Download pdf - GT4R4A1 Slurry for use on slurry shield TBM

Slurry for use in SLURRY SHIELD TBM

GT4R4A1

www.aftes.asso.fr

ASSOCIATION FRANÇAISE DES TUNNELSET DE L’ESPACE SOUTERRAIN

Organization member of the AFTES

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Text prepared by P. LONGCHAMP (Bouygues Travaux Publics)

with the close collaboration of:Antony BESQ (LEA) - Fabrice BONIN (JF Tech) - Bruno DEMAY (Vinci) - Michel GUERIN (Calypso)

Alexandre GUILLAUME (MS) - Anne PANTET (ESIP-Poitiers) - Jean-Marie ROGEZ (RATP)André SCHWENZFEIER (CETU)

Other members of Working Group GT 4 (Mechanized Excavation) also contributed to these Recommendations:Alain AMELOT (SPIE) - Daniel ANDRE (SNCF) - François BERTRAND (Chantiers Modernes) Laurent CHANTRON (CETU) - Didier CUELLAR (SNCF) - Jean-Marc FREDET (Pergolese)

Jean-Louis GIAFFERI (EDF) - Christian MOLINES (Eiffage) - Philippe MONNET (ESIP Poitiers) - Laurent NICOLAS (JF Tech) - Paul RENAULT (Razel) and Jean-François ROUBINET (Vinci).

The Working Group acknowledges the help of J. GUILLAUME (Razel) - B. GODINOT ( GTM Construction) - G. PIQUEREAU (CAP)

of the Technical Committee for reviewing and validating the text.

Grateful to D.P. Richards (Parsons Brinckerhoff International) for his translation review

AFTES RECOMMENDATIONS CONCERNING

SLURRY FOR USE IN SLURRY-SHIELD TBM

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1 – FOREWORD – PURPOSE OF THE RECOMMENDATIONS - - 1652 – BASIC ROLES AND FUNCTIONS - - - - - - - - - - - - - - - - - - - - - 166

2.1 - General - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1662.2 – Primary functions of slurry - - - - - - - - - - - - - - - - - - - - - - - - 1662.3 – Secondary functions and qualities of slurry - - - - - - - - - - - 166

3 – BASIC PRINCIPLES FOR USING SLURRY - - - - - - - - - - - - - - - 1673.1 – Confinement function - - - - - - - - - - - - - - - - - - - - - - - - - - 167

3.1.1 – General - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1673.1.2 – Filtration - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1673.1.3 – Stiffening of slurry - - - - - - - - - - - - - - - - - - - - - - - - - - 1673.1.4 – Types of cake - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1673.1.5 – Speed of cake formation - - - - - - - - - - - - - - - - - - - - - 167

3.2 – Transport function - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1673.3 – Separation function - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1673.4 – Changes to slurry during excavation - - - - - - - - - - - - - - - - 167

3.4.1 – Physical contamination of slurry by solid load - - - - - - 1673.4.2 – Chemical contamination of slurry - - - - - - - - - - - - - - - 167

4 – BASIC PROPERTIES OF SLURRY AND THEIR MEASUREMENT 1674.1 – Rehological properties - - - - - - - - - - - - - - - - - - - - - - - - - - 168

4.1.1 – Introduction - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1684.1.2 – Apparent viscosity (AV) - - - - - - - - - - - - - - - - - - - - - - - 168

4.1.3 – Plastic viscosity (PV) - - - - - - - - - - - - - - - - - - - - - - - - 1684.1.4 – Yield point - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1684.1.5 – Thixotropy and gels - - - - - - - - - - - - - - - - - - - - - - - - 169

4.2 – Physical properties - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1694.2.1 – Density of slurry - - - - - - - - - - - - - - - - - - - - - - - - - - - 1694.2.2 – Sand content (particles > 80 µm) - - - - - - - - - - - - - - 1694.2.3 – Filterability - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1694.2.4 – Quality of cake - - - - - - - - - - - - - - - - - - - - - - - - - - - 169

4.3 – Chemical properties - - - - - - - - - - - - - - - - - - - - - - - - - - 1694.3.1 – Presence of electrolytes - - - - - - - - - - - - - - - - - - - - - 1694.3.2 – pH variation - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 170

