform, which is a fast-rising step function, is sent down the a coaxial cable. If the pulse encounters a change in the characteristic impedance of the cable, it is reflected. Changes in the characteristic cable impedance are called "cable faults." These can take the form of kinks, foreign substances such as water, or breaks in the cable. Cable heating can also be detected (Steiner and Weeks, 1990). The returned pulse is compared with the emitted pulse, and the reflection coefficient (in rho's or millirho's) is determined. If the reflected voltage equals the transmitted voltage, the reflection coefficient is +1 and the cable is broken. If the opposite occurs, and the cable is shorted, all the energy will be returned by way of the ground, and the reflection coefficient will be -1. If the cable has a change of impedance, the reflection coefficient will be between -1 and +1. If the pulse experiences a decrease in impedance, the reflection coefficient will be negative. If the pulse experiences a higher impedance the reflection coefficient will be positive.
In a vacuum, electrical energy travels at the speed of light. The speed at which it travels in a cable is somewhat less depending on the impedance of the cable. This speed is known as the velocity of propagation (Vp) and is a property of each cable. When the cable propagation velocity and time delay between transmitted and measured pulses are known, the distance to any cable fault can be found. The type and severity of the fault can also be determined (Su, 1987).
1.2.2 Cable Response to Deformation
Coaxial cables, FIGURE 1-2, are composed of a central metallic conductor surrounded by an insulating material, a metallic outer conductor surrounding the insulation, and a protective jacket. The cables have a characteristic impedance determined by the thickness and type of insulating material between the cables. This insulating material is called the "dielectric," and may be made of almost any non-conducting material. Common dielectric materials are PVC-foam, Teflon, and air. If the cable is faulted, the distance between the inner and outer conductors changes, as does the impedance at that point. The TDR cable tester can then determine the location of the fault.
The data consist of series of TDR signatures or strip charts, FIGURES 1-3 and 1-4. Different wave reflections are received for different cable deformations. The length and amplitude of the reflection indicates the severity of the damage. For a cable in shear, a voltage reflection spike of short wavelength is recorded. The wavelength increases in direct proportion to the shear deformation. A distinct negative spike occurs just before failure. After failure, a permanent positive reflection is recorded. For cables in tension, the wave reflection is a subtle, trough-like voltage signal that increases in length as the cable is further deformed. At tensile failure a small necking trough appears, making it is easy to distinguish from a shear failure (Dowding et al., 1988; 1989). These results were verified by Aimone-Martin et al. (1994) who also quantified combined shear and tension in copper/air and copper/foam corrugated cables.
Attenuation of the pulse with distance along the cable can be a problem when using TDR to determine shear displacement. Pierce et al. (1994) found that deformation could be quantified to a distance up to 268 m (880 ft) and detected to a distance of 530 m (1740 ft). Each deformation in the cable reduces the strength of following (downstream) reflections.
1.3 TDR in Geotechnical Engineering
Rock mass movements deform the grouted cable, changing the cable impedance and, as a result, the reflected waveform of the voltage pulse, FIGURES 1-3 and 1-4. The time delay between a transmitted pulse and the reflection from a cable deformity determine the damage location. Also, as described in Section 2 of this report, the time, sign, length, and amplitude of the reflection pulse defines the location, type, and severity of the cable deformation.
TDR is used successfully in monitoring soil moisture conditions (Topp and Davis, 1985). This application so far has found use only in agricultural and environmental projects, but is also suitable to geotechnical engineering.
TDR has been used in several other geotechnical applications. Most significantly, it is utilized in coal mining ground control. One of the earliest applications was in determining rock fracture after detonation of a nuclear device (Sisemore and Stefani, 1971). Other applications include monitoring ground movements in the Waste Isolation Pilot Project (Aimone-Martin and Oravecz, 1994), above abandoned mine stopes (Aston, 1995), and slope stability monitoring in open-pit mines (Lord et al., 1991).
In the Appalachian coal fields, Hasenfus et al. (1988) used two RG-59 cables and a twisted pair cable in each of four 220 m (720 ft) deep boreholes to monitor strata movement during subsidence above a coal mine. All four holes had cable failures at a depth of about 152 m (500 ft). The cables failed between rock interfaces and within the weak claystone and coal strata. It was believed that the cable failures were due to shear slippage along roughly horizontal planes within the rock layers. The slippage was due to bending of the rock strata during caving of the overburden. This hypothesis was verified by finite element analysis.
