Cement Bentonite Grout Backfill for Borehole Instrument -- P.Erik Mikkelson

Cement-Bentonite Grout Backfill forBorehole Instruments

P. Erik Mikkelsen

Current Use of Bentonite

Materials and TechnologyAlthough this article advocates the useof cement-bentonite grout it is appropri-ate to review current bentonite backfilland sealing products to illustrate whytheir use should be limited. To the unini-tiated, there is a confusing array of vari-ous pellets, chips, granulated and pow-der-forms of sodium bentoni tecommercially available in North Amer-ica and elsewhere. Calcium type mont-morillonite and opalite are also used.The proper use of these products is amature and complex technology em-ployed by the environmental, wa-ter-well and petroleum industries. Basicresearch has been done and their prop-erties are known. For example, BaroidIndustrial Products (1994), a manufac-

turer of about two-dozen drilling-prod-ucts gives a five-day workshop in Hous-ton, Texas including a hands-onlaboratory day. Here, the user learnsabout the difference between drillingmuds and bentonite used for sealing(single-component grouts) and how toplace the materials. However, drillersand geo-professionals who installpiezometers and other borehole instru-ments for the geotechnical industrygenerally do not have this backgroundand often have limited understanding ofbentonite products outside of powderused to make mud and dropping chipsdown the hole. Going “high-tech” re-quires more knowledge, better equip-ment, and a higher level of quality con-trol, which appears to be unrealistic forborehole instrumentation.

Solid Bentonite SealsInstallation of bentonite balls, pellets orchips as seals above a sand pocket havedominated piezometer installation pro-cedures over the last 50 years. It is clas-sic procedure for open standpipepiezometers. Installation is usually verytime consuming, particularly on deeperboreholes or when caving occurs. Whenyou manage to get such seals installedwithout bridging the hole, there is usu-ally no question about their permeabil-ity being adequately low. These sealshave a very low permeability, oftenlower than many in-situ clays. Estab-lishment of proper procedures forplacement of such materials has beenimportant not only because sealing isimportant, but also because the installa-tion conditions are often difficult andthe procedures cumbersome. Many in-stallations end up less than satisfactory.Over the last decade, experience hasshown that bentonite chips (as opposedto balls and pellets) are the easiest toplace. These chips look like crushedgravel and hydrate very slowly. How-ever, the fine clay-dust unavoidablymixed in with the chips can make condi-tions increasingly sticky as filling pro-ceeds, leading to bridging and blockagehigher in the borehole. For relativelysimple installations where the sealheights and volumes are not too exten-sive, this material usually does notbridge and is often the seal of choice forenvironmental observation wells andopen standpipe piezometers.

Pellets, chips and polymer-sus-pended granules can also be tremied tothe desired location in the boring. Thepotential for clogging is always a hazard,but at least a clogged tremie-pipe can bewithdrawn and discarded. If clogging orbridging occurs while dropping the ma-

GEOTECHNICAL INSTRUMENTATION NEWS

Geotechnical News, December 2002

The backfill for a borehole instrument is often an item that receives adisproportionate lack of attention. The behavior of the backfill, thematerial that is in the most intimate contact with both the formationand the instrument, is critical for obtaining correct measurements. Inmany situations, instrument observations may just reflect unstablebackfill, lack of backfill or backfill that is too stiff or too soft. Sand,gravel and various bentonite products have proven to be both toodifficult to place and often entirely inappropriate. Experience hasshown the author that cement-bentonite grout is the most universallyapplicable material for successfully backfilling a borehole instrument.Single-component bentonite grouts have been used in related indus-tries a long time, and have been adopted for borehole instrumentationwith mixed success. Their uses are more involved and, as explainedbelow, should be avoided. The use of fly ash as a substitute for cementpromises to be a good way for reducing grout stiffness when required.

terials directly into the borehole, there isno recourse except to start over.

Bentonite GroutsThese products are the least desirablefor sealing or backfilling. They aremade from water and powdered benton-ite mixed into slurry-like drilling mud,but to a higher density with the aid ofadditives and specialized grout mixingunits. The higher the bentonite sol-ids-content is, the lower the permeabil-ity is. The water-content of such slurryis extremely high and it never really setsup to anything more than thick paste,not a solid like the chip-seals. A numberof bentonite sealing grouts are avail-able, but none appear to set up to a solidform. They are sensitive to over-mixing(leading to a flash set) and can be diffi-cult to pump down the small diametergrout pipes (3/4 inch) often used forpiezometers and other geotechnicalborehole instruments where space is at apremium. Their working time tends tobe too short, and mix dilution to circum-vent mixing and pumping problems willlead to a permanently soupy backfill.

