Monitoring Deformation in Rock and Soil with TDR Sensor …...bles will indicate the location and mag-nitude of subsurface shear strain. TDR sensor cables are especially sensitive

Monitoring Deformation in Rock and Soilwith TDR Sensor CablesPart 1. Concept and Case History

Charles H. DowdingMatthieu L. DussudWilliam F. KaneKevin M. O’Connor

Historical BackgroundTime Domain Reflectometry (TDR) isa remote sensing electrical measure-ment technique that has been used formany years to determine the spatial lo-cation and nature of various cablefaults. In the 1950s TDR technologywas adapted to locate and identifyfaults in power and communication ca-bles. As a result, TDR cable testers areconsidered standard equipment inthese industries. In the 1970s TDRtechnology began to be applied togeomaterials and has been adapted foruse by soil scientists, agricultural engi-neers, geotechnical engineers and en-vironmental scientists. This articleconcentrates on the geotechnical appli-cation of monitoring subsurface defor-mation in soil. If there is sufficient in-terest, future articles in GIN couldfocus on use of TDR for monitoringmoisture content and pore water pres-sure.

TDR Concept and CableInstallationIn concept, TDR is similar to radaralong a cable. As shown in Figure 1b, a

voltage pulse, produced by a TDRpulser, travels along a two-conductorcoaxial metallic cable until it is partiallyreflected by deformation of the cable.The distance to the deformation can becalculated knowing the propagation ve-

locity of the signal in the cable and thetime of travel of the voltage pulse fromthe disruption to the cable tester. Asshown in Figure 1a, a cable is groutedinto a borehole, then rock or soil move-ment shears the grout and deforms the

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Geotechnical News, June 2003

Figure 1. Shearing mechanism and induced reflection on a grouted TDR sensorcable

cable, which changes the geometry(thus impedance) between the innerand outer conductors. This change inimpedance produces the reflected volt-age pulse shown in Figure 1c. Thetravel time of the reflected pulse deter-mines the location of the shearingzone. The amplitude of the voltage re-flection is proportional to the amountof cable deformation that is correlatedwith the rock or soil movement.

Initially, TDR was geotechnicallyapplied to monitor rock mass deforma-tion, which occurs predominantlyalong joint interfaces (Dowding et al.,1988). The large stiffness of rock andthe high degree of strain localizationalong rock joints allow installation ofstiff cable with standard drilling andgrouting procedures. As a result, thetechnique has been adopted worldwideby the mining industry.

At the opposite end of the spectrumof geomaterials, the low stiffness of softsoil and the relatively small strain local-ization in the early stages of failure insoft soils, complicate the application ofTDR technology. For TDR to be effec-tive in soil, a shear band must occur toproduce the localized strain necessaryto locally deform the cable. Deforma-tion occurring along a shear band in soilmust be transferred to the cable through

the grout. Thus, the compositesoil-grout-cable must faithfully transferthe relative soil displacement to the ca-ble. Ideally the grout should be no morethan 5 to 10 times stronger than the sur-rounding soil (Blackburn, 2002). Agrout that is too strong may not fail withthe soil and thus smears or widens theshear band, whereas a grout that is tooweak will not kink or distort the cable.

A coaxial cable consists of a solidcore (inner conductor) and a cylindricalshield (outer conductor), separated by adielectric such as foam polyethylene.As shown in Figure 2, two main types ofcoaxial cables are recommended forTDR application. Bare solid aluminumor copper outer conductor cable are themost common types; however, morecompliant copper braid outer conductorcables are also being developed for usein soft soils (Cole, 1999). At this time,the stiffer cables are commerciallyavailable while the compliant cable isunder development and fabricatedmanually in short lengths.

Grout for TDR cable installation istypically a lean cement mix with thebentonite and water content adjusted toachieve various compressive strengths.Ideally it's viscosity should be lowenough to be pumped with a drill rigwater pump, but it is common to use a

grout pump. The viscosity can be re-duced (f luidi ty increased) byintroducing additives such as IntrusionAid R, which also acts as an expansionagent to reduce shrinkage. Refer toMikkelsen (2002) for an excellent dis-cussion of grout mixing procedures andstrength, as well as field crew errors inusing grout mixes with higher watercontent and bleeding.