Haramy and Fejes (1992) used TDR in a similar study in a western Colorado longwall mine. The goal of the research was to compare several types of instrumentation for characterizing overburden response during mining. A 335 m (1,100 ft) coaxial cable was installed in a borehole. The TDR system allowed the researchers to correlate failure of a sandstone layer 213 to 229 m (700 to 750 ft) above the mine with the location of the mining face.
Dowding and Huang (1994) described the installation and monitoring of a 175 m (575 ft) long vertical cable in front of a longwall mining panel in southern Illinois. Data was acquired remotely, using telephone lines and modems, from 500 km (310 mi) away. Results from the project indicated deformation of the strata just in advance of mining that section. Correlations were made with surface subsidence that occurred at the same time.
Kawamura et al. (1994) also used TDR in coal mines in Illinois. They monitored three TDR cables above two longwall mine panels. A comparison was made of TDR with inclinomters and borehole extensometers. The cables were able to locate shear planes more accurately than the extensometers. The extensometer performance was a function of grout stiffness which caused localized stress concentrations and false readings. The inclinometers functioned well until they deformed to such an extent that the probe could not pass down the casing.
1.3.2 Waste Isolation Pilot Project
The Waste Isolation Pilot Project (WIPP) in Carlsbad, New Mexico, is a system of 650 m (2,150 ft) deep tunnels developed in bedded evaporite deposits for the purpose of storing nuclear waste material. Delays in the permitting of the project have made time-dependent movements of the rock salt a critical factor in the stability of the workings. As part of the rock mass monitoring program, five TDR cables were installed in the roofs of two rooms (Francke et al. 1994). Observation boreholes monitored with video cameras, roof extensometers, and convergence meters were also installed to correlate with TDR data. Cable behavior was correlated with laboratory testing. Results indicated that the TDR shear rate was ±4.3 mm/yr (0.69 in/yr) which agreed with borehole observations. The TDR cables were also effective in locating offsets between beds of rock.
1.3.3 Abandoned Hard Rock Mines
The Canada Centre for Mineral and Energy Technology (CANMET) used TDR to monitor the stability of the surface above abandoned metal mines in Ontario and Nova Scotia (Aston et al., 1994). They used 12.7 mm (½ in) O.D. smooth-walled aluminum cable (Cablewave Systems ½" Foamflex FXA) grouted into a borehole with portland cement. The cables were crimped to aid in locating distances. Results indicated that TDR could locate deformations along rock discontinuities.
The use of TDR in monitoring slope movements is relatively new. Only a few studies have been done.
Most slope monitoring using TDR has been done with mine highwalls. The stability of such slopes is essential when large, expensive draglines are located at the crest. Most monitoring is done using inclinometers. The exposure of personnel on potentially unstable slopes, the lack of real-time reading capability, and the expense of inclinometer installations has led to the trial of coaxial cables as a means of monitoring slope stability.
Lord et al. (1991) and O'Connor et al. (1992) described the use of TDR to monitor the stability of a highwall at the Syncrude Canada Ltd. oil sand mine in northern Alberta. TDR using coaxial cables was compared with optical TDR (OTDR) using optical fibers, and electrolytic bubble sensor strings. Although the TDR method using coaxial cables seemed viable, problems occurred with data acquisition (O'Connor, 1991). Cable signatures were recorded using strip charts which made the comparison of cable changes over time difficult. O'Connor (1991) also noted that grout/cable bond in laboratory tests was a factor in whether the cable was sheared or merely slipped within the grout matrix. However, in the field, the cables indicated movements similar to those measured by an inclinometer installed nearby. O'Connor (1991) concluded that laboratory calibrations might not be reliable indicators for field behavior.
CANMET installed coaxial cables in five boreholes in the National Gypsum mine near Milford, Nova Scotia (Hayward et al., 1995). The purpose of the project was to monitor three highwalls which were close to several structures. Although the highwalls had been stable for up to forty years, there was some concern as to their stability. The TDR cables were monitored bi-weekly to monthly from June 1994 until September 1995. The cables showed that no movement in the highwalls occurred during that period.
