geotechnical instrumentation news - john dunnicliff

Geotechnical Instrumentation News

John Dunnicliff

IntroductionThis is the fifty-fifth episode of GIN.Two subjects this time, both based onpapers that were presented at the Inter-nat ional Symposium on FieldMeasurements in Geomechanics(FMGM) in Boston in September lastyear. I selected these for re-publicationin GIN because I want to maximize theaudience for two very important contri-butions. There is also a review of a bookabout fibre optic sensing.

Those of you who have already readthe FMGM paper on fully-groutedpiezometers by Contreras et alpleasereplace it with this version, in whichseveral issues have been corrected.

The Use of the Fully-groutedMethod for PiezometerInstallationThe first article, in two parts, is by IvnContreras, Aaron Grosser, and RichardVer Strate of Barr Engineering Com-pany in Minneapolis. Ive been waitingfor this for 39 years, ever since I readPeter Vaughans 1969 technical note inGotechnique, A Note on SealingPiezometers in Boreholes. That maysound flippant, but its true!

In 1949 the Journal of the Boston So-ciety of Civil Engineers published Ar-thur Casagrandes paper, SoilMechanics in the Design and Construc-tion of Logan Airport. He describedthe installation of open standpipepiezometers (Casagrande

piezometers) in boreholes bysurrounding them with a sand pack andplacing bentonite pellets over the sand.The drill casing was left in place and thebentonite seal was within the casing, sono grout was placed over the bentonite.A few years later it became normalpractice to withdraw the drill casing andplace grout over bentonite pellets orchips, and were still doing this forpiezometers installed in boreholes.

For open standpipe piezometers thesand pack is necessary because a sizableintake volume is required for obtaininga pore water pressure reading withoutsignificant time lag. So this normalpractice is still appropriate today, ex-cept that in my view the grout should beplaced directly over the sand, omittingthe bentonite pellets or chipsI explainthis in my discussion of the article byContreras et al.

Since the development of dia-phragm piezometers, usually pneu-

matic or vibrating wire, most of us havefollowed this same normal practice,with a sand pack, bentonite pellets orchips, and overlying grout. Forget thesand and bentonite seal! This is nolonger the way to go! Use theful ly-grouted method! Thefully-grouted method entails installinga piezometer tip in a borehole which isbackfilled entirely with cement-benton-ite grout. Its taken several years of dis-cussion and argument for me to arrive atthis conclusion because I feared that thegrout surrounding the tip might preventthe piezometer from responding cor-rectly to changes in pore water pressure.If you have the same fears, read the arti-cle, the discussion and the authors re-ply to the discussion, and become abeliever!

If any reader has other data, pro orcon, about the fully-grouted method, Idvery much welcome hearing about it,and will consider it for publication in alater episode of GIN.

Geotechnical Alarm SystemsThe second article is by KevinOConnor, and focuses on alarm sys-tems based on the technology of timedomain reflectometry (TDR).

My reason for regarding this as avery important contribution is be-cause the message is an umbrella one,applicable to all alarm systems, notonly those based on monitoring withTDR.

I have very clear memories of Kevin

Geotechnical News, June 2008 29

GEOTECHNICAL INSTRUMENTATION NEWS

Forget the sand andbentonite seal! This isno longer the way to

go! Use thefully-grouted method!

The Use of the Fully-grouted Method forPiezometer InstallationPart 1

Ivn A. ContrerasAaron T. GrosserRichard H. Ver Strate

IntroductionThe fully-grouted method described inthis ar t ic le entai ls instal l ing apiezometer tip in a borehole which isbackfilled entirely with cement-benton-ite grout. Part 1 of this article presents adetailed discussion of the fully-groutedmethod, including the installation pro-cedure and theoretical background, aswell as a seepage-model analysis usedto evaluate the impact of the differencein permeabilities between surroundingground and cement-bentonite grout.Part 2 describes laboratory test resultsfor six cement-bentonite grout mixesand field examples of applications ofthe fully-grouted method. Both parts ofthis article are based on the paper, TheUse of the Fully-grouted Method forPiezometer Installation, presented atFMGM 2007: Seventh International

Symposium on Field Measurements inGeomechanics, and are published inGIN with permission from ASCE.

A crucial parameter for the successof the fully-grouted method is the per-meability of the cement-bentonitegrout. Vaughan (1969) postulated thatthe cement-bentonite grout should havea permeability no greater than two or-ders of magnitude higher than the sur-rounding soil in order to obtainrepresentative pore-water pressurereadings. Unfortunately, there is limitedpublished data on the permeability ofcement-bentonite grout mixes.

Figure 1a shows the typicalpiezometer installation commonlyknown as a Casagrande or standpipepiezometer. With this installation, thetip of the piezometer (e.g., slotted PVCpipe or porous stone filter) is sur-

rounded with a high permeability mate-rial, commonly referred to as sand pack.Above the sand pack is a bentonite sealtypically consisting of bentonite chipsor pellets. The installation is finishedwith cement-bentonite grout to theground surface. This installation relieson a sizable intake volume and a narrowriser-pipe diameter to obtain a pore-wa-ter pressure reading in the riser pipewithout significant time lag (Hvorslev,1951).

With the development of diaphragmpiezometers (e.g., pneumatic and vi-brating wire), the method developed forstandpipe piezometers was used for dia-phragm piezometer installations(Dunnicliff, 1993). This has been acommon practice for decades and theresulting installation is shown on Fig-ure 1b. However, because of the


30 Geotechnical News, June 2008

Instrument sent to me from Australia by Craig Johnson:I was in an antique storeon the weekend, saw this artifact (circa late 20th century) and thought of you.Enjoy! Thank you Craig.

laying down some ground rules both atthe 2007 instrumentation course inFlorida and during his presentation ofthis subject at the FMGM symposium inBoston. He talked about responding toalarms and said, with enormous empha-

sis, If there is an alarm, you have torespond. Failure to respond is not anoption.

ClosurePlease send contributions to this col-

umn, or an article for GIN, to me as ane-mail attachment in MSWord, [email protected], or byfax or mail: Little Leat, Whisselwell,Bovey Tracey, Devon TQ13 9LA, Eng-land. Tel. and fax +44-1626-832919.

low-volume operation of diaphragmpiezometers, the sand pack around theinstrument tip is unnecessary, and thediaphragm piezometer can be installedin the borehole surrounded by ce-ment-bentonite grout. This procedure iscommonly known as the fully-groutedmethod (Mikkelsen and Green, 2003)and is shown on Figure 1c.

Fully-grouted MethodFigure 1c shows a piezometer installa-tion using the fully-grouted method, inwhich a diaphragm piezometer tip is setin a drilled borehole and entirely back-filled with cement-bentonite grout. Thefollowing is a detailed description of theinstallation procedure for a vibrat-ing-wire sensor t ip in typicalgeotechnical boreholes (i.e., 140 mm),including preparation of piezometer as-sembly and materials, grout mixing,piezometer construction, and theoreti-cal background.