For best results, the cable should beinstalled in its own dedicated boreholeand the grout must be strong enough toshear the cable, but weak enough to befailed by the surrounding soil, (Pierce,1998). For installation in rock, this rela-tive strength and stiffness considerationis not important because of the rela-tively high strength and stiffness ofrock. In order to maximize cable/groutcomposite sensitivity in soil, it has beenhypothesized that the shear capacity ofthe grout should be less than the bearingcapacity of the soil just outside the lo-calized shear plane. This may be as highas 5 to 10 times the shear strength of thesoil. Installations in soft natural soilsand fills indicate the need to carefullycalibrate the stiffness of the grout withthe soil. More research is needed in thisregard.

Deformation ModesCrimping and localized shearing of a co-axial cable will produce a distinct TDRreflection spike such as the one in Figure1c. If the cable is severed by shear, thereis a large positive reflection immediatelyfollowing the negative spike.

If the cable is simply cut off with asaw or severed in tension, there will notbe a negative spike preceding the largepositive reflection. Consequently, incases where TDR has been used to mon-itor strata movement in mines it hasbeen possible to determine if the strataseparate in extension or shear at jointsor rock mass discontinuities. It has alsobeen possible to quantify the tensile de-formation by monitoring changes indistance between crimps made in thecable prior to installation in drill holes(O’Connor and Dowding, 1999).



Figure 2. Two most common types of coaxial TDR sensor cables.

Correlation Between TDRReflection Magnitude andInclinometer Displacements inSoilTDR technology provides a method ofdeformation measurement that can beemployed as a complement to, andcomparison with, inclinometer mea-surements. The two technologies havedifferent advantages and disadvantages.For brevity, the present discussion con-centrates on the issue of localized shear-ing.

Inclinometers and TDR sensor ca-bles respond differently when subjectedto localized shearing. TDR sensor ca-bles are most sensitive to highly local-ized shear, and have been foundespecially useful in rock where defor-mation occurs along thin joints. On theother hand, inclinometers are more sen-sitive to general shear or gradualchanges in inclination. Localized shear-ing of inclinometer casing causes it tokink so it cannot be profiled with an in-clinometer probe. Thus in situations in-volving both general shear andlocalized shear, the two technologies re-spond differently. These differenceshave been documented for four cases in“Comparison of TDR andInclinometers for Slope Monitoring”(Dowding and O’Connor, 2000).

There are two alternative methods ofevaluating inclinometer response: 1) to-tal displacement or deformation profileof the casing, and 2) incremental dis-placement or slope of the deformationprofile. Dowding and O’Connor (2000)compared inclinometer incremental

displacement (IID) with TDR reflectionmagnitude. IID is also the inclination ofthe inclinometer probe, and therefore ameasure of the local shear strain.

The difference in response of thesetwo approaches results from the spanover which relative displacement ismeasured. IID is the change in angulardisplacement every 60 cm (2 ft) whichis the wheel-base of the standard incli-nometer probe. Thus a IID of 1 mm over60 cm (0.04 in. over 24 in.) is a slope orshear strain of 0.0017. However, thisshear strain is averaged over a distanceof 60 cm (24 in), which is a fairly largegage length when measuring localizedshear within a discrete plane or shearband.

Conversely, the sensitivity of TDRsensor cables decreases as the shearzone increases from a thin band to alarge mass undergoing general shear.O’Connor et al. (1995) reported that re-flections decline by a factor of 2 whenthe thickness of a shear zone in the labo-ratory was increased from 1 mm to 40mm, and declined by a factor of 20when the shear zone thickness was in-creased to 80 mm. Thus these datacould be interpreted to imply that theTDR sensor responded optimally to lo-calized shear zones with thickness of1/100 to 1/10 times the gage length ofan inclinometer.