5 - MATERIELS AND PROCEDURES FOR MEASURING SLURRY QUALITY - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 170

5.1 – Test equipment - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1705.2 – Procedures - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 170

6 – CHOICE OF MAIN SLURRY CHARACTERISTICS - - - - - - - - - 1706.1 – Properties - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1706.2 – Guiding the choice of slurry type - - - - - - - - - - - - - - - - - 170

7 – COMPONENTS OF SLURRY - - - - - - - - - - - - - - - - - - - - - - - - 1717.1 – Composition - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1717.2 – Choice of components - - - - - - - - - - - - - - - - - - - - - - - - - 171

CONTENTSCONTENTS

TUNNELS ET OUVRAGES SOUTERRAINS – HORS-SERIE N° 1 – 2005

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➬ For vertical drilling:

• The volume and sectional area of the bore-hole are relatively small, and any ground insta-bility is localized.

• The mud pressure is governed directly by itsdensity and the hydrostatic head.

• Cuttings from the cutterhead are carried up

the hole by laminar flow to prevent erosion ofthe borehole walls. This calls for low flow velo-cities and a mud whose viscosity enables it tocarry the cuttings at that speed.

• The borehole generally goes through a seriesof sub-horizontal geological strata, but gene-rally crosses them at nearly perpendicularangles.

➬ For tunnelling:

• Support conditions are generally more diffi-cult , with a high risk of instability.

• Slurry pressure is produced and controlledmechanically.

• Spoil is carried by a turbulent flow regimewhich requires high flow velocity; the suspen-

1 - FOREWORD – PURPOSE OF THE RECOMMENDATIONS

Slurry is an indispensable aspect in the use of closed face slurry-shield tunnel boring machines (TBMs), for tunnel support and for slurry remo-val. The AFTES Recommendations on the choice of mechanized tunnelling techniques (TOS No. 157, January/February 2000, paragraphs4.3.4, 6.3.3, 7.3.3, and 8.5) provide more information on this.

AFTES Working Group GT 4 dealing with mechanized excavation considered it important to publish recommendations on slurry.

The science of slurry for this kind of application is indeed relatively complex. It consists of a combination of experience and theory, as the slurryitself combines rehological, physical, and chemical phenomena. It also has to take account of another range of complex sciences, the earthsciences.

Use of slurry for tunnelling can undoubtedly trace its origins back to drilling fluid (mud) for vertical holes. Because of their immense resources andthe economic issues involved, oil companies played a leading role in the development of the technology. Civil engineering thus benefited from awealth of prior knowledge, first in respect of deep foundations and drill holes for ground improvement, and then, some years later, for excavationof tunnels in loose water bearing granular soils.

While the composition of tunnelling slurry is essentially the same as that of the drilling mud used in the oil industry, it does have its own specialrequirements. Some examples are given below:

PagesPages

8 – TREATMENT OF EXCESS SLURRY (see Appendix 6) - - - - - - 1719 – SLURRY SCHEDULE - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 172

9.1 – Objectives - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1729.2 – Preparation - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 172

9.2.1 – Regulatory considerations: environment / health and safety - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 172

9.2.2 – Site conditions: project geometry, geology, geotechnics, and landtake - - - - - - - - - - - - - - - - - - - - - - 172

9.2.3 – Plant - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1729.2.4 – Quality of components (water, additives) - - - - - - - - - - 1729.2.5 – Consumption estimate - - - - - - - - - - - - - - - - - - - - - - - 172

9.3 – Testing implementation and follow-up - - - - - - - - - - - - - - 1729.3.1 – Documentary system and checks - - - - - - - - - - - - - - - 1729.3.2 – Personnel - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 172

10 – USING SLURRY - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 17310.1 – Reception of basic products - - - - - - - - - - - - - - - - - - - - - 17310.2 – Manufacture - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 173

APPENDIX 1: GLOSSARY - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 173APPENDIX 2: POLYMERS AND BENTONITES FOR TUNNELLING SLURRIES - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1741- General - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 174