Aston (1994a, 1994b, 1995) described the installation and monitoring of five TDR cable at the BC Hydro Checkerboard Creek site near Revelstoke, British Columbia. The cables were installed along with inclinometers and piezometers. The data indicated that no ground movement was apparent. Some small signal variations were observed but were not significant enough to indicate ground movement.
Caltrans installed a cable attached to an inclinometer (Kane and Beck, 1994) in the Last Chance Grade landslide beneath U.S. Highway 101 in Del Norte County, California. The cable and inclinometer casing were installed in an 85.3 m (280 ft) deep borehole backfilled with coarse sand. When read several months later, the TDR signature showed a spike at approximately 39.6 m (130 ft), the same depth as the deflection in the inclinometer casing. Another Caltrans installation in a landslide along U.S. 101 at Cloverdale is currently being monitored and is described by Freeman (1996).
Neil O. Anderson & Associates installed cables and an inclinometer in an embankment slope in Antioch, California and in at the crest of a San Joaquin River Delta levee near Stockton, California (Kane et al., 1996). TDR results from the Antioch embankment correlated well with the inclinometer in that no movement was indicated. Four coaxial cables were installed in the Delta levee. The first cable was a jacketed, 6 mm (0.24 in) braided cable, the second was identical but with the outside jacket removed, the third was a 16 mm (0.63 in) corrugated copper cable with a plastic jacket, and the fourth was a 13 mm (0.51 in) aluminum cable with a plastic jacket. The cables were separated by spacers and grouted with cement into a 12.2 m (40 ft) borehole. An inclinometer casing was installed on the crest about 8 m (26 ft) from the cables.
FIGURE 1-5 shows the inclinometer profile approximately four months after installation. A slide plane is clearly developing at 6.4 m (21 ft). FIGURE 1-6 shows TDR cable readings for June, August, and October for the corrugated copper cable. June was the initial reading after installation. The October reading was taken on the same day as the inclinometer profile. A spike is visible in the August and October readings, developing at the same depth as the base of the sliding mass shown by the inclinometer. The other cables showed a similar depth and development pattern. Gwinnup-Green (1996) showed that there is a correlation between the surface movement of the slide and the growth of the spike in the corrugated copper cable.
One aspect of using TDR cables in soil slopes is the interaction between the cables, grout, and soil. Dowding and Pierce (1994a) addressed this issue. They felt that the grout must be stiff enough to stabilize the borehole, but compliant enough not to affect the movement of the soil mass. In other words, the grout should approximate the soil in strength and stiffness. To accomplish this, they proposed using a controlled low strength material (CLSM) (American Concrete Institute, 1989). It consists of a mixture of cement, fly ash, fine aggregate, and water with moduli between 70 MPa (71 psf) and 7 GPa (7050 psf) . It should be noted that Dowding and Pierce (1994a) presented only a theoretical view on grout/soil interaction. Kane et al. (1996) in their California Delta levee installation used a sand/cement/water mixture with satisfactory results. It was believed that, a failure of any size, even in a soil slope, would mobilize enough force to fracture the grout column and deform the cable.
Another consideration of using TDR cables in soil slopes is the behavior of the cable in an ungrouted hole. Logan (1989) investigated the TDR signatures of cables embedded in sand and gravel. The cables were 12.7 mm (½ in), smooth-walled, aluminum outer conductor with a copper-coated aluminum inner conductor surrounded by a polyethylene foam dielectric. His work was confined to laboratory tests and indicated that there was no change in the TDR signature when the cable was embedded in sand. Gravel, however, was effective in causing a change with shear. The reflection change with displacement was a function of the relative density and confining pressure of the backfill.
Caltrans has installed a number of cables in slopes since this research began. Some of the installations are described in Kane et al. (1996).
1.3.5 Other Geotechnical Applications
TDR was used in 1971 as part of the "cliper" (Collapse Location Indication by Pulsed Electromagnetic Radiation) system described by Sisemore and Stefani (1971). This method was used to locate the area of rock fractures and vaporization surrounding an underground nuclear blast. They used RG213U coaxial cable with lengths of 150 to 2750 m (500 to 9000 ft). They suggested that longer lengths could be used with 22 mm (7/8 in) air-filled helical cables or modifying the cable tester to accommodate a different cable. They were able to correlate the travel of the shock front and chimney collapse in the rock mass due to the explosion.