Piezometer AssemblyConstruction of the piezometer assem-bly commonly begins with attachmentof the sensor tip to a sacrificial groutpipe. The sacrificial grout pipe, whichcan be either belled-end electrical con-duit or threaded PVC well casing, isgenerally constructed or laid out on theground in manageable lengths for han-dling. The piezometer location is se-lected by reviewing the soil stratigra-phy. The sacrificial grout pipe willgenerally extend to the bottom of theborehole for support; therefore, it is

possible to determine the location of thepiezometer tip from the top or bottom ofthe borehole since the pipe is left inplace.

After drilling a borehole, thepiezometer tip is attached to the groutpipe at the appropriate location. Forboreholes with a diameter of 140 mm, atypical grout pipe (such as 25.4-mm di-ameter PVC well casing) is used.Large-diameter or stronger grout pipemay be required for deeper installationswith higher pumping pressures.

The sensor tip, which has been satu-rated following the manufacturers di-rections, is typically set with the sensorin the upward position to minimize thepossibility of desaturation. The cableconnected to the sensor tip is attached tothe pipe at approximate intervals alongthe grout pipe, leaving some slack in theline. The grout pipe, sensor tip, and ca-ble are then lowered into the boreholewith the grout pipe placed on the bottomfor support. The piezometer tip is nowlocated within the desired monitoringzone. The cable is brought to the surfacewhere readings are taken with a readoutdevice.

One advantage of the fully-groutedmethod is that it can be used for installa-tion of nested piezometers. In a nestedpiezometer configuration, more thanone piezometer tip is attached to thesacrificial grout pipe. The authors havesuccessfully installed up to fourpiezometer tips in a borehole. Duringinstallation the drill casing should be re-

moved carefully to prevent damage tothe cables and the cables should be sep-arated around the grout pipe to prevent adirect seepage path along a bundle ofcables.

Another advantage of thefully-grouted method is the feasibilityof using a single borehole to installmore than one type of instrument. Forexample, the piezometer tips can be at-tached to an inclinometer casing, and asingle borehole is used for measuringboth deformation and pore-water pres-sures, resulting in reduced drillingcosts. However, the inclinometer casingjoints must be sealed. This techniquehas been used successfully by the au-thors on several projects.MaterialsThe cement-bentonite mixes describedin this article use Type I Portland ce-ment and sodium bentonite powdersuch as Baroid Aquagel Gold Seal orQuickgel. The water used in the mixesshould be potable water to prevent pos-sible interaction of chemical constitu-ents in the water with the cement-ben-tonite mixture.

Grout MixingThe mixing procedure described in thisarticle assumes the availability of a ca-pable drill-rig pump and a high-pres-sure, jet-type nozzle attachment on theend of a mixing hose. In most cases, thedrill-rig pump provides enough pres-sure for the jet-mixing required to ob-tain a desirable mixture. Other methods



Figure 1(a). Traditional standpipe piezometer with sand pack.Figure 1(b). Diaphragm piezometer with sand pack.Figure 1(c). Fully-grouted piezometer. Figure 2. Schematic computer model to simulate seepage

around a fully-grouted piezometer (borehole not to scale).

may use actual grout-mixing plants.Generally, the cement-bentonite mix isprepared in a barrel or mud tank usingthe drill-rig pump to circulate the batchwith a suction hose and return line.Occasionally, a hydraulically-operated,propeller-type mixer is used. However,it has been the authors experience that,in some cases (depending on the mixviscosity, pump operability on the drillrig, or grout volume), the use of a groutmixer/pump may be required. Typicalbatch sizes are 200 to more than 2,000liters.

The mixing process begins withcalculation of the amount of grout re-quired to fill the borehole. A measuredquantity of potable water is pumpedinto the mixing barrel first and circula-tion begins. During circulation, the wa-ter and cement are mixed first so that thewater:cement ratio remains fixed andthe properties of the grout mix are morepredictable. The measured quantity ofcement is gradually added to the wateruntil both components have been thor-oughly mixed. This is the most impor-tant step in the mix preparation and runscontrary to the common practice ofmixing bentonite and water first. An ini-tial measured quantity of powderedbentonite, based on a mix design, isadded into the barrel near the jet streamto minimize the formation of clumpswithin the mix. Typically, additionalbentonite is added as mixing continuesto achieve a creamy consistency.Mikkelsen (2002) describes the consis-tency as drops of grout should barelycome off a dipped finger and shouldform craters in the fluid surface.

Piezometer ConstructionAt the completion of the grout-mixingprocess, and after measuring the finaldensity of the mix, the piezometer tipassembly is lowered into the borehole.In shallow boreholes (e.g., typicallyless than 30 m deep), grout is thenpumped into the borehole through thesacrificial grout pipe until it reaches theground surface. In deeper boreholes,staged grouting using multiple groutpipes or multiple port pipes may be re-quired so the piezometers are notover-pressurized during installation. Incased boreholes, the drill casing is

slowly retrieved so that no gap is left be-tween the top of the grout and the bot-tom of the casing. Typically, the entireprocess takes approximately one hourfor a 30-m borehole. The hole is typi-cally completed with concrete and aprotective top.

The field engineer should take pres-sure readings during and immediatelyafter installation. One benefit of vibrat-ing-wire technology is that readings canbe taken quickly. The readings obtainedduring grouting can be used to deter-mine if the device has beenover-pressurized during grouting. Themeasured pressures should approxi-mately correspond to the pressure ex-erted by the column of grout above thetip, provided the sensor and grout are atnearly the same temperature, astemperature equalization may take sev-eral minutes. However, with time, thispressure decreases as the cement-ben-tonite mix sets up and pore-water pres-sure readings are taken at the tiplocations. Typically, grout set-up takesone to two days.

Theoretical BackgroundMcKenna (1995) clearly describes thetwo basic requirements for anypiezometer to perform its function. Themeasured pore-water pressure must befairly representative of the actualpore-water pressure at the measurementlocation (i.e., small accuracy error), andthe hydrodynamic time lag must beshort. At first glance, it does not appearthat the fully-grouted method will sat-isfy these requirements. It would seemthat the cement-bentonite grout sur-rounding the tip might prevent thepiezometer from responding quickly tochanges in pore-water pressures in theground due to its low permeability. Onthe other hand, if the cement-bentonitegrout is too permeable to enhance shorthydrodynamic time lags, there wouldbe significant vertical fluid flow withinthe cement-bentonite grout column.