2 - Types of polymers used in tunnelling slurry - - - - - - - - - - - - - 1742.1 – Starches and amylase derivatives - - - - - - - - - - - - - - - - - 1742.5 – Polyacrylamides - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1742.6 – Polyacrylates - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 174

3 - Bentonite - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1743.1 – Structure of bentonite- - - - - - - - - - - - - - - - - - - - - - - - - - 1753.2 – Bentonite properties - - - - - - - - - - - - - - - - - - - - - - - - - - 1753.3 – Bentonite for underground works- - - - - - - - - - - - - - - - - 175

APPENDIX 3: PRINCIPLE FOR EVALUATION OF THE RHEOLOGICAL CHARACTERISTICS OF TUNNELLING SLURRIES 176

APPENDIX 4: SLURRY PARAMETER TABLE - - - - - - - - - - - - - - - 177

APPENDIX 5: SLURRY RHEOLOGY - - - - - - - - - - - - - - - - - - - - - 1771 – Definition of rheology - - - - - - - - - - - - - - - - - - - - - - - - - - - 1772 – Rheology of tunnelling slurries - - - - - - - - - - - - - - - - - - - - - 1773 – Definitions and terminology - - - - - - - - - - - - - - - - - - - - - - 1774 – Components models- - - - - - - - - - - - - - - - - - - - - - - - - - - - 178

APPENDIX 6: PRINCIPLE OF OPERATION OF A SLUDGE TREATMENT PLANT- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 179

APPENDIX 7: PERMEAMETER - - - - - - - - - - - - - - - - - - - - - - - - 181

APPENDIX 8: STANDARD SLURRY-MONITORING DOCUMENTATION - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 182

CONTENTS CONTENTS

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ded solids (cuttings) carrying capacity isgoverned by the dynamics of the turbulence .

• Since the geological strata encountered aresub-horizontal and sub-parallel to the axis ofexcavation, ground at the face is often strati-fied and heterogeneous.

• The excavation volumes involved and thesectional face areas of the excavation arelarge with

The basic solids mixing component formud/slurry is bentonite, a type of clay withvery special physical and components pro-perties.

Other products such as polymers or defloc-culants are being used increasingly. Theycomplement and improve the basic proper-ties of the slurry. They are therefore addressedin these Recommendations.

Geological and hydro-geological conditionsand dimensions can vary widely from oneproject to another and therefore entail project-specific problems.

The purpose of these Recommendations isto provide help for preparing, managing,and checking the slurries used in conjunctionwith closed face slurry-shield TBMs.

2 – BASIC ROLES ANDFUNCTIONS

2.1 - GENERAL

Tunnelling slurry fulfils several roles. Theseare principally groundwater control and sup-port of the excavated surfaces. Other secon-dary roles must also be taken into account.They are dealt with in Section 2.3.

In the simplest cases, only the function ofslurry removal is required (hydraulic muc-king). Plain water (with no additives) can fulfilthis purpose effectively.

In the majority of cases, however, the condi-tions under which slurries are used are lessfavourable and are associated with a combi-nation of:

• the presence of unstable ground

• moderate to high in-situ permeability

• high ground water pressure, which aggra-vates the above two difficulties.

A slurry consisting of water, bentonite, andpossibly some special additives must then bedesigned to overcome these difficulties.

2.2 – PRIMARY FUNCTIONSOF SLURRY

The fundamental function of tunnelling slurry

is to impart to the tunnel walls and excava-tion face the qualities that will hold up theexcavation. To achieve this, its componentsand physical properties must:

• create an impermeable membrane at theground interface (at the face and around theshield, in the case of overcutting). Theground interface must be made imper-meable so that the slurry can develop ade-quate confinement pressure, or, more preci-sely, extra confinement pressure above thehydrostatic ground water pressure.

• create and maintain confinement pressuregradient at the ground interface sufficient toensure stability (see Appendix 3).

It as sometimes been claimed that the use ofslurry and the associated development ofconfinement pressure improve the mechani-cal characteristics of the ground around thetunnel by adding extra cohesion and/or anspecific state of stress. However, with currentknowledge of the phenomena involved, suchimprovement cannot be quantified becauseit is so complex, and accordingly it is not takeninto account in tunnel stability calculations.