O'Connor (1989) described the installation of a TDR cable to monitor subsurface fracturing and caving with respect to solution mining. A mining company was experiencing well loss during the heating and removal of sulfur at depth. Consolidation of the limestone ore during heating was responsible for the loss of production. The corrosive sulfuric environment made TDR a viable alternative to more conventional methods, such as inclinometers. A 152 m (500 ft), 22 mm (7/8 in) diameter aluminum cable (Cablewave FXA 78-50J) was installed to a depth of 146 m (478 ft) and grouted in cement. No results have been published.
Another use for TDR was suggested by O'Connor et al. (1987) to locate and quantify potential collapse in karst areas. Although proposed, no application has been attempted.
TDR also has been widely used in soil moisture measurements. Look and Reeves (1992) examined the use of TDR in monitoring moisture conditions in pavements and embankments. They stated that TDR was used by the Materials and Geotechnical Services Branch of the Queensland (New South Wales, Australia) Department of Transport. They described the application, but did not publish any data .
Appendix B Bibliography of Time Domain Reflectometry Applicable to Geotechnical Engineering is a nearly complete listing of references pertaining to TDR for geotechnical uses.
1.3.6 Structural Engineering Applications
The application of TDR to structural engineering applications is beyond the scope of this review. Additional information can be obtained from Huston et al. (1994) who described experiments using optical time domain reflectometry (OTDR). The authors embedded optical fibers in concrete and noted the response of the fiber light-transmission capabilities as a function of damage to the concrete. Schoenwald and Beckham (date unknown) described a system of OTDR for monitoring structures. Zimmerman et al. (date unknown) also discussed the theoretical aspects of using OTDR to monitor large structures. Research on the use of TDR to monitor for bridge scour is described by Dowding and Pierce (1994b) and others.
Aimone-Martin, C. T., and Oravecz, K. I. (1994). "Time Domain Reflectometry Calibration for the Waste Isolation Pilot Project." Preprint No. 94-144, for presentation at the SME Annual Meeting, Albuquerque, NM, 7 p.
Aimone-Martin, C. T., Oravecz, K. I., and Nytra, T. K. (1994). "TDR Calibration for Quantifying Rock Mass Deformation at the WIPP Site, Carlsbad, NM." Proceedings, Symposium on Time Domain Reflectometry in Environmental, Infrastructure, and Mining Applications, Evanston, IL, 507-517.
American Concrete Institute (1989). "State-of-the-Art on Controlled Low Strength Materials (CLSM)." ACI Publication, Committe 229.
Anderson, N. O. (1995). Personal Communication.
Andrews, J. R. (1994). "Time Domain Reflectometry." Proceedings, Symposium and Workshop on Time Domain Reflectometry in Environmental, Infrastructure, and Mining Applications, Northwestern University, U.S. Bureau of Mines Special Publication SP-19-94, 4-13.
Aston, T. (1994a). "Installation and Monitoring of Three TDR Cables, Checkerboard Creek Site, Near Revelstoke, B.C.: July 1993 to September 1993." Report No. 93-052 (CL), Submitted to B.C. Hydro by CANMET-MRL, 33 p.
Aston, T. (1994b). "Installation and Monitoring of Three TDR Cables, Checkerboard Creek and Marble Shear Block Sites, Near Revelstoke, B.C.: September 1993 to May 1994." Report No. 94-006 (CL), Submitted to B.C. Hydro by CANMET-MRL, 41 p.
Aston, T. (1995). "Installation and Monitoring of Three TDR Cables, Checkerboard Creek and Marble Shear Block Sites, Near Revelstoke, B.C.: May 1994 to October 1994." Report No. 94-034 (CL), Submitted to B.C. Hydro by CANMET-MRL, 48 p.
Aston, T., Bétournay, M. C., Hill, J. O., and Charette, F. (1994). "Application for Monitoring the Long Term Behaviour of Canadian Abandoned Metal Mines." Proceedings, Symposium and Workshop on Time Domain Reflectometry in Environmental, Infrastructure, and Mining Applications, Northwestern University, U.S. Bureau of Mines Special Publication SP-19-94, 518-527.