However, the fully-grouted methoddoes satisfy both of McKennasrequirements . A diaphragmpiezometer, such as a vibrating wirepiezometer, generally requires only avery small volume equalization to re-spond to water pressure changes (10-2

to 10-3 cm3), and the cement-bentonitegrout is able to transmit this small vol-ume over the short distance that sepa-rates the piezometer tip and the groundin a typical borehole. A practical way toreduce this distance is to set up the tipclose to the wall of the borehole by re-ducing the thickness of grout betweenthe tip and ground using pre-manufac-tured, expandable piezometer assem-blies.

Grout PermeabilityRequirementsVaughan (1969) introduced thefully-grouted method and developedclosed-form solutions which showedthat the error in the measured pressure issignificant only when the permeabilityof the borehole backfill is two orders ofmagnitude greater than the permeabil-ity of the surrounding ground. If thepermeability of the cement-bentonitegrout is lower than the permeability ofthe surrounding ground, measuredpressures will be without error. As a re-sult, for the fully-grouted method towork, the grout mix used to backfill theborehole must meet certain permeabil-ity requirements. A seepage model wasdeveloped by the authors to better un-derstand those requirements.

Computer ModelingA finite-element computer modelsimulating seepage conditions around afully-grouted piezometer installationwas used to evaluate the impact of groutpermeability on the accuracy of thepiezometer reading. The seepage modelwas conducted using SEEP/W, a com-puter-modeling program developed byGEO-Slope International.

Figure 2 shows the conceptualmodel developed to simulate the seep-age around a piezometer installed usingthe ful ly-grouted method. Theaxisymmetric flow model includes a7-cm radius, cement-bentonite-groutcolumn surrounded by soil of constantpermeability. The simulated ce-ment-bentonite grout column extends27.5 m and the soil layer extends 33 mbelow the ground surface with a radiusof 50 m. Underlying the soil, a sandlayer was incorporated to simulate thelower boundary conditions.



The seepage analyses were per-formed simulating upward and down-ward flow using two sets of imposedtotal head conditions (i.e., 10 and 20 m)that induce flow under steady-state con-ditions. This set of boundary conditionscorresponds to the one-dimensionalflow condition in the vertical direction.In all cases, fully saturated conditionswere used for all the materials in themodel. The error, , defined as the dif-ference in computed pore-water pres-sure between the soil and the grout, wasdetermined during each model run at

points in the soil and grout 20 m belowthe ground surface, as shown on Fig-ure 2.

Results of Computer ModelingSeveral model runs were made in whichthe permeability ratio, kgrout/ksoil, wasvaried from 1 to 107. Figure 3 shows theresults of the seepage simulations interms of the normalized error, i.e., di-vided by the pore-water pressure in soil,usoil, against the permeability ratio. Fig-ure 3 also shows that the normalized er-ror is zero for all practical purposes up

to permeability ratios of 1,000 fordownward and upward flow and the twosets of imposed total heads. As the per-meability ratio increases beyond 1,000,the normalized error increases up toabout 10 percent at permeability ratiosof 10,000. As the permeability ratiocontinues to increase to 107, the nor-malized error also increases up to about23 and 40 percent, respectively, for the10-m and 20-m imposed total heads.

In summary, the finite-element com-puter model revealed that the perme-ability of the grout mix can be up tothree orders of magnitude greater thanthe permeability of the surroundingground without introducing significanterror. This finding differs from previousassessments, which indicated that thepermeability of the grout mix shouldonly be one or two orders of magnitudegreater than the permeability of the sur-rounding ground. The minimum per-meabil i ty that is l ikely to beencountered in natural soils is on the or-der of 10-9 cm/s. As a result, the ce-ment-bentonite grout mix used in thefully-grouted method needs to have apermeability of, at most, 10-6 cm/s.

Part 2 of this article will discuss lab-oratory test results of six cement-ben-tonite grout mixes and field examples ofapplications of the fully-groutedmethod.



Figure 3. Normalized error versus permeability ratio.

The Use of the Fully-grouted Method forPiezometer InstallationPart 2

Laboratory Testing ProgramA laboratory testing program was de-veloped to evaluate the range in perme-ability and strength of cement-benton-i te grout for piezometerinstallationsusing the fully-groutedmethod. The test program was designed

so that small batches of grout could bemixed in a controlled environmentwithout large grout-batch mixingequipment. Six mix designs were cho-sen to represent a wide range of valuesthat would reasonably be used on pro-jects.

Sample PreparationMixing the grout used for laboratorytesting began with calculating the de-sired quantities of material andthenweighing individual portions of ce-ment, water, and bentonite. Additionalbentonite was prepared in anticipation

of adjusting the mix viscosity. Theproperties of the individual mix compo-nents used in the laboratory testing arelisted in Table 1.

To begin, the cement was added to

the water slowly while mixing. Thebenefit of adding the cement first in themixing process is that it ensures the cor-rect water:cement ratio before addingthe bentonite.

After the cement and water weremixed and the water-cement paste ap-peared uniform, which generally tookfive minutes, bentonite was slowlyadded to the bucket. The cement-ben-tonite grout was then mixed for approx-imately five additional minutes until itappeared uniform and did not containlumps. Viscosity was measured at vari-ous times during mixing to evaluate thecondition of the mix. Samples of the fi-nal mix were taken using plastic moldsand the density was measured.

After a short cure period, the sam-ples were carefully extruded out of theplastic molds and stored until the testdate. For the Unconfined CompressiveStrength testing (UCS), a set of twospecimens were tested at 7, 14, and 28days. Permeability testing was com-pleted on specimens from each mix at 7and 28 days under three different con-fining stresses. In addition to strengthtests, basic index properties, such asmoisture content and dry density of thesamples, were also measured.

Laboratory Test ResultsTable 2 summarizes the final ce-ment-bentonite grout proportions usedin this study. The results of the labora-tory testing are presented in a series offigures.

Figure 4 summarizes test results asthe average UCS at 28 days versus thewater:cement ratio by weight. It showsthat the UCS decreases with increasingwater:cement ratios. In fact, the UCS at28 days is approximately 1700 kPa at awater:cement ratio of 2:1; it then de-creases to approximately 90 kPa withincreasing water:cement ratio. Also in-cluded on Figure 4 are data presented byMikkelsen (2002), which show a rela-tively strong correlation with the data ofthis study.

