FEDERATION OF PILING SPECIALISTS BENTONITE SUPPORT FLUIDS .
FEDERATION OF PILING SPECIALISTSBENTONITE SUPPORT FLUIDSINCIVIL ENGINEERINGJanuary 2006www.fps.org.uk
Federation of Piling Specialists – January 2006 (2nd edition)(first published April 2000)
.7.19.7.29.89.99.1010The use of bentonite support fluids in civil engineeringThe production and properties of bentonite powderBentonite specificationsBentonite slurry propertiesThe effect of slurry properties on required functionsExcavation supportRetention within the excavationSuspension of solidsDisplacement by concreteCleaningPumpingPreparation of bentonite slurryCleaning bentonite slurryRe-use of bentonite slurryDisposal of bentonite slurryTestingDensityTest procedureCalculation of slurry density for mix proportionsGrain specific gravity of the bentonite powderSand contentTest procedureRheological measurementsTest procedure for the electrically driven viscometerTest procedure for the hand cranked viscometerChecking viscometersCalculation of resultsFlow conesThe Marsh funnelThe ShearometerPhTest procedureTypical pH valuesFiltrate lossTest procedureTest resultsBleedingMoisture contentWater compatibility testingBibliography Federation of Piling Specialists – January 2006 (2nd edition)(first published April 91920202122222323242525252627
ACKNOWLEDGEMENTSThe Federation of Piling Specialists acknowledges the contributions made in thepreparation of this document by the following members of the Working Group:D. J. BallSteetley Bentonite and Absorbents LimitedM. T. HutchinsonKvaerner Technology LimitedS. A. JefferisGolder Associates (UK) LimitedP. G. ShottonKvaerner Cementation Foundations LimitedL. Stansfield (Chairman)Bachy Soletanche LimitedA. J. WillsKvaerner Technology Limitedand also the assistance provided by Dr. M. Stocker, Chairman of the Technical WorkingGroup of the European Federation of Foundation Contractors and Chairman of CEN/TC288: Special Geotechnical Works. Federation of Piling Specialists – January 2006 (2nd edition)(first published April 2000)
PREFACEBentonite is the name used for a range of clays that can swell and gel when dispersed inwater. The name “bentonite” originates from the discovery of this type of clay near FortBenton, USA, in the 19th Century. This was a natural sodium bentonite, and has beenmined extensively for many years in Wyoming and Dakota for oil well drillingapplications.Bentonite is now used extensively throughout the world in civil engineering, but the costof transporting original “Wyoming” bentonite from the USA has led to the use ofalternatives from other sources. A large proportion of bentonite now used is thereforefrom other parts of the world.It is important to recognise that the properties of bentonites from different sources vary,and to take these variations into account when deciding on the suitability of a particularbentonite for a specific purpose.The purpose of this document is to provide information that will enable a decision to bemade as to whether or not a particular bentonite will produce a satisfactory support fluid,and to give guidance on the preparation, use, re-use and disposal of the bentonite slurry,and also on methods of testing.Since the first edition of this guide the use of polymer support fluids as an alternative tobentonite has become relatively common. Polymer support fluids are fundamentallydifferent to bentonite support fluids and their scope is too great to be covered in this guide. Federation of Piling Specialists – January 2006 (2nd edition)(first published April 2000)
Federation of Piling Specialists – January 2006 (2nd edition)(first published April 2000)
1 THE USE OF BENTONITE SUPPORT FLUIDS IN CIVIL ENGINEERINGBentonite support fluids are widely used in civil engineering.One of their main uses is to support the sides of panel excavations for diaphragm walls. Inthis application, the bentonite must be capable of forming a barrier or “filter cake” on thesides of the excavations to prevent loss of fluid into the ground and provide a surface layeragainst which the pressure of the fluid can act in order to resist external pressures from thesoil and groundwater.Bentonite support fluids are also widely used in the construction of large diameter boredpiles. This application is similar to that for diaphragm wall construction except for theshape of the excavation.Small diameter boreholes for site investigation work or other purposes are often boredthrough unstable strata using bentonite support fluid as an alternative to temporary casings.Another widely used application is in the construction of cut-off walls below ground toform barriers to groundwater or to surround areas of contaminated land where leachatesmust be contained. In this application, cement and/or other materials are added to thebentonite to form a slurry which remains fluid for several hours before setting to form arelatively soft barrier wall. A thin flexible membrane is sometimes inserted into the slurryin the excavation before it sets. This application is not treated in this document.Bentonite support fluids are also used in some cases to support the excavation face in frontof tunnelling machines and to transport the cuttings to desanding equipment where they