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GEOTECHNICAL INSTRUMENTATION NEWSThe Use of the Fully-grouted Method forPiezometer InstallationPart 1Iván A. ContrerasAaron T. GrosserRichard H. Ver StrateIntroductionThe fully-grouted method described inthis article entails installing apiezometer tip in a borehole which isbackfilled entirely with cement-bentonitegrout. Part 1 of this article presents adetailed discussion of the fully-groutedmethod, including the installation procedureand 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 InternationalSymposium on Field Measurements inGeomechanics, and are published inGIN with permission from ASCE.A crucial parameter for the successof the fully-grouted method is the permeabilityof the cement-bentonitegrout. Vaughan (1969) postulated thatthe cement-bentonite grout should havea permeability no greater than two ordersof magnitude higher than the surroundingsoil 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 surroundedwith a high permeability material,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-waterpressure reading in the riser pipewithout significant time lag (Hvorslev,1951).With the development of diaphragmpiezometers (e.g., pneumatic and vibratingwire), the method developed forstandpipe piezometers was used for diaphragmpiezometer installations(Dunnicliff, 1993). This has been acommon practice for decades and theresulting installation is shown on Figure1b. However, because of the30 Geotechnical News, June 2008

GEOTECHNICAL INSTRUMENTATION NEWSFigure 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 seepagearound a fully-grouted piezometer (borehole not to scale).low-volume operation of diaphragmpiezometers, the sand pack around theinstrument tip is unnecessary, and thediaphragm piezometer can be installedin the borehole surrounded by cement-bentonitegrout. 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 installationusing the fully-grouted method, inwhich a diaphragm piezometer tip is setin a drilled borehole and entirely backfilledwith cement-bentonite grout. Thefollowing is a detailed description of theinstallation procedure for a vibrating-wiresensor tip in typicalgeotechnical boreholes (i.e., 140 mm),including preparation of piezometer assemblyand materials, grout mixing,piezometer construction, and theoreticalbackground.Piezometer AssemblyConstruction of the piezometer assemblycommonly begins with attachmentof the sensor tip to a sacrificial groutpipe. The sacrificial grout pipe, whichcan be either belled-end electrical conduitor threaded PVC well casing, isgenerally constructed or laid out on theground in manageable lengths for handling.The piezometer location is selectedby reviewing the soil stratigraphy.The sacrificial grout pipe willgenerally extend to the bottom of theborehole for support; therefore, it ispossible 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 diameterPVC well casing) is used.Large-diameter or stronger grout pipemay be required for deeper installationswith higher pumping pressures.The sensor tip, which has been saturatedfollowing the manufacturer’s directions,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 cableare 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 installationof 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 removedcarefully to prevent damage tothe cables and the cables should be separatedaround 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 attachedto an inclinometer casing, and asingle borehole is used for measuringboth deformation and pore-water pressures,resulting in reduced drillingcosts. However, the inclinometer casingjoints must be sealed. This techniquehas been used successfully by the authorson several projects.MaterialsThe cement-bentonite mixes describedin this article use Type I Portland cementand sodium bentonite powdersuch as Baroid Aquagel Gold Seal orQuickgel. The water used in the mixesshould be potable water to prevent possibleinteraction of chemical constituentsin the water with the cement-bentonitemixture.Grout MixingThe mixing procedure described in thisarticle assumes the availability of a capabledrill-rig pump and a high-pressure,jet-type nozzle attachment on theend of a mixing hose. In most cases, thedrill-rig pump provides enough pressurefor the jet-mixing required to obtaina desirable mixture. Other methodsGeotechnical News, June 2008 31

GEOTECHNICAL INSTRUMENTATION NEWSFigure 3. Normalized error versus permeability ratio.The seepage analyses were performedsimulating upward and downwardflow using two sets of imposedtotal head conditions (i.e., 10 and 20 m)that induce flow under steady-state conditions.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, ε, defined as the differencein computed pore-water pressurebetween the soil and the grout, wasdetermined during each model run atpoints in the soil and grout 20 m belowthe ground surface, as shown on Figure2.Results of Computer ModelingSeveral model runs were made in whichthe permeability ratio, k grout /k soil ,wasvaried from 1 to 10 7 . Figure 3 shows theresults of the seepage simulations interms of the normalized error, i.e., ε dividedby the pore-water pressure in soil,u soil , against the permeability ratio. Figure3 also shows that the normalized erroris zero for all practical purposes upto permeability ratios of 1,000 fordownward and upward flow and the twosets of imposed total heads. As the permeabilityratio increases beyond 1,000,the normalized error increases up toabout ±10 percent at permeability ratiosof 10,000. As the permeability ratiocontinues to increase to 10 7 , the normalizederror also increases up to about23 and 40 percent, respectively, for the10-m and 20-m imposed total heads.In summary, the finite-element computermodel revealed that the permeabilityof 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 surroundingground. The minimum permeabilitythat is likely to beencountered in natural soils is on the orderof 10 -9 cm/s. As a result, the cement-bentonitegrout mix used in thefully-grouted method needs to have apermeability of, at most, 10 -6 cm/s.Part 2 of this article will discuss laboratorytest results of six cement-bentonitegrout mixes and field examples ofapplications of the fully-groutedmethod.The Use of the Fully-grouted Method forPiezometer InstallationPart 2Laboratory Testing ProgramA laboratory testing program was developedto evaluate the range in permeabilityand strength of cement-bentonitegrout for piezometer installationsusing the fully-grouted method. Thetest program was designed so that smallbatches of grout could be mixed in acontrolled environment without largegrout-batch mixing equipment. Six mixdesigns were chosen to represent a widerange of values that would reasonablybe used on projects.Sample PreparationMixing the grout used for laboratorytesting began with calculating the desiredquantities of material and thenweighing individual portions of cement,water, and bentonite. Additionalbentonite was prepared in anticipationGeotechnical News, June 2008 33