The void ratios of the samples werecomputed based on the measured watercontent of the specimens and the spe-cific gravity of the grout-mix constitu-ents. The computed void ratios of themixes are relatively high, in fact, theseare higher than soils with similarstrength and permeability. However, thedata show that the amount of cementcontrols the strength characteristics ofthe grout mix. Bentonite appears to in-



Table 1. Properties of grout constituents

Mix Component Brand Specific Gravity MoistureContent (%)

Portland Cement Type I LaFarge 3.15 Bentonite Quickgel

(Mixes 1-4)Baroid 2.41 to 2.45 11

Aquagel Gold Seal Ben-tonite (Mixes 5 and 6)

Baroid 2.4 10

Table 2. Summary of cement-bentonite grout mixes used in the study

Mix Water : Cement :Bentoniteby Weight

Marsh FunnelViscosity (sec)

Bentonite Type

1 2.5 : 1: 0.35 50 Quickgel2 6.55 : 1: 0.40 54 Quickgel3 3.99 : 1: 0.67 60 Quickgel4 2.0 : 1: 0.36 360 Quickgel5 2.49 : 1: 0.41 56 Aquagel Gold Seal6 6.64 : 1: 1.19 60 Aquagel Gold Seal

Figure 4. Variation of unconfined compressive strength versus water:cementratio.

fluence the amount of bleed water andvolume change of the specimen duringcuring. Additional information on thestrength and deformation properties ofcement-bentonite mixes can be found inContreras, et al. (2007).

Figure 5 summarizes the test resultsin terms of the permeability of the spec-imens at seven days for various confin-ing pressures. The data show thatsamples with a higher water:cement ra-tio or void ratio have higher permeabil-ity than those with lower water:cementratios.

Figure 6 shows the permeability in

the same format for specimens at 28days. Data are very similar, showingthat the permeability is relatively con-stant or decreases slightly with confin-ing pressure. One important result isthat, from seven to 28 days, the perme-ability continues to decrease. For exam-ple, mixes with 2.49 water:cement ratioindicate a permeability greater than1.0x10-6 cm/sec at 7 days and less than1.0x10-6 cm/sec at 28 days. The data in-dicate that, as hydration of the cementoccurs, the permeability of the mix de-creases. The high void ratio and lowpermeability are two reasons the

fully-grouted method works; it allowstransmission of a low volume of waterover a short distance yet maintains over-all low permeability in the vertical di-rection.

Figure 7 shows the variation in per-meability data with respect to void ratio.The data indicate that specimens withlower void ratios typically exhibit lowerpermeability, while those with highervoid ratios exhibit higher permeability.With grout mixes, the cement has agreater influence on the void ratio thanthe bentonite and is considered the con-trolling factor in the permeability of thegrout. The difference between the sevenand 28-day permeability is relativelysmall, as shown on Figure 7.

Field ExamplesThis section describes three field exam-ples in which the fully-grouted methodwas successfully applied. The first ex-ample compares pressure readings be-tween one installation using thefully-grouted method in a nested con-figuration and the traditional approachwith individual piezometer installationsin separate boreholes. The second ex-ample descr ibes use of thefully-grouted method with the installa-tion of nested piezometers in an up-ward-flow condition. The third exam-ple is for a nested, fully-grouted methodinstallation in a downward-flow condi-tion.

Example 1. Comparison BetweenNested and Individual InstallationsThis field example compares two meth-ods of installation: Three vibrating-wire piezometers in

a single borehole using thefully-grouted method.

Four individual pneumaticpiezometers in separate boreholesusing the traditional sand packaround the piezometer tips.The two installations were within 7.5

m of each other. As a result, some differ-ences in the pressure readings were ex-pected. Figure 8 shows a comparison ofthe pore-water pressure profile with ele-vation for both installations. The figureillustrates a fairly similar response con-sidering the distance between the twosets. Similar data have been presented



Figure 5. Variation of permeability versus confining pressure at 7 days.

Figure 6. Variation of permeability versus confining pressure at 28 days.

by McKenna (1995) , fur therconfirming the val idi ty of thefully-grouted method.

Example 2. Upward-FlowConditionsThis field example illustrates the use ofnested piezometers using thefully-grouted method in upward-flowconditions. The site is in an area wherethree distinct stratigraphy units arefound (alluvial deposits, Huot Clay For-mation, and Red Lake Falls Formation).The upward-flow conditions play a ma-jor role in the slope instability of thearea (Contreras and Solseng, 2006).

Figure 9 shows the pore-water pres-sure and total-head profiles at the site,illustrating the upward-flow conditions.Two vibrating-wire piezometer tipswere installed in the Huot Formationand one in the Red Lake Falls Forma-tion. The Huot Formation is fairly uni-form and has a permeability in the rangeof 1.2x10-8 to 1.9x10-8 cm/s. The ce-ment-bentonite grout mix used in thenested installation had a 2.66:1:0.27water:cement:bentonite ratio with apermeability of approximately 2.0x10-6cm/s. This example presents the resultsof the fully-grouted method in alow-permeability unit.

Example 3. Downward-FlowConditionsFinally, this field example demonstratesthe use of nested piezometers with theful ly-grouted method in down-ward-flow conditions. A total of fourpiezometer tips were installed in threeunits, with permeability ranging from1.0x10-3 cm/s to 9.49x10-7 cm/s. Wherethere is a wide range of permeability,the least permeable unit controls the ce-ment-bentonite grout permeability. Asa general rule, the less permeable the ce-ment-bentonite grout, the better, and asshown by the computer model, for mostsoil, a cement-bentonite grout with apermeability of 1.0x10-6 cm/s will beadequate. Figure 10 shows the pore-wa-ter pressure and total-head profiles atthe site, illustrating the downward-flowconditions. This example presents theresults of an installation of nestedpiezometers with up to four piezometertips in a single borehole.

Summary and ConclusionsThis two-part article presents a detaileddiscussion of the fully-grouted methodfor piezometer installation, including theprocedure and theoretical background. Italso discusses the results of a laboratorytesting program on six cement-bentonitegrout mixes, along with an evaluation ofa computer model to determine the im-pact of the difference in permeabilities

between the cement-bentonite groutbackfill and the surrounding ground.The following summarizes the articlesmain issues and findings: The practice of installing diaphragm

piezometers in a sand pack with anoverlying seal of bentonite chips orpellets could be discontinued.

The fully-grouted method is a fairlysimple, economical, and accurateprocedure that can be used to mea-sure pore-water pressures in soilsand fractured rock. It allows easy in-stallation of a nested piezometerconfiguration, resulting in drillingcost savings. It can also be used incombination with other instrumenta-tion (e.g., inclinometers) to measuredeformation and pore-water pres-sures, provided the inclinometerjoints remain sealed.

The permeability of the cement-ben-tonite grout mix can be up to threeorders of magnitude greater than thepermeability of the surroundingground without a significant error inthe pore-water pressure measure-ment. This finding differs from pre-vious assessments.

Laboratory test results show that thepermeability of the cement-benton-ite grout mixes is a function of thewater:cement ratio. As the water:ce-ment ratio (void ratio) decreases, thepermeability decreases.

Bentonite has little influence on thepermeability of the mix, but ratherappears to stabilize the mix, keepingthe cement in suspension and reduc-ing the amount of bleed water.