areremoved before the fluid is re-circulated for further use.The properties of bentonites from different sources vary, and it is important to understandthat a property which may be required for one application may not be required for another.For example, gel strength is important if material has to be kept in suspension while thefluid is at rest, but may not be important if the fluid is agitated continuously in acirculatory system. Differences in the properties of available bentonites should thereforebe considered before deciding which bentonite to use for a particular application.Polymer support fluids, as an alternative to bentonite, have grown in usage for bored pilingoperations in recent years. They behave and have different properties to to bentonitesupport fluids and their scope is too great to be covered in this guide. Engineers may wishto consider their use, but should always seek the necessary specialist advice on their usageand application.2 THE PRODUCTION AND PROPERTIES OF BENTONITE POWDERCommercial bentonites are hydrated alumino silicates, and comprise predominantly themineral montmorillonite. The name “montmorillonite” is derived from the discovery ofthis type of clay near Montmorillon in France.3 Federation of Piling Specialists – January 2006 (2nd edition)(first published April 2000)
Bentonite occurs as a clay ore containing up to 50 % moisture. Commercially viabledeposits consist of accessible clay seams, low in accessory minerals, which can be cleanlyworked to minimise unwanted inclusions such as sand. The characteristics of the clayvary, and selection is based on factors such as yield and gelling ability.There are three common types of bentonite, namely: Natural sodium bentonite Natural calcium bentonite Sodium-activated bentoniteAll bentonites have a capacity to exchange cations which is much greater than that of otherclays such as china clay, ball clays and attapulgite.Natural sodium bentonite is characterised by very high swelling ability, high liquid limitand low filter loss. This bentonite was used as the standard by which all other bentoniteswere measured for many years. The predominant exchangeable cation in natural sodiumbentonite is the sodium cation but there may also be significant amounts of other cationspresent.Natural calcium bentonite, where calcium is the predominant exchangeable cation, ismined world-wide. It has much lower swelling ability and liquid limit, and much higherfilter or fluid loss than natural sodium bentonite.Sodium-activated bentonite is produced by the addition of soluble sodium carbonate tocalcium bentonite. This effects a base exchange on the surfaces of the clay particles,replacing calcium ions with those of sodium. The result is a bentonite exhibiting many ofthe typical characteristics of a natural sodium bentonite.Most bentonites used in civil engineering to produce support fluids are sodium-activated.Natural sodium bentonite is rarely used because of its high cost. Natural calcium bentoniteis usually not suitable for this purpose.Processing methods used in the production of sodium-activated bentonite depend on thedeposit and its geographical location. They could include several of the following: Selective mining. Field laying and rotavation. This is a process used in hot, dry climates where thebentonite is spread out and rotavated with the addition of sodium carbonate. Crushing to 50 mm maximum size at the production plant. Extrusion. In wet climates, where field laying is not possible, a blend of rawbentonite and sodium carbonate is extruded to promote activation. Drying by rotary louvre drier to optimum moisture contents in the range 11 to22 % of dry weight. Milling to a particle size that promotes good powder flow but does not diminishclay performance. Generally, powders with 95 % of particles less than 150microns (dry sieve basis) are used.4 Federation of Piling Specialists – January 2006 (2nd edition)(first published April 2000)
The rheological characteristics of bentonites (i.e. their behaviour as bentonite slurries whenmixed with water) influence their suitability for use in civil engineering applications.Natural sodium bentonite and sodium-activated bentonite, when dispersed in water underconditions of high shear mixing, break down into minute plate-like particles, negativelycharged on their surfaces and positively charged along their edges. Typically, if 3% ormore bentonite powder is dispersed in water, a viscous slurry is formed which is thickwhen allowed to stand but thin when agitated. This phenomenon is known as thixotropy,and results from the orientation of the plate-like particles within the slurry. When theslurry is allowed to stand, the plate-like particles become orientated as shown in Figure1(a). Electrical bonding forces between the particles form an interlocking structure whichcauses the slurry to gel. When the gel is agitated, the electrical bonds are broken and theslurry becomes fluid, with the particles orientated in random fashion as shown in Figure1(b).Fig. 1(a)Fig. 1(b)3 BENTONITE SPECIFICATIONSBentonite powder is normally satisfactory for use in support fluids in civil engineering if itcomplies with one of the following specifications: API Specification 