Cement-Bentonite Grouts

BasicsA bentonite grout backfill consisting ofjust bentonite and water may not be vol-umetrically stable and introduces un-certainty about locally introduced porewater pressures caused by the hydrationprocess. Introducing cement, even asmall amount, reduces the expansiveproperties of the bentonite componentonce the cement-bentonite grout takesan initial set. The strength of the setgrout can be designed to be similar tothe surrounding ground by controllingthe cement content and adjusting themix proportions. Controlling the com-pressibility (modulus) and the perme-abi l i ty is not so easy. Weakercementitious grouts tend to remainmuch stiffer than normally consolidatedclays of similar strengths. The bentonitesolids content has the greatest influenceon the permeability of cement-benton-ite grout, not the cement content.

Cement-bentonite grouts are easierto use than bentonite grouts, provide along working time before set and are

more forgiving should the user deviatefrom the design recipe or mixing equip-ment and method. It is easier to adjustthe grout mix for variations in tempera-ture, pH and cleanliness of the water.Pure bentonite grouts must be mixedand deployed by strictly following mea-sured quantities and procedures that arenot common practice among drillers do-ing test borings.

Strength and DeformationThe general rule for grouting any kindof instrument in a borehole is to mimicthe strength and deformation character-istics of the surrounding soil rather thanthe permeability. However, while it isfeasible to match strengths, it is unfeasi-ble with the same mix design to matchthe deformation modulus of ce-ment-bentonite to that of a clay for ex-ample. The practical thing to do is to ap-proximate the strength and minimizethe area of the grouted annulus. In thisway the grout column would only con-tribute a weak force in the situationwhere it might be an issue.

Strength data collected informallyfrom various sources by the author overthe years are summarized in Figure 1. Atrend line drawn through the data pointsillustrates the decrease in strength withincreasing water-cement ratio. The wa-ter-cement ratio controls the strength ofthe set grout (Marsland, 1973).

Marsland’s rule-of-thumb is to makethe 7-day strength of the grout to matchone quarter that of the surrounding soil.

Water and cement in proportionsgreater than about 0.7 to1.0 by weightwill segregate without the addition ofbentonite or some other type of fillermaterial (clay or lime) to suspend thecement uniformly. In all cases sufficientfiller is added to suspend the cement andto provide a thick-creamy-but-pump-able grout consistency. The bentonitedoes not add significant strength to thegrout. The background data for Figure 1also suggests the amount and type ofbentonite or hydrated lime does not in-fluence strength as long as the grout isnon-bleeding and pumpable. If thegrout bleeds the water-cement ratio de-creases and strength increases. If fly ashwere to be used as a substitute for ce-ment the strength and modulus wouldbe expected to drop. Fly ash containsless cementing agents (calcium andgypsum).

Grout PermeabilityPermeability of the grout is mainly anissue that is limited to piezometer in-stallations, and is the subject of a paperto be published in the near future. It isgeneral practice to grout the boreholeabove a bentonite seal placed above thepiezometer “intake zone” (sand thatsurrounds the piezometer), but the pa-



Figure 1. 28-day cement-bentonite grout strength vs. water-cement ratio. Data fromauthor’s personal files.

per argues that boreholes may beful ly-grouted for diaphragmpiezometers, omitting both the sand andthe bentonite seal. Two articles byVaughan (1969 and 1973) make thepoint clear in theory and practice. Thisprocedure not only simplifies difficultinstallation situations, but also im-proves the quality of the installation. Itis really not a question of whether or notdiaphragm piezometers work whenfully surrounded with grout, but rather itis a question of making a grout with asuitable permeability. Cement-benton-ite grout is generally well suited to ac-complish this task.

The cement-bentonite fabric whenset is an irregular honeycomb structureheld together with both cured cementand colloidal bonds. It is a highly po-rous solid with a low permeability thatlies somewhere in the cement and ben-tonite range, from 1x10-5 to 1x10-9

cm/sec. Typical published values ofpermeability are listed in Table 1.Vaughan (1973) quotes a coefficient ofpermeability for a pumpable ce-ment-bentonite grout mix on the orderof 5x10-8 cm/sec. For low bentonitesolid contents the permeability can beexpected to be close to 1x10-6 cm/secand for higher bentonite solids contentit would be close to 1x10-8 cm/sec. Thisis an area for further testing and re-search where the water-cement ratio,bentonite solids content and permeabil-ity should be established.