It is important to remember that these funda-mental functions must be maintained inorder to provide excavation face stabilityduring man access to the cutterhead cham-ber for hyperbaric maintenance operations.

The conditions ensuring face stability byconfinement are represented on the follo-wing Mohr diagram:

Excavation of the tunnel produces a changein the state of initial stress around the ope-ning. σv is assumed to be constant whereasσh tends to 0. The initial Mohr circle (➀)expands until it reaches the critical circle ➃beyond which stability is no longer guaranteed.

This diagram has been simplified. It corres-ponds to the stability of an infinitely largetunnel face with a two dimensional stressstate. In practice, stability calculations involvethree-dimensional stress mechanisms.

2.3 – SECONDARY FUNC-TIONS AND QUALITIES OFSLURRY

Although referred to as “secondary” here,these functions and qualities are very impor-tant and can be decisive in some cases.

Such cases include:

• Mucking: the viscosity of the moving slurrymust enable it to be pumped at high velocityin the pipes of the slurry circuit.

• Lubrication: the slurry must limit wear ofplant (cutting tools, pipes, valves, pumps,etc.) by reducing plant/slurry friction.

• Encapsulation of muck: The slurry mustreduce hydration of the muck in order to limitits propensity to stick and to flocculate (this isparticularly important when excavating insilty-clayed materials with a water contentbelow the plastic limit).

• Inertness at treatment plant: the slurrymust be designed to facilitate its treatment,i.e. separation of slurry from the muck itconveys.

• Environmental inertness: water, muck, andresidual slurry must be acceptable in terms ofenvironmental regulations.

➀ Initial state

➁ Characteristic curve of ground (C, ϕ)③ Case of excavation without confinement (unstable)➃ Critical circle corresponding to minimum confinement pressure ensuring stability (without

safety margin and without water pressure)τ Shear stressσ Normal stressσh Horizontal normal stressσv Vertical normal stressσc Stress resulting from confinement pressure (without hydrostatic pressure)

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3 - BASIC PRINCIPLES FOR USING SLURRY

3.1 – CONFINEMENT FUNCTION

3.1.1 – General

Slurry penetrates the ground at the face andalong the tunnel walls (especially in the caseof overcutting).

In accordance with the permeability of theground and its intrinsic properties, theslurry’s penetration into the ground will gra-dually slow down, then stop, as a result oftwo interdependent, simultaneous pro-cesses: filtration (pressure filtration) and anincrease in the stiffness of the slurry.

3.1.2 – Filtration

The pressure in the cutterhead chamberforces the slurry through the soil at the tunnelface and walls and into the ground. As it per-meates the ground, the liquid and solid com-ponents of the slurry are separated, gra-dually leaving the solids at or near thesurface,

The filtered solids themselves enhance thefiltering process until the barrier is imper-meable. When the face is excavated, thefilter is partially or totally destroyed by eachcutting tool, but the process resumes imme-diately once the tool has passed.

This continuing process results in creation ofa filter cake of varying thickness, dependingon the permeability of the ground and thecharacteristics of the slurry.

3.1.3 – Stiffening of slurry

The filtration effect of slurry penetrating intothe ground immediately reduces the velocityof slurry flow into the ground.

Bentonite slurry is a non Newtonian (ref.chapter 4). Therefore, as a consequence ofits reduction in flow velocity, it gradually stif-fens and “sets”.

Setting is directly linked the conventionalyield point or “yield value” of the slurry.

3.1.4 – Types of cake

There are two types of cake:

• Membrane cake: this is obtained in groundof low permeability by using a relatively stiffslurry. The slurry does not penetrate very farinto the ground, and creates only surfaceimperrmeabilization of the ground interface.It will be several millimetres thick.

• Impregnation cake: in moderately tohighly permeable ground of varying porosity,the slurry penetrates relatively deeply intothe pores in the ground before it stops andsets. It is this impregnation that constitutesthe impermeable interface. This zone canvary between several tens of centimetres andseveral metres thick.

3.1.5 – Speed of cake formation

As commented above in § 3.1.2, the filtercake at the tunnel face is partially or totallydestroyed with each sweep of a cutting tool,but re-forms behind it immediately.

Rheograms (cf. Section 4.1) show that theyield point of the slurry after agitation islower than its initial value.