13A, Fifteenth Edition, May 1, 1993, Section 6 (OCMA gradebentonite) The Engineering Equipment and Materials Users Association (EEMUA)Publication No. 163 entitled “Drilling Fluid Materials”, last reprinted in 1988.The API Specification and the EEMUA Specification differ slightly in some respects. Themain differences in the specifications are in the requirements for the rheological propertiesand filtrate loss of the slurry. The rheological properties of the slurry at different rates ofshear are determined using a direct reading viscometer. Filtrate loss is determined using afilter press. Test methods are described in detail in Section 8.Tests to determine the properties of a bentonite slurry in accordance with the APISpecification are carried out on a 6.4% suspension of bentonite in deionized water, agedfor up to 16 hours. This specification requires a minimum viscometer dial reading of 30 at5 Federation of Piling Specialists – January 2006 (2nd edition)(first published April 2000)
600 rpm, and a maximum Yield Point/Plastic Viscosity Ratio of 6. Expressed anotherway, this equates to a minimum dial reading of 30 at 600 rpm, and a maximum dial readingof 0.875 times the 600 rpm reading at 300 rpm. The filtrate volume must not exceed 16 mlin 30 minutes.The EEMUA Specification expresses the rheological properties in terms of Yield (not tobe confused with Yield Point). This specification requires the yield of a bentonite/distilledwater slurry, aged for 24 hours and having an apparent viscosity of 15 cP (centipoise), tobe not less than 16 m3/tonne. An Apparent Viscosity of 15 cP equates to a 600 rpmreading of 30, and a Yield of 16 m3/tonne equates to a 6.4% bentonite suspension, which isthe same as that used in the API Specification. Thus, the requirement of the EEMUASpecification can be re-written as follows: A 6.4% suspension of bentonite in distilled water, aged for 24 hours, should have aminimum viscometer dial reading of 30 at 600 rpm.This is the same as the API Specification except that the API Specification only requiresthe bentonite suspension to be aged for up to 16 hours. The EEMUA Specification doesnot specify a maximum Yield Point/Plastic Viscosity Ratio therefore does not require aviscometer dial reading to be taken at 300 rpm. The filtrate volume is measured on a 7.5%suspension, aged for 24 hours, and should not exceed 15 ml in 30 minutes.The maximum moisture content of the bentonite powder is specified as 13% in the APISpecification and 15% in the EEMUA Specification. This difference will not affect theperformance of the bentonite, therefore the requirement of the API Specification could, ifnecessary, be relaxed to 15% to accommodate some bentonites in common use.Both Specifications require the residue greater than 75 microns (US standard sieve No.200) not to exceed 2.5% by weight. The EEMUA Specification has the additionalrequirement that the amount of bentonite passing through a dry 100 mesh (150 micron)screen shall be at least 98% by weight.Where applicable, testing procedures should be carried out in accordance with the latestedition of API Publication RP13B “API Recommended Practice - Standard Procedure forTesting Drilling Fluids”.4 BENTONITE SLURRY PROPERTIESBentonite slurries of the type normally used to support excavations, can vary widely intheir physical and chemical properties. They must, however, perform the followingfunctions:a) Support the excavation by exerting hydrostatic pressure on its wallsb) Remain in the excavation, and not flow to any great extent into the soilc) Suspend detritus and prevent sludgy layers building up at the base of the excavation6 Federation of Piling Specialists – January 2006 (2nd edition)(first published April 2000)
In addition, they must allow:d) Clean displacement by concrete, with no significant interference with the bond betweenreinforcement and set concretee) Cleaning to remove suspended detritus, by passing through screens and hydrocyclones,in preparation for re-usef) Easy pumpingIn general, items a) to c) require thick, dense slurries, while items d) to f) need very fluidslurries. There are therefore conflicting requirements which must be resolved before anacceptable specification for slurry properties can be drawn up.In the following paragraphs, consideration is given to the effect of slurry properties oneach function. This will assist in the establishment of limits for most of the slurryproperties and in defining a slurry which is acceptable for each function and also for theoverall excavation process.The primary aim of any bentonite slurry specification must be to ensure that the slurry iscapable of fulfilling functions a) to d) without deleterious effects on the finished pile, wallor other form of construction. In addition, for economic and environmental reasons, themaximum re-use and minimum disposal of used slurry are required. The specification forthe slurry properties should be as wide as possible, consistent with achieving satisfactoryresults. In most cases, for any given function, maximum and minimum values can bechosen which then enable limits to be derived as the basis for the specification.4.1 The effect of slurry properties on required functions4.1.1 Excavation supportIn order to