Typically used BentoniteDrilling contractors in the US who dotest borings in soil usually carry a highyielding sodium bentonite such asSupergel or Quik-Gel brands on theirrigs. This is a finely ground, powered

form of bentonite that yields as muchdrilling mud per sack as possible with-out additives. Other bentonite powderproducts have additives to enhance cer-tain mud characteristics such as viscos-ity, density and filtration. Such addi-tives are probably not detrimental tomaking a suitable cement-bentonitegrout, but are not really relevant or costeffective in such a context. What seemsnot to be well understood is the feasi-bility of using coarser grains of benton-ite to increase solids content for lower-ing the permeabi l i ty incement-bentonite grout. Would poly-mer additives be needed for mixing sta-bility, for example?

As a side-note, sodium bentonite ab-sorbs more water than calcium benton-ite. According to clay mineralogists(Papp, 1996) the presence of sodium asthe dominant exchangeable ion facili-tates many interlamellar water layers tobe absorbed into the crystalline struc-ture, a phenomenon which does not oc-cur with calcium or magnesium asdominant ions. Sodium bentonite is

characterized as capable of absorbing atleast five times its weight in water andexpands when fully saturated with wa-ter to a volume 12 to 15 times its origi-nal dry size.

Sodium bentonite powder appears tobe the most practicable and efficient touse in this context, but this does notmean that other fillers of different grainsizes and composition could not beused. It is matter of availability andconvenience of a good product for thisapplication. For example, in a 50-gal-lon batch of cement-lime grout, 150pounds of hydrated lime can be re-placed by using about 25 pounds ofbentonite powder.

Mix Design RulesIn order to keep field procedures simplethe emphasis should be on controllingthe water-cement ratio. This is accom-plished by mixing the cement with thewater first. This is contrary to proceduresused at more sophisticated grout plantsfor compaction grouting and sealing pur-poses. When water and cement aremixed first, the water-cement ratio staysfixed and the strength/modulus of the setgrout is more predictable. If bentoniteslurry is mixed first, the water-cementratio cannot be controlled because theaddition of cement must stop when theslurry thickens to a consistency that isstill pumpable.

Making cement-bentonite grout inthe field is a straightforward process.The most effective mixing is done in abarrel or tub with the drill-rig pump, cir-



Table 1. Permeability, k, of some grouts

Grout Type Characteristics k (cm/sec) Source

Neat cement w/c ratio = 0.89 to 0.53 10-5 to 10-7 Baroid

Bentonite chips hydrated 10-8 Baroid

Bentonite slurry 6 % solids 10-5 Baroid

Bentonite slurry 20 % solids 10-8 Baroid

Cement-bentonite water/solids = 4 to 1 10-6 Vaughan, 1969

Cement-bentonite w : c : b = 4 : 1 : 1 5 x 10-8 Vaughan, 1973

Table 2. Cement-bentonite grout mixes

Application Grout for Medium to Hard Soils Grout for Soft Soils

Materials Weight Ratio byWeight

Weight Ratio byWeight

Water 30 gallons 2.5 75 gallons 6.6Portland Cement 94 lbs.

(1 sack)1 94 lbs.

(1sack)1

Bentonite 25 lbs.(as required)

0.3 39 lbs.(as required)

0.4

Notes The 28-day compressive strengthof this mix is about 50 psi, similarto very stiff to hard clay. Themodulus is about 10,000 psi.

The 28-day strength of thismix is about 4 psi, similarto very soft clay.

culating the batch through the pump in50 to 200 gallon quantities. The rigpump provides the kind of jet-mixingrequired for getting the job donequickly. Any kind of bentonite powderused to make drilling mud combinedwith Type 1 Portland cement and watercan be used, but the appropriate quan-tity of bentonite will vary somewhat de-pending on grade of bentonite, mixingsequence, mixing effort (agitation), wa-ter pH and temperature.

Grout mixes should be controlledby weight and proportioned to give thedesired strength of the set grout. Theconversion factors contained in Ap-pendix H.10. in Dunnicliff (1988,1993) are very helpful in mix design.Two mixes are given in Table 2 that var-ies in 28-day strength from 50 psi to 4psi for water-cement ratios of 2.5 to 6.6respectively.