Example Comparison: LandfillSlope DeformationA case history involving slope move-ment that occurred in an industrial land-fill provides a useful comparison be-

tween inclinometer and TDR responsein soil. The slide mass was some severalhundred meters long and tens of metershigh. As shown by the soil profile inFigure 3, the landfill rests on a very thinlayer of silt and sand which is underlainby 9 to 12 m of soft, glacial lake clay,and a lower stiffer clay.

In accordance with standardgeotechnical practice, inclinometersand piezometers were installed to de-fine the extent of the slide mass and as-sess the effective stress within thefailure “plane.” As a field trial of TDRtechnology to detect and quantify shearwithin soft clays, an aluminum outerconductor coaxial cable was installed ina separate borehole 35 m from aninclinometer.

The lower bulge in the IID profile atthe right of Figure 3 indicates 3 mm ofincremental subsurface deformationwithin a shear zone at a depth of approx-imately 30 m within the soft to mediumstiff clay layer. This depth correspondsto the zone of maximum total displace-ment adjacent to the IID. As shown bythe 07/10/98 TDR record, there is a 5mrho reflection spike just below the dis-tance-calibration crimp at a depth of 22m. This is the interface between the fillmaterial and the underlying soft claylayer. A year later, TDR reflectionspikes appeared at depths of 28 m and31 m. These reflections are locatedwithin the bulge in the IID profile be-tween depths 27 m and 37 m. The largeIID at 8 m depth may correspond with asliding block boundary that did not in-tersect the TDR cable as the



Figure 3. Comparison of TDR sensor cable and Inclinometer Response in Soft to Medium Stiff Glacial Lake Clay

inclinometer and TDR cable areseparated by 35 meters.

These field measurements indicatethat abrupt changes in shear strains atthe boundaries of thick shear bands insoft to medium clay with large relativedisplacements will produce TDR sen-sor cable response. TDR sensor cableresponse and subsequent computermodeling (Blackburn, 2002) indicatethat shearing is sufficiently large at thisboundary to cause a TDR reflectionspike at each boundary of the localizedshear. The responses at 28 m and 31 min Figure 3 may define the thickness ofthe failure zone at the bottom of the slid-ing mass. This observation is not incon-sistent with that of O’Connor et al.(1995) whose laboratory data were ob-tained with no confinement of the groutbetween the laboratory shear rings. Inthe field the grout is confined by the soilin the shear zone, which would changethe deformation regime considerably.

SummaryBoth inclinometers and TDR sensor ca-bles will indicate the location and mag-nitude of subsurface shear strain. TDRsensor cables are especially sensitive toshear in rock, or in soil at locations ofhighly localized shear strains. On theother hand, inclinometers are especiallysensitive to gradual, general shear andrespond to early stages of plastic defor-mation in soils undergoing generalshear. TDR sensor cables may also re-spond at abrupt changes in shear strainat the boundaries of thick localizedshear zones.

The case presented here illustratesthat TDR sensor cable can be used to lo-cate and quantify localized shearing insoft soil, at least when the deformationsare large. Other cases (Dowding andO’Connor, 2000) demonstrate thatTDR sensor cables have detected defor-mation at locations where inclinom-eters did not detect deformation and

vice versa. These differences do not im-ply that either method is more correct,but the two methods respond optimallyto different degrees of shear localiza-tion. The real challenge is to explainthese different responses moreprecisely.

TDR sensor cables provide anotherinstrument to supplement and/or verifysubsurface deformation measured byinclinometers. One approach that hasbeen adopted, combines the technolo-gies by installing TDR cables and incli-nometers in separate holes and remotelyinterrogating TDR cables using an auto-mated data acquisition system con-nected to a phone or radio modem.When the TDR cable indicates thatmovement has occurred, an independ-ent measurement is then made by profil-ing the inclinometer casing.