It therefore takes some time period beforethe slurry returns to its initial consistency.This time should be as short as possible inorder to reinstate watertightness to its fullvalue. It is characterized by the “0-10 min.gel strength” test (cf. § 4.1.5).

3.2 – TRANSPORT FUNCTION

The apparent viscosity of the slurry (cf. 4.1.2)must be as low as possible for the hydraulicmucking system. One of the properties ofBingham fluids is that above the yield point,the apparent viscosity decreases with increa-sed agitation. Sufficiently intense agitation isobtained by creating a turbulent flow regimein the slurry pipes.

3.3 – SEPARATION FUNCTION

Use of slurry to excavate and support a tunneland to remove excavation spoil necessarilyimplies that the slurry is treated in order to:

• separate solid spoil from the slurry so thatthe slurry can be recycled into the muckingcircuit

• separate in excess slurry as new slurry isadded

• treat in excess slurry for disposal.

The first two of these operations are carriedout in real time, and the separation rates aregoverned by the instantaneous penetrationrate of the TBM. Slurry in excess can be trea-ted as a background task, and is governed bythe mean advance rate (limited to a certaintime interval), taking account of shutdowns,lining erection, etc.

The components and physical characteristicsof a slurry must take into account this requi-rement for separation. These characteristicsmay be inconsistent with the characteristics

required for tunnel support. A trade-off musttherefore be found to meet the opposingrequirements of ground support and solidsseparation.

The basic operation of a slurry separationand treatment plant is described inAppendix 6.

3.4 – CHANGES TO SLURRYDURING EXCAVATION

A variety of external contaminates blend intothe slurry as excavation proceeds, and canmodify its initial characteristics.

3.4.1 – Physical contamination ofslurry by solid load

Slurry treatment plants cannot filter out thefinest soil particles. The concentration ofexternal contaminates particles in the recy-cled slurry therefore increases with continueduse of the slurry and the density of the slurrygradually increases, generally to the detri-ment of its properties (yield point, filtrate,etc.) (cf. § 4.2).

It then becomes necessary to regenerate theslurry by adding fresh slurry.

3.4.2 – Chemical contamination ofslurry

Minerals or pollutants in the soil and/orgroundwater can also modify the characteris-tics of the slurry. They should therefore betaken into account when the slurry compo-nents are chosen (cf. section 4.3).

Attention should also be paid to the possibi-lity of contamination of the slurry by cementand other chemical products (silicates etc.)used for ground improvement prior to tunnelexcavation.

4 – BASIC PROPERTIES OFSLURRY AND THEIR MEASU-REMENTThe slurry used for closed face shield TBMs isa water-based fluid with a variety of solidsadded to form a colloidal suspension. Thesolid components are essentially:

• mineral colloids (chiefly bentonite)

• organic colloids (chiefly polymers).

There are hundreds of branded productsthroughout the world described as bento-nites or polymers. This can lead to conside-rable confusion, for although they may besimilar, they often have very different physico-chemical, rehological, or filtration properties.

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4.1 – RHEOLOGICAL PROPERTIES

4.1.1 - Introduction

Rheology is the science devoted to the studyof deformations and flow of matter under theeffects of internal and external stresses. Inrehological terms, tunnelling slurries are defi-ned by a number of characteristics whosemeasurement and verification are of thegreatest importance since they govern:

• stabilization of the tunnel face and controlof ground water

• muck removal

• slurry preparation and treatment perfor-mance.

Refer to Appendix 5 (Slurry rheology) formore information.

4.1.2 – Apparent viscosity (AV)

The apparent viscosity of slurry is its “real vis-cosity”. However, since these kinds of fluidsare non-Newtonian, their viscosity dependson their flow velocity. Their viscosity is highfor low velocities (during filtration into theground, in storage tanks), but conversely, islow for high velocities (in muck pipes, inhydrocyclones), as can be seen from the fol-lowing graphs:

Viscosity-velocity gradient graphs are notoften used. Since viscosity is determined bythe shear

Viscosity-velocity gradient graphs are notoften used. Since viscosity is determined bythe shear forces at play between layers ofliquid, it has become customary to use a gra-phic representation of the shear stress ver-sus velocity gradient; commonly called arheogram. Viscosity is no longer explicitlydiscussed.