Dowding, C. H. and Huang, F. C. (1994). "Early Detection of Rock Movement with Time Domain Reflectometry." under review, Journal of Geotechnical Engineering, American Society of Civil Engineers, 120 (8), 1413-1427.
Dowding, C. H. and Pierce, C. E. (1994a). "Measurement of Localized Failure Planes in Soil with Time Domain Reflectometry." Proceedings, Symposium and Workshop on Time Domain Reflectometry in Environmental, Infrastructure, and Mining Applications, Northwestern University, U.S. Bureau of Mines Special Publication SP-19-94, 569-578.
Dowding, C. H. and Pierce, C. E. (1994b). "Monitoring of Bridge Scour and Abutment Movement with Time Domain Reflectometrry." Proceedings, Symposium and Workshop on Time Domain Reflectometry in Environmental, Infrastructure, and Mining Applications, Northwestern University, U.S. Bureau of Mines Special Publication SP-19-94, 569-578.
Dowding, C. H., Su, M. B., and O'Connor, K. (1988). "Principles of Time Domain Reflectometry Applied to Measurement of Rock Mass Deformation." International Journal of Rock Mechanics, Mining Sciences, & Geomechanics Abstracts, (25), 287-297.
Dowding, C. H., Su, M. B., and O'Connor, K. (1989). "Measurement of Rock Mass Deformation with Grouted Coaxial Antenna Cables." Rock Mechanics and Rock Engineering (22) 1-23.
Francke, J. L., Terrill, L. J., and Francke, C. T. (1994). "Time Domain Reflectometry Study at the Waste Isolation Pilot Plant." Proceedings, Symposium on Time Domain Reflectometry in Environmental, Infrastructure, and Mining Applications, Evanston, IL, 555-567.
Freeman, E. L. (1996). "Time Domain Reflectometry at Cloverdale Landslide, U.S. Highway 101, Sonoma County, California." Technical Research Report CE-96-03, Department of Civil Engineering, University of the Pacific, Stockton, CA, 32 p.
Gwinnup-Green, M. D. (1996). "Monitoring of Embankment Stability Using Embedded Coaxial Cables." Technical Research Report CE-96-02, Department of Civil Engineering, University of the Pacific, Stockton, CA, 15 p.
Haramy, K. Y., and Fejes, A. J. (1992). "Characterization of Overburden Response to Longwall Mining in the Western United States." Proceedings, Eleventh International Conference on Ground Control in Mining, The University of Wollongong, N.S.W., 334-344.
Hasenfus, G. J., Johnson, K. L., and Su, D. W. H. (1988). "A Hydrogeomechanical Study of Overburden Aquifer Response to Longwall Mining." Proceedings, Seventh International Conference on Ground Control in Mining, West Virginia University, Morgantown, 149-162.
Hayward, M. M., Felderhof, S. C., and Hill, J. D. (1995). "Monitoring of Highwall Stability at the National Gypsum Mine, N.S. Using Time Domain Reflectometry." Final Report, Contract No. 23440-4-0163/01-SQ, Canada Centre for Mineral and Energy Technology, Ottawa, Ontario, 27 p.
Huston, D. R., Fuhr, P. L., and Ambrose, T. P. (1994). "Damage Detection in Structures Using OTDR and Intensity Measurements." Proceedings, Symposium on Time Domain Reflectometry in Environmental, Infrastructure, and Mining Applications, Evanston, IL, 484-493.
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.
Kane, W.F., and Beck, T.J., 1996, "Rapid Slope Monitoring." Civil Engineering, American Society of Civil Engineers, New York, p. 56-58.
Kane, W.F., Beck, T.J., Anderson, N.O., and Perez, H., 1996, "Remote Monitoring of Unstable Slopes Using Time Domain Reflectometry." Proceedings, Eleventh Thematic Conference and Workshops on Applied Geologic Remote Sensing, Las Vegas, NV, ERIM, Ann Arbor, MI, p. II-431-II-440.
