APPLICATIONS OF TIME DOMAIN REFLECTOMETRY TO LANDSLIDE AND SLOPE MONITORING William F. Kane, Timothy J. Beck, and Jeremy J. Hughes KANE GeoTech, Inc., P.O. Box 7526, Stockton, CA 95267;
[email protected],
[email protected],
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ABSTRACT Time domain reflectometry (TDR) is now an accepted tool for slope and landslide monitoring. As yet, there are no accepted methods or procedures for installing cables or presenting data. Installations, including cable testers, cables, and grout types, vary significantly from job to job, and installer to installer. Applications have included both on-site data collection and remotely accessed systems. In an effort to add to the current knowledge base, this paper summarizes one group’s six years of work using TDR for slope and landslide monitoring. This paper describes case studies documenting TDR installation procedures and the development of software that work in given applications. Experience in California and Nevada included Tektronix 1502 and Campbell Scientific TDR100 cable testers, both as stand-alone field units and remotely accessed automated monitoring installations. Local installations included single and multiple TDR cables grouted in boreholes to locate failure planes and delineate the extent of moving areas. Cables were installed on the outside of inclinometer casings as well as grouted inside failed inclinometers in an attempt to gain additional data from existing boreholes. Remote data acquisition has used TDR to monitor slope movement. This included coupling TDR with additional instrumentation, such as electrolytic bubble inclinometers and vibrating wire piezometers. TDR also has been used to trigger an alarm based on the amount of cable deformation. Case studies and installation descriptions are included ranging from using TDR as an inexpensive tool to locate shear failure in a slope to sophisticated automated monitoring system.
INTRODUCTION
One of the new developments in landslide monitoring during the 1990's was the use of time domain reflectometry (TDR) to monitor slope and landslide movement. TDR measures the strength reflections of a voltage pulse traveling along a coaxial cable. Originally developed around 1930 to find faults in transmission and power cables, TDR was used in the 1980's to find the limits of caving zones in longwall coal mines. It was not until the mid-1990's that widespread use and general acceptance of this tool for monitoring slope movement took place. The principal advantage of TDR over other methods of slope monitoring is that it is a quick and economical means of determining the extent of movement of an earth mass. In addition, the digital nature of the data makes it possible to transmit the information by telemetry allowing remote monitoring. TDR can be easily combined with other electronic instrumentation such as rain gages, vibrating wire piezometers, extensometers, and electrolytic bubble inclinometers to obtain a wide range of information about a particular slope without the necessity of physically traveling to the location. In addition, the entire process can be automated to record and collect data at specific times and intervals. Following are case studies describing various applications of TDR monitoring installations. Application case studies are divided into field applications using physical on-site data collection and remote installations where TDR signatures are collected automatically and downloaded to a monitoring station. TIME DOMAIN REFLECTOMETRY (TDR) Basic Principle of TDR Radar is a form of time domain reflectometry (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. 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. The U.S. Bureau of Mines began research with TDR in the 1960's for locating electrical faults in power cables and extended the technology to ground control problems (O’Connor and Wade, 1994). Time domain reflectometry (TDR) is a relatively new approach to monitoring slope movement (Kane and Beck, 1996; Mikkelsen, 1996; O’Connor and Dowding, 1999). The cable tester sends an electrical pulse down a coaxial cable grouted in a borehole, Figure 1. When the pulse encounters a break or deformation in the cable, it is reflected. The reflection shows as a “spike” in the cable signature, Figure 1. The relative magnitude and rate of displacement over time, and the location of the zone of deformation can be determined immediately Figure 1. Schematic of TDR cable tester operation.
and accurately. The size of the spike increase correlates with the magnitude of movement, Figure 2. A laptop computer is connected to the tester and cable signatures are written to disk for future reference.
Delta Island Levee 5 0
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Equipment and Software
Depth (Feet)
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TDR cables generally are read using one of two cable testers: the Tektronix 1502B/C or the Campbell Scientific TDR100. The Tektronix 1502B/C is a heavy, relatively large, rugged cable tester which has a screen to allow viewing of the cable signature. The signature is digitized and transferred to a laptop computer data file in ASCII format using Tektronix SP232 software. SP232 also allows the file to be recalled for viewing. Two signatures can be compared using SP232. The data can be rearranged to plot it using a conventional spreadsheet, or plotted using a program such as TDRPlot2000 (KANE GeoTech, Inc., 2001). Figure 2 was plotted using a Tektronix 1502B, laptop computer, and TDRPlot 2000.
-20 -25 -30 Legend A - 22 May 96 B - 14 Jun 96 C - 08 Oct 96 D - 17 Jan 97 E - 30 Oct 97
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Figure 2. Increased TDR signature spike and broken cable with continued slope movement. Cables show shear zone in California Delta levee (see text).