The amount of bentonite that is re-quired for the above mixing procedurewould vary due to factors mentioned ear-lier. The amount of bentonite shown inTable 2 should only be used as a guide,but is also handy for estimating materialquantities to be shipped to the site. Withthis method more bentonite is requiredthan if water and bentonite were mixedfirst. This is an advantage from the stand-point of wanting a low permeability.When the bentonite solids content in-creases, the density increases and thepermeability is lowered. A lower perme-ability is generally preferred since ce-ment-bentonite grouts have a higherpermeability than high-density bentonitegrout or chip seals. Thus, another goodreason for mixing water and cement be-fore adding bentonite.

Old habits die hard, so that some us-ers will insist on mixing water and ben-tonite powder first. This is normally theway drilling mud is mixed and it yieldsmore slurry per sack of bentonite thanthe above method. Also, use of hydratedbentonite with cement added last iscommon practice in grouting technol-ogy for ground improvement. Suchmixes are highly thixotropic and rely onindustrial type mixing plants and meth-ods. The cement content is difficult tocontrol under ordinary borehole instal-lation circumstances.

Mixing ProcedureA rig pump with one suction hose and areturn hose fitted with a jet nozzle and a50-gallon barrel, shown in Figure 2, arethe minimum requirements for circula-tion batch mixing of grout. Paddle orhigh shear mixers can also be used. Ameasured quantity of clean water goesinto the mixing tub/barrel first and the

pumping and circulation starts. Thenthe cement is gradually added to the wa-ter and mixed thoroughly. At this stagethe mix is like gray water. Next, benton-ite powder is slowly added into the jet-ting area of the barrel, slowly enough soclumps of bentonite do not form. Thisshould be constantly checked by scrap-ing the bottom with a shovel. Whenclumps form, slow down and do not addany more powder until they are dis-solved. Keep adding bentonite until thewatery mix transitions to an oily/slimyconsistency. Observe the consistencywhile mixing and let the grout thickenfor another five to ten minutes. Gen-erally, the mix thickens some more withadded mixing time. Add more bentoniteas required. When it is smooth and likethick cream or pancake batter, it is asheavy as is it feasible to pump. Drips ofthe grout should then barely come off adipped finger and should form “craters”in the fluid surface. That is the correctconsistency for pumping the groutbatch down the tremie-pipe. When pos-

sible, withdraw the tremie after eachbatch an amount corresponding to thegrout level in the boring to keep thepumping pressures as low as possible.When mixing grout 100 feet or morefrom the borehole, thinner consistencywould be required, but at the risk ofsome bleeding.

Additional Considerations

Strength is often used to characterize agrout for deformation-type instruments,but modulus of deformation should ide-ally be the basis for judging compatibil-ity with ground conditions. The groutcolumn in a borehole will carry a totalaxial force smaller or greater than thematerial it replaced, according to itsstiffness. When there is too much stiff-ness or force, displacements will be di-minished and axial measurements canbe less than displacements of the sur-rounding ground. Thus extra grout stiff-ness for extensometers is much moreundesirable than for inclinometers, forexample. More care should be taken inmaking a grout for axial borehole defor-mation measurements than for lateraldeformation measurements.

Instrumentation installations oftenencounter the combination of both softand hard ground in the same borehole.Obviously, staged grouting to match therequired properties would be the ideal



Figure 2. Circulation batch mixing of grout (minimum configuration). Aftermixing grout is tremied to bottom of the borehole.

procedure, but this is seldom warrantedor practicable. Most of the design and in-stallation challenge lies with deforma-tion measurements in the axial directionof the borehole where large volumes ofgrout backfill must be placed. So, for ex-tensometers, it is better to err on thesofter side of the spectrum.

Lateral displacements of an incli-nometer casing are generally unaffectedby added grout stiffness. Where thegrout column is too stiff the displace-ments will be distributed over a greaterdepth interval, but not be diminished inoverall magnitude. The same is proba-bly true if the grout is too soft, but thereis the additional concern for lack of lat-eral confinement. Since inclinometercasings generally are under compres-sion, lack of backfill or confinement canproduce localized shifts in the borehole,masking smaller actual displacements.So, for inclinometers, it is better to erron the stiffer side of the spectrum.