AcknowledgmentsThe support provided by the InnovationCommittee of Barr Engineering Com-pany is gratefully acknowledged. Thecareful performance of the laboratorytesting by Soil Engineering Testing ofBloomington, Minnesota, is greatly ap-preciated. The continual assistancefrom Erik Mikkelsen and his thoughtfulinsight and contributions from the be-ginning of the research program havebeen extremely helpful in pursuing theresearch and use of the fully-groutedmethod. John Dunnicliffs thorough re-view and comments on this manuscriptare also greatly appreciated.



Figure 7. Void ratio versus permeability.

ReferencesContreras, I.A., Grosser, A.T., and

VerStrate, R.H. 2007. BasicStrength and Deformation Proper-ties of Cement-Bentonite GroutMixes for Instrumentation Installa-tion. Proceedings of the 55th An-nual Geotechnical EngineeringConference University of Minne-sota. pp. 121-126.

Contreras, I.A., Grosser, A.T., andVerStrate, R.H. 2007. The Use ofthe Fully-grouted Method forPiezometer Installation. Proceed-ings of the Seventh InternationalSymposium on Field Measurementsin Geomechanics. FMGM, 2007.Boston, MA. ASCE GeotechnicalSpecial Publication 175.

Contreras, I.A. and Solseng, P.B. 2006.Slope Instabilities in Lake AgassizClays. Proceedings of the 54th An-nual Geotechnical Engineering Con-ference. University of Minnesota.pp. 79-93.

Dunnicliff, J. 1993. Geotechnical In-strumentation for Measuring Field

Performance.J. Wiley, NewYork, 577 pp.

Hvorslev, M.J.1951. TimeLag and SoilPermeabilityin Groundwa-ter Observa-tions. BulletinNo. 36,U.S. Water-ways Experi-ment Station, Vicksburg, MI.

McKenna, Gordon T. 1995.Grouted-in Instal la t ion ofPiezometers in Boreholes. Cana-dian Geotechnical, Journal 32, pp.355-363.

Mikkelsen, P.E. and Green, E.G. 2003.Piezometers in Fully Grouted Bore-holes. International Symposium onGeomechanics. Oslo, Norway. Sep-tember 2003.

Mikkelsen, P. Erik. 2002. Ce-ment-Bentonite Grout Backfill for

Borehole Instruments .Geotechnical News. December2002.

Vaughan, P.R. 1969. A Note on SealingPiezometers in Boreholes .Geotechnique, Vol. 19, No. 3,pp. 405-413.

Ivn A. Contreras, Aaron T. Grosser,Richard H. Ver Strate, Barr Engineer-ing Co., 4700 W. 77th Street, Minneapo-lis , MN 55435, 952-832-2600,[email protected], [email protected], [email protected]



Figure 8. Comparison between a nested fully-groutedinstallation and individual separate installations.

Figure 9. Field example of fully-grouted method in upwardflow condition.

Figure 10. Field example of fully-grouted method indownward flow condition.

Discussion of The Use of theFully-grouted Method for PiezometerInstallation

Ivn A. ContrerasAaron T. GrosserRichard H. Ver Strate

Geotechnical News, Vol. 26 No. 2June 2008

John Dunnicliff

Thank you to all three authors for theirexcellent practical contribution. Ivebeen waiting for this for 39 yearsseethe last reference citation in Part 2 ofyour article, Vaughan (1969)!

Other Experiences with theFully-Grouted MethodIn my view, the rationale for acceptingthe fully-grouted method is very con-vincing. Despite that view, owners andtheir consultants may tend to be wary ofwhat they consider to be a new and rad-ical method. As youll see below,theres plenty of positive experienceout there, and if were to convinceowners and their consultants, we needas much supportive information as pos-sible. Among other experiences are: The engineers at Applied

GeoKinetics, located in Irvine, Cali-fornia (www.appliedgeokinetics.com) have used the fully-groutedmethod successfully on approxi-mately 400 installations since 1990.Several of these installations havebeen to depths of approximately 500feet, with up to ten piezometers tipsinstalled within a single borehole.For more information, please con-tact Glenn Tofani at [email protected].

Colleagues in Austral ia ,Geotechnical Systems Australia PtyLtd. , (www.geotechsystems.com.au) have used the fully-groutedmethod with very good results onabout 15 sites since 2001. Installa-tions have been up to 500m deep. For

more information, please contactMatt Crawford, matt@geotech sys-tems.com.au.

Geometron of Seattle (Bellevue) andKleinfelder of Denver installedabout 40 fully-grouted vibratingwire piezometers on a multiple-damproject in Southern Oregon over thelast three years, with real-time-mon-itoring. It includes several in unsatu-rated embankments, some reactingto rainfall recharge. For more infor-mation please contact ErikMikkelsen, [email protected].

Many firms in Washington State reg-ular ly specify and use themethod, including Camp DresserMcKee (CDM), CH2M Hill, andJacobs Associates. It is in current useon major transportation and tunnelprojects.

The US Army Corps of Engineers inOmaha started using fully-groutedpiezometers on Oahe Dam on theMissouri River, SD in 2000, particu-larly for piezometers installed in Pi-erre Shale. A pilot relief wellprogram showed that the vibratingwire piezometers responded betterthan conventional open standpipes.

Syncrude Canada Ltd., in FortMcMurray, Alberta have usedfully-grouted piezometers success-fully in 83 vibrating wire piezometertip installations since 2003 in stiff insitu soils using Syncrude installationprocedures . Syncrude hasalso grouted in about 65 vibrating

wire tips when installing in com-pressible fills, but in those cases useda bentonite seal within 3m above thetip to protect against the potential ofthe grout cracking due to the settle-ment.

Strata Control Technology, a miningconsulting firm in Australia that alsospecializes in geotechnical instru-mentation, have used the method andconclude, Fully-grouted vibratingwire piezometers are proving an ex-cellent tool for investigating the im-pact of coal mining on groundwatersystems. For more information,please contact Ken Mills ,[email protected].

DeJong et al (2004) describe com-parative tests between a single vi-brating wire piezometer installed ina soft varved clay deposit with a sandpack, and a ful ly-groutedpiezometer. They conclude, Theperformance of the fully-groutedpiezometer was shown to be nearlyidentical to that of the sand packedinstrument. This paper also de-scribes the use of pre-manufactured,expandable piezometer assemblies,to which the authors refer in Part 1 oftheir article, in the context of reduc-ing the distance between thepiezometer filter and the wall of theborehole.If any reader has other data, pro or

con, about the fully-grouted method, Idvery much welcome hearing about it,and will consider it for publication in alater episode of GIN.