The Campbell Scientific TDR100 is a small lightweight cable tester designed to work with a Campbell Scientific CR10X datalogger in remote automated data acquisitions systems. However, the TDR100 can be incorporated into a field system by placing it in a weatherproof case with a rechargeable battery system. This type of field unit is available commercially. Data output for field monitoring consists of tabular ASCII digitized cable signatures. The output from the remote application is standard datalogger comma-delimited ASCII. This data can be easily plotted using a spreadsheet or with TDRPlot 2000. Installation Depending on the application and characteristics of the site, two types of coaxial cables can be used: flexible RG59/U cable television cable or stiffer, higher quality, foam-filled communications cable. RG59/U cable is relatively inferior in quality and suffers from signal attenuation. This is seen in as a slight angle of the cable signatures to one side of the plot. In addition RG cable is elastic and may relax over time making the signature spike appear smaller. For these reasons, it is not recommended for very deep holes, greater than 100-ft, or long-term monitoring. However, as long as its limitations are understood, it can be used in these applications. In each of the following case studies, one or both types of TDR cables were grouted in boreholes using a 10% bentonite/90% cement or 100% lean cement grout. ON-SITE TDR DATA COLLECTION TDR data can be collected in a manner similar to inclinometer data, that is, personnel can visit the site and
interrogate the TDR cable, saving the data to a file. This approach still has significant advantages over using an inclinometer. A coaxial cable, no matter how deep can be read in just a few minutes as opposed to 45 minutes for the shallowest inclinometers and hours for very deep ones. Another advantage is that deformation of the cable, indicating slope movement, can be viewed immediately rather than waiting until data has been downloaded from a logger and plotted on computer. Levee Embankment San Joaquin/Sacramento River Delta, California Numerous islands in the California Delta, at the confluence of the San Joaquin and Sacramento Rivers are below sea level. To protect property from flooding, a system of levees has been developed. The soft peat soil beneath the levees causes them to settle differentially and become unstable so that small slides develop inward toward the island. Repair is done by compacting soil on top, and buttressing the sides of the levee. This has the effect of further contributing to instability. In an area that was undergoing slope failure, three cables were installed in boreholes at the top, middle, and bottom of the levee. The purpose of the monitoring was to determine the location of the shear zone so that a buttress repair could be completed above the toe of the slide. The cables were monitored at intervals and the location determined. Two sections of the levee showed signs of distress including tension cracking and the formation of small scarps in the levee roadway. Settlement of the roadway was as much as eight inches. Borehole data indicated that the levee profile at this location consisted of 15 feet of medium silty clay. Foundation soils were composed of 10 feet of peat underlain by 7 feet of soft clay followed by a very stiff clay. Before initiating levee repair, it was necessary to determine the extent of the failure mass. To delineate the mass required at least three monitoring holes as shown in Figure 3. To reduce installation and monitoring costs, TDR cables were installed and run in trenches to a monitoring location on the roadway shoulder. In this way, the boreholes could be read quickly from one location. Six inexpensive RG59/U coaxial cables with twist-on BNC connectors at the ends were installed. Cable lengths ranged from approximately 40 feet at the levee crown to 20 feet on the inboard toe. All cables were grouted in a weak cement grout. A Tektronix 1502B cable tester was used to collect data using Tektronix SP232 software. TDR signatures were plotted using TDRPlot2000 and signatures arranged to obtain the plot shown in Figure 2.
Figure 3. Relative locations of TDR cables for levee monitoring.
TDR monitoring showed that only the cable nearest the roadway in both locations was undergoing slight deformation. Therefore, the failure plane was relatively shallow and could be repaired with an earth buttress.
15 Legend A - 02 Mar 00 B - 21 Mar 00 C - 20 Apr 00
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Small Landslide in Golf Course – San Francisco Bay Area
Large Landslide in Development – San Francisco Bay Area Although not generally recommended because of attenuation problems, RG59/U cable can be used in deeper holes, especially when the long-term data outlook is not promising.. Earth moving operations in a valley resulted in a large relatively fast moving landslide. Inclinometer casing and RG59/U TDR cables were installed in boreholes. The slide quickly rendered the inclinometers unuseable. Figure 5 shows the TDR cable signatures for one of the cables. They were collected using a Tektronix 1502B cable tester. Note the relative rapid development of the TDR signature spike. In this case, movement of the slide was so rapid that the flexible cable had no time to relax. The pronounced slant to the cable signatures is due to attenuation over the relatively deep, 170-ft, cables.
Depth (Feet)
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Figure 4. TDR signatures over time showing cable deformation due to landslide shear plane.