Kawamura, N., Bauer, R. A., Mehnert, B. B., and D. J. Van Roosendaal (1994). "TDR Cables, Inclinometers and Extensometers to Monitor Coal Mine Subsidence in Illinois." Proceedings, Symposium on Time Domain Reflectometry in Environmental, Infrastructure, and Mining Applications, Evanston, IL, 528-539.
Logan, J. W. (1989). "Calibration of Time Domain Reflectometry Monitoring Cable in Granular Material." Report AFWL-NTE-TN-12-89, Civil Engineering Research Division, Weapons Laboratory, Kirtland AFB, NM, 34 p.
Look, B. G., and Reeves, I. N. (1992). "The Application of Time Domain Reflectometry in Geotechnical Instrumentation." Geotechnical Testing Journal, 15 (3), 277-283.
Lord, E., Peterson, D., Thompson, G., and Stevens, T. (1991). "New Technologies for Monitoring Highwall Movement at Syncrude Canada Ltd." Preprint CIM/AOSTRA 91-97, paper presented at CIM/AOSTRA 1992 Technical Conference, Banff, April 21-24, 97-1 to 97-8.
O'Connor, K. M. (1989). "Monitoring of Rock Mass Response to Solution Mining." Final Report, State Mining and Mineral Resources Institute, Grant No. 9C-BD-03-9-0000, New Mexico Tech, Socorro, NM, 26 p.
O'Connor, K. M. (1991). "Development of System for Highwall Monitoring Using Time Domain Reflectometry." Summary Report, U.S. Bureau of Mines Twin Cities Research Center, Minneapolis, MN, 75 p.
O'Connor, K. M., and Wade, L. V. (1994). "Applications of Time Domain Reflectometry in the Mining Industry." Proceedings, Symposium on Time Domain Reflectometry in Environmental, Infrastructure, and Mining Applications, Evanston, IL, 494-506.
O'Connor, K., Dowding, C. H., and Su, M.-B. (1987). "Quantification of Rock Caving Within Sinkholes by Time Domain Reflectometry." Proceedings, Second Multidisciplinary Conference on Sinkholes and the Environmental Impacts of Karst, Orlando, FL, 157-160.
O'Connor, K. M., Peterson, D. D., and Lord, E. R. (1992). "Development of a Highwall Monitoring System Using Time Domain Reflectometry." Proceedings, Ninety-fifth National Western Mining Conference, Denver, CO, 3 p.
Pierce, C. E., Bilaine, C., Huang, F.-C., and Dowding, C. H. (1994). "Effects of Multiple Crimps and Cable Length on Reflection Signatures From Long Cables." Proceedings, Symposium on Time Domain Reflectometry in Environmental, Infrastructure, and Mining Applications, Evanston, IL, 540-554.
Rohrig, J. (1931). "Location of Faulty Places by Measuring with Cathode Ray Oscillographs." Elektrotech Z., 8.
Schoenwald, J. S. and Beckham, P. M. (date unknown). "Distributed Fiber-Optic Sensor for Passive and Active Stabilization of Large Structures." Reference Unknown, 565-558.
Sisemore, C. J. and Stefani, R. E. (1971). "Rock Fracture Measurements: A New Use for Time Domain Reflectometry." Journal of Applied Physics, 42 (7), 2701-2710.
Steiner, J. P., and Weeks, W. L. (1990). "Time-Domain Reflectometry for Monitoring Cable Changes. Feasibility Study." Project RP2308-18, Electric Power Research Institute, Palo Alto, CA, 59 p.
Su, M.-B. (1987). "Quantification of Cable Deformation with Time Domain Reflectometry Techniques." Ph.D. Dissertation, Northwestern University, Evanston, IL, 112 p.
Topp, G. C., and Davis, J. L. (1985). "Measurement of Soil Water Content Using Time Domain Reflectometry (TDR): a Field Application." Journal of Soil Science Society of America, 49, 19-24.
U.S. Bureau of Mines (1995). "Early Detection and Technical Animation of Rock Movements Using Time Domain Reflectometry." Technology News, No. 449, April 1995, 2 p.
Zimmerman, B. D., Murphy, K. A., and Claus, R. O. (date unknown). "Local Strain Measurements Using Optical Fiber Splices and Time Domain Reflectometry." Reference Unknown, 553-558.
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