It may not be possible to achieve asuitable grout with cement for softerclays. Fly ash promises to be a goodsubstitute for reducing the modulus, butmore testing is needed in this regard.

Conclusions

Grout backfill should ideally be selectedaccording to the field instrument type be-ing used and the given ground condi-tions. The reality however is that grout-ing practices will remain relatively crudeand, at best, with only marginal controlover the grout properties. Drillers, geolo-gists and engineers alike still have a lot tolearn about what is appropriate. We can-not just borrow ideas from drilling mudand grouting technology that have no rel-evance to what is needed for instrumen-tation functionality.1. Avoid using a bentonite alone for a

borehole grout. It is not a volumet-rically stable material and can in-fluence both piezometer anddisplacement measurements whenit keeps hydrating or desiccating. Itis often very difficult to place suc-cessfully.

2. A stable grout can be made using ce-ment or fly ash with bentonite. Rela-tively small amounts of cement or

fly ash are used as compared togrouts used for other geotechnicalpurposes such as compaction grout-ing and sealing of seepage. Groutingfor instrumentation has differentproperty priorities.

3. Grouting for field instrumentationshould remain a relatively simpleendeavor, using materials com-monly available to drillers. How-ever, when working in soils likenormally consolidated clay, moreattention should be paid to the mixdesign. Since little information isavailable on softer grouts, particu-larly those mixed with fly ash, a fewtrial batches in the laboratory areappropriate to determine basiccharacteristics for use on such pro-jects.

4. Grout mixing should start with wa-ter and cement (or fly ash) first.Strength and modulus are more pre-dictable that way. Also, and just asimportant, is that more bentonitesolids can be added to the mix tolower permeability where requiredfor sealing.

5. Grout permeability is an issue forpiezometers installed in clay. Thegrout should have permeability nogreater than one (possibly two) or-ders of magnitude above the clay toget representative readings.

6. Grout stability is very importantduring both the liquid and set condi-tions. The liquid grout consistencyshould be as thick as possible, yetliquid enough to be pumpable. Thisis a property that requires field expe-rience. Field crews tend to err to themore liquid end of the spectrum, re-sulting in bleeding and possiblycracking when set.

7. Strength is often used to character-ize a grout for deformation-type in-s t ruments , but modulus ofdeformation should ideally be thebasis for judging compatibility withground conditions. There is very lit-tle in the literature to help us select agrout mix for sealing piezometers inboreholes. Further testing and re-search is needed (see editor’s notebelow).

Acknowledgements

The author acknowledges the technicalcontributions of Gordon E. Green andthe editorial contributions by JohnDunnicliff. They both gave a significantamount of their time to make this a morevaluable and readable product.

References

Bariod Industrial Products (1994),“Drilling and Boring Fluids Work-shop”, Houston, TX.

Dunnicliff , John, (1988, 1993),“Geotechnical Instrumentation forMonitoring Field Performance”, J.Wiley, New York, 577 pp.

Marsland, A. (1973), “Discussion, Prin-ciples of Measurement”, in Field In-strumentation in GeotechnicalEngineering, British GeotechnicalSociety, Halsted Press, a Division ofJohn Wiley, pp. 531-532.

Papp, J.E. (1996), “Sodium Bentonite asa Borehole Sealant”, Chapter 12, inSealing of Boreholes and Under-ground Excavations in Rock. Editedby K. Fuenkajorn and J.J.K. Daemen,Chapman & Hall, London, UK.

Vaughan, P. R. (1969), “A Note onSealing Piezometers in Boreholes”,Geotechnique, Vol. 19, No. 3, pp.405-413.

Vaughan, P. R. (1973), “Discussion,Principles of Measurement”, inField Instrumentat ion inGeotechnical Engineering, BritishGeotechnical Society, Halsted Press,a Division of John Wiley, pp.542-543.

P. Erik Mikkelsen, Consulting Engineer,Geometron, 16483 SE 57th Place,Bellevue, WA 98006Tel: (425) 746-9577e-mail: [email protected]



Editor’s Note:John Dunnicliff, Erik Mikkelsen,andAllen Marr have decided to plan andconduct a test program to mix vari-ous proportions of cement and ben-tonite, also fly ash and bentonite,and test for strength, permeability,compressibility and volume stability.The results will be published in GINas soon as they are available.

Cement Bentonite Grout Backfill for Borehole Instrument -- P.Erik Mikkelson

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