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A small slope failure adjacent to the fairway of a golf course needed repair. Budgetary constraints precluded the installation and labor intensive use of conventional probe inclinometers. Instead two RG59/U TDR cables were installed, one in the middle and one at the base of the slide. Cables were read using a Tektronix 1502B cable tester. Figure 4 shows the TDR signatures over time. The lower limit of the slide is clearly seen at 48-ft.
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Figure 5. Deep RG59/U cables in fast moving landslide.
Routine Landslide Investigation – San Francisco Bay Area
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Cable Connectors landsliding and instability. It was desired to locate the shear zone under difficult geologic conditions. The ore body was situated beneath soft volcanic tuff sediments and a thick, brittle welded tuff. The actual shear zone was believed to be RG59/U TDR Cable PVC Casing located in the tuff sediments. However, large scale movement of the welded tuff and the deep nature of the shear zone made inclinometers impractical. In addition, it was impossible to position a drill rig Cement Grout close to the head scarp because of large tension cracks in the crest. Consequently, FLC12-50J TDR Cable angled boreholes were required, some up to 300 m. Figure 8. TDR installation for Nevada gold mine. Upper portion FLC12-50J cable is protected from slope The deep holes, angle borings and the movement by annular space. RG59/U cable used to large number of readings that would be required, made inclinometers impractical. monitor for upper slope movement. Six TDR cables were installed initially to monitor continued movement of the pit wall. Cables were read every day to determine locations of deformation. No data was stored. Another problem at this site was due to the fact that movement of the upper welded tuff would pinch off any inclinometer or cable before a the smaller movements in the tuff sediments member could be monitored. A system was devised to grout a ½” FLC12-50J foam-filled coaxial cable into the tuff sediments and protect it in the upper part of the hole with an annular space formed by 150 mm plastic pipe casing in the welded tuff. This is shown in Figure 8. The casing could take up movement in the welded tuff before movement would affect the cable. A flexible RG59 cable was attached to the outside of the casing to monitor casing deformation. Using TDR with Inclinometer Casing – Santa Clara County, California Many users of TDR prefer to install a TDR cable, usually a flexible RG cable, to the outside of an inclinometer or piezometer. The California Department of Transportation (Caltrans) routinely uses this method for a large number of its inclinometer installations. The effect of this approach is to reduce the sensitivity of the TDR cable to movement, but to allow slope monitoring to continue when the inclinometer has been sheared off. Figure 9 shows the inclinometer reading for a large landslide in the Coast Range of California, mostly in sandstones of the Franciscan formation. The inclinometer was installed with an RG59/U TDR cable attached to the outside. Slope movement eventually rendered the inclinometer unreadable at 52-ft. Figure 10 is the plot of TDR signatures for the cable obtained with Tektronix 1502B and Campbell Scientific TDR100 cable testers. No movement was detected until after the inclinometer was sheared. Additional movement was also picked up at 18-ft long after the inclinometer was abandoned.
AUTOMATED REMOTE TDR DATA ACQUISITION TDR is very effective when combined into a system with other geotechnical instruments and a datalogger. It is relatively easy to program dataloggers to trigger alarms for a broken or deformed TDR cable. Telephone lines, cellular telephones, and radios can be used to communicate with the system. TDR/Inclinometers – Monterey County, California
Soon after installation, slight movement of the inclinometers triggered the telephone dialer and personnel were paged. TDR cable readings showed the development of a spike in the cable at a depth of 9-m indicating movement, Figure 11. Observation of tension cracks in the ground surface verified the fact that movement had taken place.
Depth (Feet)
Numerous slides along California Highway 1 in San Luis Obispo and Monterey Counties closed portions of the road throughout the winter of 1998. One slide, known as Grandpa’s Elbow landslide, in Monterey County was a reactivation of an older, much larger landslide Figure 9. Probe inclinometer prior to shearing at complex. To protect motorists and clean-up 52-ft. crews, the California Department of Transportation (Caltrans) instrumented the slide with four downhole, electrolytic Crothers Road Landslide Santa Clara County, California inclinometers attached to a FLC12-50J TDR TDR-4 cable in a 61 m borehole. The 10 inclinometers were placed at the 46 m, 31 B C F E F G D 0 m, 15 m, and 3 m. Any movement of the slide changed the tilt of the inclinometers -10 and the Campbell Scientific CR10X -20 datalogger triggered a warning by phone dialer and hard-wire telephone line. The -30 system could also be monitored remotely by -40 computer and modem.
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Figure 10. TDR signatures from cable attached to the outside of inclinometer casing. Compare with Figure 9.