Monitoring Deformation in Rock and Soilwith TDR Sensor CablesPart 2. Lessons Learned Using Time DomainReflectometry

Charles H DowdingMatthieu L. DussudWilliam F. KaneKevin M. O’Connor

IntroductionListed below are the top TDR sensor ca-ble installation and communication les-sons learned from installations byNorthwestern University, KANEGeotech Inc., and GeoTDR Inc. Instal-lations involved a wide range of situa-tions that called for TDR monitoring ofthe deformation of:• Bridge piers and abutments• Landfills & embankments• Rock/soil masses (sinkhole and min-

ing-induced deformation)• Excavations in soft soils



Figure 1. Installation of horizontal TDR sensor cable in a grouted trench over astabilized sinkhole

Top 11 “TDR Sensor CableInstallation” Lessons

1. Monitoring Large Surface Areas

Long (> 300m) TDR sensor cables canbe installed horizontally beneath/besidehighways, above mines, near land-slides, etc. to monitor more surface areawith fewer cables. Installation has beenaccomplished both by trenching as wellas horizontal boring. Figure 1 shows theinstallation of a horizontal 36 m (120 ft)long TDR sensor cable in a shallowgrouted trench parallel to a road sub-jected to sinkhole subsidence. Detailedinformation about this project can befound at:http://www.iti.northwestern.edu/tdr/operational/florida.

2. Monitoring at Great Depth

As shown in Figure 2, deep (> 500m)vertical TDR sensor cables are being in-stalled to monitor mine-induced defor-mation at great depths (O’Connor andWade, 1994).

3. Solid Aluminum Outer ConductorCoaxial Cable

The current preferred cable for installa-tion in rock and stiff to medium stiff soilis the 75 Ohm, 22 mm diameter, barealuminum outer conductor, foam poly-ethylene dielectric cable (CommScopeParameter III 875 or equivalent). In or-der to investigate the sensitivity of amore flexible cable in soft soil a compli-ant cable was made by Cole (1999) bystripping the solid aluminum outer con-ductor from a cable. The exposed poly-ethylene foam was fitted with a flexiblecopper braided outer conductor. Studiesare continuing to assess the relativestrength and stiffness of similar, moreflexible cables. Photographs of the ca-bles are in Figure 2, Part 1.

4. Cables Installed in DedicatedBoreholes

TDR sensor cables must be installed intheir own hole especially in soil. Strap-ping flexible cables to inclinometercasing degrades TDR sensitivity formonitoring soil deformation. Local-ized shearing response is reduced by thestiffening provided by the grouted incli-nometer casing. Figure 3 comparesthree installation geometries. The



Figure 2. Cross section of the installation of a deep TDR sensor to monitor mine-in-duced deformation above a long wall coal mine

Figure 3. Comparison of geometrics of a TDR sensor cable in its own hole (recom-mended) and TDR sensor cable strapped around an inclinometer casing (not recom-mended).

Figure 4. Comparison between response of inclinometer and strapped TDR sensorcable

leftmost two geometries are too oftenchosen to save the cost of two holes (onefor the inclinometer and another for theTDR sensor cable) and are not recom-mended.

While not recommended, some re-sults may be obtained in rock by strap-ping TDR sensor cables outside ainclinometer casing. Figure 4 comparesthe response of such an installation in alandslide that occurred in the CaliforniaCoast Range in heavily sheared andbroken Franciscan sandstone. It wasparticularly fast-moving and the incli-nometer casing became kinked at 15.8m (52 ft). The inclinometer probe couldnot be lowered below this depth afterFebruary 10, 2000. The TDR sensor ca-ble (RG50/U), however, remained us-

able for some months afterward. TheTDR sensor did not show a reflectionuntil at least April 11, 2000 after a sig-nificant amount of movement occurredin the inclinometer casing. Because theTDR extended the usable life of thehole, it was able to detect an additionalshear displacement at a depth of 5.5 m(18 ft) seven months after the inclinom-eter casing had been abandoned.