Use of Bentonite Pellets andChips over Sand Packs forDiaphragm PiezometersThe authors conclude, The practice ofinstalling diaphragm piezometers in asand pack with an overlying seal of ben-tonite chips or pellets can be discontin-ued. As I wrote at the beginning of thisdiscussion, owners and their consul-tants may tend to be wary of thefully-grouted method. In these cases Isuggest that we ask them how confidentthey are that bentonite pellets or chipsarrive at the depths shown so neatly inyour Figure 1(b). Ive had numerous ex-periences of these infuriating thingsarching part way down the borehole,and have little confidence that Figure1(b) represents reality. As an aside here,Ive tried to retard the onset of swellingso that they dont become sticky for afew minutes, including freezing, coat-ing with hydraulic oil, and sprayingwith hair sprayforget it!

If owners and their consultants fullyappreciate these uncertainties, perhapsthey may be more willing to accept thefully-grouted method.

Use of Bentonite Pellets andChips over Sand Packs forOpen Standpipe PiezometersFigure 1(a) shows a bentonite sealabove the sand pack for an openstandpipe piezometer. For the reasonsgiven above, I believe that a more reli-able installation method is to omit thebentonite seal and to place the grout di-rectly over the sand pack. To preventcontaminating the sand pack with grout,a bottom plug should be used on thegrout pipe, side-discharge holes drillednear the bottom of the pipe, and thegrout pumped very slowly.

Use of the Method in SoftGround below FutureEmbankmentsWhen there is a need to monitor porewater pressure in highly-compressibleground as an overlying embankment isconstructed, it is usual to do so withpiezometers at various elevations. Thishas been done where vertical compres-sion in the soft ground has been up to35% (e.g. Handfelt et al, 1987). Be-cause it allows several piezometers tobe installed in a single borehole, the

fully-grouted method is very attractivein this application, but I dont know ifthis has been done. Some of the installa-tion procedure that is described by theauthors would have to be changed forthis application.

First, a sacrificial grout pipe couldntbe used, because it would impede con-formance as vertical compression pro-gresses, and I believe that a telescopinggrout pipe would introduce too manyproblems. Perhaps the piezometerscould be attached to aircraft cable(stranded flexible and thin stainlesssteel cable), which would readily copewith the vertical compression. Aflush-coupled (inside and outside) PVCgrout pipe would be used, and with-drawn after grouting. If drill casing hasbeen used, care would need to be takento maintain piezometer depths whenpulling the casing.

Second, arrangements would have tobe made to ensure that the piezometertubing or cable doesnt fail in compres-sion. I know that this can happen. Forpneumatic piezometers the tubing canbe pre-spiraled as shown in Figure 9.31in the red bookthis was for a projectin Hong Kong, described by Handfelt etal (1987), where the pneumatic tubeswere wrapped around a 3 steel pipe,placed in very hot water for a few min-utes, removed and allowed to cool. Butfor a future project Id prefer to use vi-brating wire piezometers, and wouldntwant to trust that the cable would movearound in the grout and survive. Variousunhappy experiences have taught me agolden rule about installation of instru-ments: if you can think of somethingthat might go wrong, deal with it bychanging the planned procedure. Ap-parently it isnt possible to pre-spiralthe types of electrical cable that are usedfor field instrumentation. One possibleway could be to coil the piezometer ca-bles loosely around the grout pipe as allcomponents are lowered together, butwould that run the risk of cable damageand possible lifting of piezometerswhen removing the grout pipe? Anotherpossible way could be for the manufac-turer to insert the cable in a plastic tubeand coil the tube as was done for theHong Kong project, but would that runthe risk of tubing damage and creation

of a bleed path for pore water pressure?Third, the compressibility of the

grout must not be less than that of thesurrounding groundthis would needto be taken care of during design of thegrout mix.

For this application it is necessary tokeep track of the elevation of thepiezometers as they settle, because themeasurements of pressure need to beconverted to piezometric elevations.This is done by monitoring settlementnearby, usually with probe extensom-eters, and ensuring that the probe exten-someters can also cope with the verticalcompression.

Id very much welcome the authorscomments on these suggestions.

Borehole DiameterIn Part 1 the authors refer to typicalgeotechnical boreholes (i.e., 140 mm).In my experience many piezometers areinstalled in smaller diameter boreholes,often as small as 76 mm. Do the authorshave any recommendations if we dothis?

Use of a Single Borehole forFully-grouted Piezometers andInclinometer CasingIn Part 1 of the article the authors saythat a single borehole can be used to in-stall both piezometers and inclinometercasing, resulting in reduced drillingcosts. They add the caveat, However,the inclinometer casing joints must besealed. I want to emphasize thatsealed must be taken very seri-ouslyany lack of sealing will create apath for dissipation of pore water pres-sure, therefore false readings.

ReferencesDeJong, J.T., Fritzges, M.B., Sellers,

J.B. and J.B. McRae (2004), PorePressure Character izat ion ofGeotechnical Experimentation SiteUsing Multilevel Vibrating WirePiezometers, Proc. 57th CanadianGeotechnical Conference, QuebecCity, Session 7B, pp 17-24.

Handfelt, L.D., Koutsoftas, D.C. and RFoott (1987), Instrumentation forTest Fill in Hong Kong, J. Geotech.Eng. Div. ASCE, Vol. 113, No. 2,Feb.., pp. 127-146.



Authors Reply

The authors appreciate the opportunityof discussing the details and concerns ofthe fully-grouted method with othercolleagues.

Other Experience with theFully-Grouted MethodThe authors have successfully installedover 100 piezometers using thefully-grouted method to depths greaterthan 100 feet since 2003. Some installa-tions have had up to four piezometertips in a single borehole.

Use of Bentonite Chips andSand PacksThe best way to obtain confidence in theuse of the fully-grouted method is toconstruct a trial application using boththe traditional sand pack method andfully-grouted method in adjacent bore-holes. This comparison test will quicklyreveal the benefits of the fully-groutedmethod regarding the ease of installa-tion, time, and cost reduction. This isthe way the authors became convincedthe method works. Arching of sandpack and bentonite chips can be a veryfrustrating and costly problem duringconstruction and the fully-groutedmethod eliminates this problem.

Use of the Method in SoftGround below FutureEmbankmentsThe authors have successfully used thefully-grouted method in soft groundwith sacrificial grout pipes; however, intheir experience, the magnitude of set-tlement has generally been much lessthan 35 percent. It is recognized that inground with high compressibility, thegrout pipe would impede conformanceas vertical compression progresses.

The application of the aircraft ca-ble installation sounds reasonable andwill be considered for future installa-tions in the authors practice. It appearsto be the easiest and most sound solu-tion to the problem.

The installation using traditionalmethods and fully-grouted method ap-pear to have the same concerns regard-ing the compressibility of the grout andthe formation.