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TDR/Piezometers/Rain Gage, Morgan County, Illinois
Grandpa's Elbow Landslide 20
Inclinometers/TDR, Orange County, California (1)
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In an effort to determine climatic conditions during slope movement, the Illinois Department of Transportation installed a remote monitoring system adjacent to a highway near Jacksonville, Illinois. The system incorporated five FLC12-50J TDR cables and five vibrating wire piezometers to monitor for ground movement and groundwater levels respectively. In addition a tipping bucket rain gage was installed to record rainfall data, and correlate it with groundwater levels. A CR10X datalogger was used. Data collection was by manually hooking a laptop computer to the datalogger and downloading the data. The system was powered with a rechargeable battery and solar collector.
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Legend A - 4 Apr 98 B - 23 Jun 98
The developer of a coastal property was cautioned by its -220 0 15 30 45 60 75 90 105 120 135 150 attorneys that slope movements adjacent to the property Relative Reflectance (millirhos) could pose a concern. The nearby city of Laguna Niguel, in Figure 11. TDR signature spike indicating particular, experienced severe damage from El Niño storms. This included a spectacular failure of a 38-m slope movement at 9-m (30-ft). which destroyed several condominium buildings at the base and caused a number of homes at the crest to plummet down the head scarp. The site, currently under development, contained some weak colluvial material as well as a remnant landslide. Construction plans called for removal of the weak material and construction of an engineered fill shear key. This meant the use of a temporary excavation with steep slopes. To alleviate fears of litigation by adjacent property owners, and to protect workers and property during construction, an ambitious remote monitoring plan was established. This included the installation of six 33-m conventional cased inclinometer borings and five 33-m FLC12-50J TDR. In addition, each of the six inclinometer casings had a removable electrolytic bubble inclinometer installed to monitor for movement between readings. Data was acquired daily and stored by a datalogger attached to a cellular phone for communications. Alarms were set for each instrument to warn of movement between scheduled automatic readings. A Tektronix 1502B cable tester, modified for automated data collection and a Campbell Scientific CR10X datalogger were used as part of the system. Communications were via cellular telephone. Power was supplied by a solar panel and rechargeable battery. Inclinometers/TDR, Orange County, California (2) Another installation in Orange County used a similar set-up as above. This time two FLC12-50J TDR cables were installed in boreholes. Each cable had two inclinometers grouted in place adjacent to a road where a previous landslide had occurred. One inclinometer was installed above a suspected slide plane and the other below. A CR10X datalogger was used but coupled with a Campbell Scientific TDR100
cable tester. Communications were via a hardwire telephone. A solar panel and rechargeable battery were used to power the unit. TDR/Inclinometers/Piezometers/Rain Gage, San Francisco Bay Area, California A large landslide threatened several residences on a ridge underlain by the Franciscan formation. An ambitious monitoring system was developed. It included two electrolytic tiltmeters attached to two structures close to the head scarp, two electrolytic bubble tiltmeters in inclinometer casings near the scarp, and two tiltmeters attached to a soldier pile retaining wall located at the base of the slide. In addition, three TDR cables were attached to inclinometer casings. To monitor groundwater and rainfall, two vibrating wire piezometers were installed in standpipes and a rain gage mounted near the instrumentation enclosure. A third piezometer was used to determine barometric pressure to correct piezometer readings. A solar collector and rechargeable battery supplied power to the system. SUMMARY TDR is an excellent tool for slope monitoring when used properly. TDR cannot reliably provide the amount of slope movement a slope is undergoing. However, it has many uses outlined in this paper. For example, 1. TDR can be used to economically monitor soil embankments. Time can be saved by routing all cables to a central location for data collection. 2. Inexpensive RG59/U cable can be installed as part of routine landslide investigations and can accurately determine the location of slide planes. 3. Coaxial cables can be attached to the outside of inclinometer casing and provide accurate monitoring information as well as extend the life of the monitoring hole. 4. TDR can be used with other electronic instrumentation as part of an automated remote system to monitor many phases of a landslide, including the use of alarms to warn of movement. REFERENCES Kane, W.F., and Beck T. J. 1996a. Rapid slope monitoring: Civil Engineering, 666:56-58. New York: American Society of Civil Engineers, 666, 56-58. KANE GeoTech, Inc.. 2001. TDRPlot 2000 user’s manual: KANE GeoTech, Inc., Stockton, California, 8 p. Mikkelsen, P.E. 1996. Field instrumentation, in A. K. Turner & R. L. Schuster, Landslides. investigation and mitigation. Washington: Transportation Research Board. O’Connor, K. M., and Dowding C. H. 1999. Geomeasurements by pulsing TDR cables and probes: Boca Raton, Florida: CRC Press. O’Connor, K. M., and Wade, L. V. (1994). Applications of time domain reflectometry in the mining industry: Symposium on time domain reflectometry in environmental, infrastructure, and mining applications, U.S. Bureau of Mines, 494-506.