5. Retrofit Kinked InclinometerCasing

Assessment of the response of cables in-stalled in kinked inclinometer casing inrock indicates that TDR sensor cablescan also extend the useful life of existinginclinometer instrumentation holes.Such retrofitting shown in Figure 5, al-lows continued monitoring deformationof critical structures without the need todrill additional holes. Solid aluminumouter conductor cable must be used and,in rapidly moving rock or soil, the cablemust be installed relatively soon after theinclinometer casing has been kinked toensure that the cable can be inserted pastthe kink in the casing. Pushing the cablepast the kink has been a problem whenusing flexible coaxial cable.

6. Pumping Grout

Specialized low strength cement benton-ite grout mixtures shown in Figure 6have been employed for TDR installa-tion in soil (Aymard, 1996 and Will,1997). For least installation cost, theyshould be able to be tremmie pumpedwith the drilling rig’s water pump. Atfirst these mixtures appear to be moreviscous than the higher strength cementonly mixtures. But low viscosities can beproduced by the addition offluidizing/expansion agents such as In-trusion Aid R. The fluidity achieved willhave to be demonstrated to the drillingcrew, who may not wish to pump it forfear of blocking their pump. A separategrout pump can also be used for cable in-stallation. Refer to Mikkelsen (2002) foran excellent discussion of grout mixingprocedures and strength, as well as fieldcrews errors in using grout mixes withhigh water content and bleeding.

7. Installation Using Hollow StemAugers

Installation with hollow stem augers



Figure 5. Installation of TDR sensor ca-ble in sheared inclinometer casing ex-tends the life of the monitoring borehole

Figure 6. Principal mix proportions of cement-bentonite-water system (Aymard,1996 and Will, 1997)

Figure 7. Plot of model shearing sensitivity of stiff and special braided flexiblecable that shows there is an optimal grout strength (Blackburn, 2002)

may lead to degradation of responsethrough two mechanisms. First, groutslumps into the large void left as the au-ger is extracted. Unless a sufficient headof grout is maintained in the auger as itis extracted, voids will exist between thecable and hole walls. Extra grout at ahigher head should be available to fillthe large annulus created as the auger isextracted. Secondly, extraction of theauger will disturb the soil around theTDR sensor cable (Dussud, 2002).

8. Soil-Grout-Cable Interaction

Use of TDR sensor cables in soft soil re-quires special cement-bentonite groutmixes with prehydrated bentonite andfluidizing agent which should be care-fully designed to match the soil proper-ties. The grout must be stiff enough tokink the cable, but not so stiff (strong)that it resists localized soil shearing.Special low loss, flexible cables will al-low use of low strength grouts in softsoils. Model results in Figure 7 showthat shearing sensitivity of stiff and spe-cial flexible grouted coaxial cable is op-timal at a ratio of grout to soil strengthof 1 to 5 (Blackburn, 2002). Shearstresses in the more compliant braidedcable are closer to the critical value,which is the model shear stress associ-

ated with the first appearance of a TDRvoltage reflection. More research willbe needed to determine optimal groutmixtures.

9. Sealing and Insertion of Cables

Appropriate techniques for cable inser-tion are dependent upon cable stiffness.Flexible cables have been inserted byattaching to the cable tip a plastic coneas shown in Figure 8 in order to catchthe stiff flush coupled PVC grout pipeas it is pushed in the hole. Alternatively

the bottom of stiff solid aluminum co-axial cables can be fitted with a me-ter-long section of PVC or steel pipe(acting as a stiffener/strengthener) andthen pushed down the hole. Before in-sertion, the bottom end of the TDR co-axial cable must be sealed to prevent in-trusion of water between the inner andouter conductor.

10. Installation using CPT Rig

As shown in Figure 9, 12.5 mm diame-ter FLC12-50J cables have been in-



Figure 8. View of the sealed end tip of a flexible TDR sensorcable also fitted with a plastic cone to catch the PVC grouttube for insertion

Figure 9. Insertion of TDR sensor cable in CPT rods after at-taching special disposable tip

Figure 10. Protective enclosure for connection between a sensor and aconnecting cable

serted in soft soil with cone penetrom-eter equipment (CPT). Afterdetermination of stratigraphy the CPTrods are reinserted and the cable placedinside. A special tip is machined for thecable and left in place as the rods arewithdrawn. The hole is grouted whileextracting the rods. Such technique wasused in a landslide in Orange County,California to install a 25 meter deep ca-ble. There are many situations in softsoils, such as investigation of levee sta-bility, where the CPT method workswell.