Borehole DiameterTypical hollow-stem auger drill casingused in the Midwest region of theUnited States has inner diameters of 82mm to 108 mm, which allows the use ofa 25 mm grout pipe and multiplepiezometer tips and cable bundles. Cas-

ing diameters less than 108 mm maycause casing removal problems as thepiezometers and cables may catch onthe casing and damage the installations.For multiple-device installations, thelarger casing is preferred. Some manu-facturers also offer protective casing forthe piezometers that helps reduce dam-age to the devices. For a single-tip in-stallation, the small-diameter casingshould be adequate.

Use of a Single Borehole forFully-grouted Piezometers andInclinometer CasingThe authorsexperience of installing in-clinometer casing and piezometers hasbeen successful. Care has been taken toseparate the tips from the joints. Addi-tionally, care has been taken to ensurethe joints are as watertight as possible.A verification test can be performed byadding and removing water inside thecasing and measuring readings at thepiezometers to identify any impact onthe readings from leaks. A stable read-ing may indicate a successful installa-tion. However, the readings should beevaluated during monitoring in theevent casing movement has caused ajoint to leak.



Geotechnical Alarm Systems Based onTDR Technology

Kevin M. OConnor

AbstractGeotechnical applications of time do-main reflectometry (TDR) are continu-ing to evolve and usage is increasing,particularly for monitoring deformationover large lateral distances when a pri-ori knowledge of movement locations islimited. Applications include monitor-ing subsidence along major roadwaysover active and abandoned mines andmonitoring movement along the toe ofdams. It has evolved into real time mon-itoring with alarms and a variety of noti-

fication schemes. Fully automated sys-tems have been installed which detectwhen measured deformation on the or-der of millimeters has exceeded speci-fied magnitudes and/or rates, and initi-ate phone calls to responsible parties.The diversity of project-specific details(e.g., cables installed in trenches, inhorizontal directionally drilled holes, inangled holes, notification via telephoneor radio, etc.) is a reflection of the rangeof site conditions and owner require-ments. This article is based on the paper,

Geotechnical Alarm Systems Basedon TDR Technology, which was pre-sented at FMGM 2007 and is publishedin GIN with permission from ASCE.

Representative ProjectsThe following projects illustrate alarmsystems utilizing TDR technology.Common features of the hardware andsoftware include: 22 mm diameter solid aluminum co-

axial cable robust datalogger and TDR unit

cable lengths up to 610 m maximized number of interrogation

points per cable baseline reading, or specified refer-

ence values, stored on datalogger difference between baseline reading,

or specified reference value, and cur-rent reading computed by datalogger

datalogger initiates alarm sequencewhen difference exceeds specifiedthreshold value(s), and

external data storage moduleWhile there are common features,

project-specific installation details,data requirements, and interrogationdetails reflect the flexibility of TDR

technology in these applications.There are also project-specific ratio-

nale and objectives that reflect the flexi-bility of TDR technology.

I-77 Summit County, OhioThis project was motivated by previousexperience of the Ohio Department ofTransportation during stabilization of asection of interstate highway impactedby abandoned mine subsidence inGuernsey County. At that location, thehighway was closed as the mine wasbackfilled with grout. As grout was in-jected, water within the mine was dis-placed and subsidence sinkholes devel-oped under the roadway.

For the project in Summit County, itwas specified that the highway remainopen for traffic during grout backfill in-jection and it was necessary to providean early warning system to detect if sub-sidence was occurring beneath the high-way as water was displaced. Holes werehorizontal directionally drilled (HDD)beneath the centerline of each lane ofthe highway. Coaxial cables werepulled back through the holes and thenconnected to a remote data acquisitionsystem. Cables were also installed in atrench along the road. When the systemdetected an alarm condition along anycable, it communicated via phonemodem with on-duty GeoTDRpersonnel.

State Route 91 Plasterco, VirginiaWhen the United States Gypsum Com-

pany was decommissioning its facilityin Plasterco, it was known that subsi-dence would occur when pumps wereturned off and water levels rose withinthe mine.

One component of the decommis-sioning plan was construction of a newalignment for SR91 outside the pro-jected influence of subsidence. Con-struction of the new alignment involvedblasting for rock excavation and com-pany personnel were concerned thatblast-induced vibrations would acceler-ate subsidence of the soft overburdenbeneath the existing highway. Coaxialcables were installed in angled holesdrilled under the road and also in atrench along the road.

In addition, mine personnel wereconcerned that subsidence would occurin the former plant area where exca-



Table 1. Installation Details

Site Orientation CablesI-77Ohio

HDD holesand trench

8 cables317 to 374m (1040 to1227 ft)

SR 91Virginia

Angledholes andtrench

18 cables36 to 447m (118 to1466 ft)

TuttleCreekDamKansas

Trench 4 cables610 m(2000 ft)

McMickenDamArizona

Trench 6 cables416 to 497m (1366 to1630 ft)

Table 2. Interrogation Details

Site Data Points IntervalI-77Ohio

1 point/ m 3 hrs

SR 91Virginia

3 points/m 3 hrs

TuttleCreekDamKansas

2 points/m 10 minutes

McMickenDamArizona

20 points/m 24 hour

Figure 1. TDR data acquisition system. Four coaxial cables areconnected to the multiplexer inside the smaller cabinet. Thedatalogger, TDR unit, external storage module, phone modem,and auto-dialer are also in the smaller cabinet.

Figure 2. Holes being drilled for grout injection intoabandoned mine along I-77 in Summit County, Ohio.

vated rock was being stockpiled. Cableswere also installed in trenches in thisarea. As each of the four remote sys-tems detected an alarm condition alonga cable, a datalogger would activate anauto-dialer to call on-duty U.S.Gypsum personnel.

Tuttle Creek Dam, Manhattan,KansasUnder contract with the U.S. ArmyCorps of Engineers, URS Corporationinstalled a warning system at TuttleCreek Dam. The concern was motivatedby a possible seismic event within theNew Madrid Fault Zone that could initi-ate movement of the downstream slopeof the dam. URS installed a multi-fac-eted instrumentation system to monitorsurface and subsurface movement inreal time. When multiple sensors detectchanges that exceed specified threshold

values, a sequence is initiated that canmobilize evacuation of downstreamresidents.

If slope movement should occur,modeling by URS has indicated bulgingof the toe would intersect the down-stream trench in which cables are in-stalled. Two adjacent cables extendwest from the data acquisition system(DAS) and two adjacent cables extendeast from the DAS. When deformationexceeds a threshold value on adjacentcables simultaneously, the dataloggeractivates one channel of a control mea-surement unit. The CMU polls severaldifferent sensors and communicateswith the base station via radio.