exert stabilising pressure on permeable walls of an excavation, bentonite slurrymust form a seal on or near the surface of the soil. This avoids loss of slurry into the soil,with consequent increase in pore pressure and reduction in shear strength, and enables theslurry to exert its maximum stabilising effect.The seal can be formed by three different mechanisms: Surface filtration Deep filtration Rheological blockingSurface filtration occurs when a filter cake is formed by the bridging of hydrated bentoniteparticles at the entrance to the pores in the soil, with negligible penetration of the bentoniteinto the soil. During and after its formation, water percolates through the filter cake from7 Federation of Piling Specialists – January 2006 (2nd edition)(first published April 2000)
the slurry into the soil. Water lost in this way is referred to as fluid loss, and reduces inrelation to the square root of time.Deep filtration occurs when slurry penetrates into the soil, slowly clogging the pores andbuilding up a filter cake within them. In this case, the seal may penetrate into the soilabout 40 to 50 mm.In both surface filtration and deep filtration, the concentration of bentonite in the filtercake is greater than in the slurry (typically 15% for a slurry containing 5% bentonite).Rheological blocking occurs when slurry flows into the soil until it is restrained by itsshear strength. In this case the slurry may flow several metres into the soil.Of these three mechanisms, surface filtration is much to be preferred, since the seal isformed very rapidly with little or no penetration of bentonite into the soil.4.1.2 Retention within the excavationRetention of bentonite slurry in excavations in clay, silt or sand should not present anyproblems provided the bentonite slurry has a minimum Marsh funnel viscosity of about 32seconds (946 ml test volume). Excavations in gravel may require a Marsh funnel viscosityof 40 to 50 seconds to limit the filtration depth into the soil. A Marsh funnel viscosity inexcess of 50 seconds will make desanding operations more difficult, and may inhibitcomplete displacement of the bentonite slurry by concrete in excavations containingcomplex steel reinforcement.It may not be possible to retain bentonite slurry in very open ground containing cobblesand boulders unless special measures are taken. These may include the addition of sand tothe bentonite to assist the blocking mechanism, or the use of bentonite-cement slurry orweak concrete to seal off strata where losses occur.4.1.3 Suspension of solidsWhile excavating under bentonite, fine soil particles will accumulate in the slurry. If thismaterial is to be kept in suspension, for example to prevent the formation of a layer ofsediment at the base of a pile bore, the bentonite slurry should have a high viscosity underquiescent conditions. A measure of this can be obtained from the 10 minute gel strengthwhich can be determined when testing the rheological properties of the slurry, or othertesting method for gel strength.4.1.4 Displacement by concreteThe bentonite slurry should have a low viscosity and contain the minimum possibleamount of suspended soil particles if it is to be displaced by concrete placed through atremie pipe or by pumping. It is therefore normal practice to use desanding equipmentand, if necessary, desilting equipment to remove soil particles from the slurry, or to replacethe8 Federation of Piling Specialists – January 2006 (2nd edition)(first published April 2000)
contaminated slurry with fresh or reconditioned slurry before the concrete is placed.Sometimes there can be a build-up of fine silt and clay particles in the slurry which cannotbe removed by conventional desanding or desilting equipment. A practical upper limit hastherefore to be set on the density of the slurry, after which it is considered to be unsuitablefor re-use.4.1.5 CleaningSuspended soil particles can be removed from a slurry more easily if the slurry has a lowviscosity. Desanding becomes increasingly difficult as the viscosity
The API Specification and the EEMUA Specification differ slightly in some respects. The main differences in the specifications are in the requirements for the rheological properties and filtrate loss of the slurry. The rheological properties of the slurry at different rates of shear are determined using a direct reading viscometer. Filtrate loss is determined using a filter press. Test methods .
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Keywords: bored piles, diaphragm walls, bentonite fluid, polymer fluid, concrete defects, pile design, trench stability, surface filtration, deep filtration INTRODUCTION Piling and other deep foundation methods require the excavation of shafts and trenches to construct of more than 50 m or 165 ft (Jefferis and Lam 2013).
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Piling and other deep foundation methods require the excavation of shafts and trenches to construct of more than 50 m or 165 ft (Jefferis and Lam 2013). Sometimes, excavations for deep foundations need to be carried out in challenging and potentially unstable ground conditions like loose to dense sands
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