11. Crimping and Connectors Details

Miscellaneous details include: 1) mak-ing distance-calibration crimps whilelowering the cable to avoid accidentalkinking at the crimp during installationand 2) ensuring top-of-hole connectorsare moisture-proofed and placed in alocked protective cover shown in Figure10 (Dussud, 2002).

TOP 8 “TDR Instrumentation”Lessons

1. Integration of Data AcquisitionComponents

PC based data acquisition systems(DAS) with off-the-shelf componentsshould be avoided because of integra-tion and reliability problems. Reliable,rugged systems should be employedsuch as those offered by Campbell Sci-entific Inc. combining a TDR 100pulser and a CR10X datalogger. Theseinstruments, shown in Figure 11, alsohave relatively low power consumption,which is an advantage for operation atremote sites.

2. Alarm Call Capability

Automated surveillance of remote sitesfrom a central polling computer (pas-sive monitoring) as well as callbackalarm notification from remote sites(active monitoring) has been success-ful ly implemented with TDR(O’Connor et al, 2002). Figure 12shows a typical DAS equipped with analarm autodialer.

3. Web-based data Display

Autonomous posting of TDR wave-forms over the internet on a daily basis

has been successfully implemented formonitoring of deformation of multiplecables at multiple locations. Examplescan be seen at http://www.iti.northwest-ern.edu/tdr (Kosnik and Kotowski,2002).

4. Telemetry

Hard-wired phone and power lines arepreferable at sites that involve real timemonitoring and callback alarms. How-ever, several truly remote operations arebeing operated using cell phone, radio

communication and solar power(Dussud, 2002).

5. Low-loss Lead Cable

Long lead cables should be of the lowloss, 75 Ohm F11 variety. The often em-ployed, standard, 50 Ohm, RGU con-necting cables should be kept as short aspossible (<50 m) to minimize attenua-tion and noise. Such problems havearisen with RG58 and RG59 lead ca-bles.



Figure 12. Remote TDR monitoring system with alarm autodialer

Figure 11. View of an integrated DAS comprising (from the left) a TDR 100pulser, a 12V battery, a CR10X datalogger and communication equipment (phonemodem and cellular phone)

6. Multisensor Monitoring Systems

Integrated multiparameter monitoringsystems have been implemented withtiltmeters and TDR sensor cables at re-mote datalogger-controlled installa-tions. These have involved monitoringof bridge pier deformation from scourand from mining induced subsidencealong highways.

7. Connector Accessibility

Connections between different cables(i.e. transmission and transducer ca-bles) are a weak link and should bemade as robust and water proof as pos-sible. N-type connectors are recom-mended, but F-type have also beenused. They should also be accessible formaintenance as shown in Figure 10.

8. Digital Data Format

If cables are interrogated manually witha Tektronix 1502 cable tester, it shouldbe equipped with a SP232 module to ac-quire digital records for display, analy-sis and quantification of TDR reflec-tions.

AcknowledgementsDirect financial support for develop-ment of TDR technology has been pro-vided by the U.S. Department of Trans-portat ion funded InfrastructureTechnology Institute of NorthwesternUniversity from 1993 to the present andthe Civil Mechanical Systems Divisionof the National Science Foundation be-tween 1995 and 2001. The authors arealso indebted to the many other individ-uals and organizations that also havesupported this development but are toonumerous to mention by name.

ReferencesAymard, N. (1996) “Low Strength

Grouts for Embedding TDR Cablesin Soil,” M.S Thesis, Department ofCivil and Environmental Engineer-ing, Northwestern University,Evanston, IL USA, December.