McMicken Dam, Maricopa County,ArizonaBased on the subsidence history of theMcMicken Dam embankment crest and

other informa-tion, AMEC andthe MaricoupaCounty FloodControl Districthave determinedthat groundstrains and fis-suring are devel-oping due toconsolidation ofthe underlyingalluvial aquifercaused byground waterw i t h d r a w a l .Based on furtherstudies, it was

determined that there exists a high prob-ability of earth fissures being present inthe soils underlying McMicken Dam.

As a component of the Fissure RiskZone Remediation Project, two adja-cent coaxial cables were installed in atrench downstream of the dam to detectdevelopment of earth fissures. Whendeformation exceeds a threshold valueon adjacent cables simultaneously, thedatalogger initiates a call via radio tothe ALERT-protocol control center.

Alarm ActivityWhen calls are received from a remotedata acquisition system, informationtransmitted includes the cable identifi-cation number and location along thecable where the alarm condition exists.Algorithms, which have been pro-grammed into the dataloggers, do notdistinguish the cause of the alarm con-dition. They only alert responsible per-sonnel to the fact that a condition existsin which the difference between the cur-rent reflection magnitude and baselinevalue is greater than the specified alarmlevel threshold. These alarms can betriggered by causes other than cable de-formation and the alarms must be fil-tered.

A context for the performance of thealarm systems is provided by some his-torical data for police alarms and debrisflow alarms. The false alarm rates forthree systems listed in Table 3 rangedfrom 70% to 90%:

Monthly alarm activity for the TDRbased systems in Ohio and Virginia issummarized in Table 4. These projects



Figure 3. Installing coaxial cable in the trench along SR91in Plasterco, Virginia.

Figure 4. Installing coaxial cable in the downstreamtrench at Tuttle Creek Dam in Manhattan, Kansas.

Figure 5. Data acquisition system at McMicken Dam inMaricopa County, Arizona.

used single cables in each borehole ortrench without redundant measure-ments, and false alarms ranged from 0to 100% with averages of 52% and76%.

Average rates are not really mean-ingful since weekly and monthly ratesprovide a more realistic assessment ofthe impact on the response of personnel.

Alarm ResponseVarious techniques have been used tofilter alarm calls. Personnel assigned re-sponsibility for responding to alarmshave developed operational proceduresto filter calls as they gained experience.Subsequent system designs have beenmodified to incorporate redundancy toimprove the reliability of alarm calls.

Time identifier - each cable is as-signed a time when it is interrogated sothe cable is identified simply by the timeat which an alarm call is received. Thedatalogger is programmed to stop call-ing after a specified number of retries,and this information has been utilized tofilter calls. If a call is not received fromthe same cable at the next assigned timefor that cable, the alarm condition istypically intermittent and not associatedwith ground movement. This techniquethat has been used to respond to alarmcalls when the cause of the alarm wasdetermined previously and the alarmcondition is being addressed.

Adjustment of alarm levels this is arelatively straightforward measure inwhich the specified threshold value isincreased either temporarily or perma-nently,

Specific portions of cable are inter-rogated it is possible to specify if theentire cable is interrogated or specific

segments of the cable are interrogated,Simultaneous deformation of adja-

cent cables two cables can be placedin one trench and the alarm condition isnot verified unless deformation has oc-curred on both cables simultaneously.

Action PlanConsider the following action plan thatwas implemented for the U.S. Gypsumproject in Plasterco.Action Level 1: Receive call from remote datalogger Down load data, identify cause of

alarm conditionAction Level 2: Visual inspection of alarm location Increased frequency of data acquisi-

tion Confirmed movement (based on vi-

sual inspection and/or redundant

measurements) Notify managementAction Level 3: Accelerating movement Confirmation with visual evidence

or redundant data Shut down highway

This type of action plan is based onreaction to an alarm call from the re-mote monitoring system. It inherentlyinvolves: decision making within acompressed time frame, and personnelon call 24/7.

During the USG project, the tasks ofmonitoring and response were assignedto in-house personnel to control costsand to expedite the decision makingprocess. For the other projects, callswere handled by an in-house centralcontrol center or out-sourced person-nel.

Monitoring a phone 24/7 is reactiveand can lead to burnout when alarmlevels are being exceeded frequently.This operational issue has been ad-dressed by the call-filtering techniquesand system design features mentionedabove. Equally significant are periodsduring which there is no alarm activity(e.g., Oct-Nov 2002 in Table 4). Foreach project, dummy cables are in placethat are used to create an artificial alarmcondition, verify system operation, andverify personnel response duringperiods when there is no alarm activity.

ClosureTDR technology is capable of monitor-ing movement over large lateral extentsand to great depths with a high densityof monitoring points. It is being used tomonitor deformation over active andabandoned mines, deformation alongdams and slopes, movement in land-slide areas, and sinkhole movement inkarst areas.

TDR-based systems are similar toother geotechnical measurement alarmsystems. The rationale for these mea-surements is significantly differentfrom the rationale for performancemonitoring where measurements aremade to compare actual and anticipatedbehavior. Alarm calls are received thatmay not be associated with actual defor-mation, but an action plan must be inplace to respond to each call and deter-



Table 3. Public Safety AlarmActivity

Location Alarms ReferenceBellevue,Washing-ton

75% - 90%filtered out

AIREF,2002

Columbus,Ohio

90% falsealarms

Andes,2005

Taiwan de-bris flow

70% falsealarms

Wu, 2005

Table 4. TDR Alarm Activity

Period TotalCalls

GroundMoving

OtherCause

Summit County, Ohio8/01 10 4 6 60%9/01 82 40 42 51%Total 92 44 48 52%

Plasterco, Virginia6/02 5 0 5 100

%7/02 17 4 13 76%8/02 14 2 12 86%9/02 7 0 7 100

%10/02 2 2 0 011/02 0 -- -- --12/02 52 0 52 100

%1/03 6 0 6 100

%2/03 0 -- -- --3/03 108 49 59 55%4/03 42 3 39 93%Total 253 60 193 76%



mine the cause. Unless this is accom-plished, the alarm calls will continue.

Proactive, scheduled data acquisi-tion and display has been the most ef-fective monitoring plan to observemovement before alarm levels were ex-ceeded.

ReferenceOConnor, K.M. (2007). Geotechnical

Alarm Systems Based on TDR Tech-nology, Proceedings of the 7th In-ternational Symposium on FieldMeasurements in Geomechanics,ASCE Geotechnical Special Publi-cation 175, Boston, Sept 24-27.

Kevin M. OConnor, Manager of theGeoTDR subsidiary of GeotechnicalConsultants, Inc., 720 GreencrestDrive, Westerville, Ohio 43081; Tel:(614) 895-1400; Fax: (614) 895-1171;email: [email protected]

geotechnical instrumentation news - john dunnicliff

Documents

bentonite pellets orchips

omittingthe bentonite

sono grout

overlying grout

ite grout

drill casing andplace

thefullygrouted method

boreholes bysurrounding