Blackburn, J.T. (2002) Finite ElementAnalysis of TDR Sensor Ca-ble-Grout-Soil Mass InteractionDuring Localized Shearing, M.S

Thesis, Department of Civil and En-vironmental Engineering, North-western University, Evanston, IL,USA, April.

Cole, R. G. (1999) “Compliant TDRsensor cable Grout Composites toMeasure Localized Soil Deforma-tion” M.S. Thesis, Department ofCivil and Environmental Engineer-ing, Northwestern University,Evanston, IL, USA, December.

Dowding, C.H. , Su, M.B. andO’Connor, K.M. (1988) “Principlesof Time Domain Reflectometry Ap-plied to Measurement of Rock MassDeformation,” Int. Journal of RockMechanics and Mineral Science,Vol. 25, No.5, pp. 287-297.

Dowding, C.H. and O’Connor, K.M.(2000) “Comparison of TDR andSlope Inclinometers for Slope Moni-toring,” ASCE Geotechnical SpecialTechnical Publication, No. 106 pp80-90.

Dussud, M. L. (2002) “Case Historiesand Field Techniques for TDR Mon-itoring of Soil Deformation,” M.SThesis, Department of Civil and en-vironmental Engineering, North-western University, Evanston, IL,USA, Dec.

Kosnik D. and Kotowski M. (2002) “In-frastructure Remote MonitoringSoftware,” Infrastructure Technol-ogy Institute, Internal Report,Northwestern University, Evanston,IL.

Mikkelsen, P.E. (2002) “Cement-Ben-tonite Grout Backfill for BoreholeInstruments,” Geotechnical News,December pp. 38-42.

O’Connor, K.M. and C.H. Dowding(1999) GeoMeasurements by Puls-ing TDR Cables and Probes. CRCPress, Boca Raton, 420p.

O’Connor, K.M., Peterson, D.E. andLord E.R. (1995) “Development of aHighwall Monitoring System UsingTime Domain Reflectometry,” Pro-ceedings, 35th U.S. Symposium onRock Mechanics, Reno, Nevada,June, pp.79-84.

O’Connor, K.M., R. Ruegsegger, andK. Beach (2002) “Real Time Moni-

toring of Subsidence Along Inter-state I-77, Summit County, Ohio”ITGAUM 4th Biennial AbandonedUnderground Mine Workshop, Dav-enport, Iowa,http://www.fhwa.dot.gov///mine/oconnor.htm

O’Connor, K.M. and Wade, L.V.(1994), “Applications of Time Do-main Reflectometry in the MiningIndustry,” Proceedings of the Sym-posium and Workshop on TDR inEnvironmental, Infrastructure andMining Applications, NorthwesternUniversity, Evanston, IL, USA, Sep-tember, pp. 494-506.

Pierce, C. E. (1998) “A Compliant Co-axial Cable-Grout Composite forTime Domain Reflectometry Mea-surements of Localized Soil Defor-mation,” Ph.D. Thesis, Departmentof Civil and Environmental Engi-neering, Northwestern University,Evanston, IL, USA, December.

Will, D. (1997), “Cement BentoniteGrouts Compatible with CompliantTDR Cables,” M.S Thesis, Depart-ment of Civil and Environmental En-gineering, Northwestern University,Evanston, IL, USA, December.

Charles H. Dowding and Matthieu L.Dussud, Professor and Graduate Stu-dent: Department of Civil & Environ-mental Engineering, NorthwesternUniversity, Evanston, IL 60208,Tel: 847-491-4338,Fax: 847-491-4338,email: [email protected];[email protected]

William F. Kane, President, KaneGeotech, Inc., PO Box 7526, Stockton,CA 95267-0526, Tel: 209-472-1822,Fax: 209-472-0802,email [email protected]

Kevin M. O'Connor, President ,GeoTDR, Inc. 720 Greencrest Drive,Westerville, OH 43081-4902,Tel: 614-895-1400,Fax: 614-895-1171,email: [email protected]



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Monitoring Deformation in Rock and Soil with TDR Sensor …...bles will indicate the location and mag-nitude of subsurface shear strain. TDR sensor cables are especially sensitive