Apparent viscosity is the relationship betweenthe shear stress and the velocity gradient.Consequently it is not a constant quantity:

Rheogram: solid lines

Apparent viscosity: dotted lines

Apparent viscosity is expressed in mPa◊sthese days, and formerly in centipoise (cP).

1 mPa.s = 1 cP

4.1.3 – Plastic viscosity (PV)

Plastic viscosity concerns non-Newtonianfluids and characterizes Bingham fluids. Itcorresponds to the gradient of the linear partof the rheogram, as shown

In the case of tunnelling slurries, the plasticviscosity is deemed to be constant, as a firstapproximation. It is determined by extrapo-lating the linear part of the rheogram (asshown above). This physical quantity isimportant for calculation of pressure losses inthe slurry pipes. It depends above all on thesolids content of the slurry and the nature ofthose solids.

Plastic viscosity is also expressed in mPa.s(or cP).

4.1.4 – Yield point

The yield point determines the ability of aslurry to “set” in the ground. When slurrypenetrates into the ground around a tunnel,since its flow velocity decreases, shearstresses become insufficient to allow it toflow further, and it “sets”.

Contrary to a Newtonian fluid, a Binghamfluid requires a certain minimum shear stressto be able to flow; it behaves like a visco-elastic fluid. At stresses below this yield

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point, and since its viscosity is infinite, itbehaves like a solid. The actual yield pointdepends on initial conditions and increaseswith time at rest. In slurry-tunnelling practice,the convention is to use a value determinedfrom the intersection on the rheogram of thestraight line obtained with the Fann rheome-ter and the vertical scale of co-ordinates.This extrapolation is called the Yield Value(cf. below).

The yield point of a tunnelling slurry is asso-ciated with the physico-chemical reactionsbetween the active particles of the suspen-sion. It also depends on the content of suchactive particles. However, increasing the den-sity and viscosity of a slurry by adding inertparticles does not necessarily result in anincrease in the yield point; on the contrary, itcan lower it appreciably. Tunnelling expe-rience shows that since the soil can containboth inert particles and active particles (clay),slurry viscosity can also increase with densityin some cases.

Yield point (YP) is generally expressed inPascals (Pa) or in pounds per hundred squarefeet (lb/ 100 ft 2). The conversion is 1 Pa = 2.084 lb/100 ft 2.

4.1.5 – Thixotropy and gels

An interesting property of some fluids or col-loidal suspensions such as bentonite slurriesis that at rest they create a structure whichincreases the stiffness of the fluid, but whichcan be destroyed by agitation. “Thixotropy”refers to the reversibility of this phenome-non. The process creating the structure is notimmediate, however. As stated in paragraph3.1.5, the effective yield point changes withthe slurry’s time at rest, which allows for gra-dual restructuring of the slurry.

Any increase in the stiffness of the slurry isassessed by measuring gel formation times.In practice, thixotropy represents the abilityof the slurry to reform a cake behind the cut-ting tools more or less quickly. It is quantifiedin the laboratory by measuring the 10-sec.

and 10-min. gel strengths (refer to the“Thixotropy” entry in the glossary).Bentonite has this thixotropic behaviour.Other commonly used additives such aswater-soluble polymers contribute other pro-perties (viscosity, filtrate reduction, electroly-tic stability, etc.).

The rheogram below illustrates the conceptsdescribed above.

4.2 – PHYSICAL PROPERTIES

4.2.1 – Density of slurry

Measurement of the density of a desandedslurry serves chiefly to check the finescontent of the slurry being recycled to theTBM. Monitoring its change can also giveinformation on the face conditions since:

• an increase in the clay or marl content ofthe slurry during excavation indicates thatsolids from the ground are remaining in theslurry, increasing its density (this can bebeneficial or detrimental to the yield value,depending on conditions).

• a decrease in the clay or marl content notsubsequent to addition of water (dilution)indicates that groundwater is entering theslurry, or that there is a chemical imbalance(action of silicates, for instance, which causemicro-foaming of the slurry).

4.2.2 – Sand content (particles >80 µm)

The sand content of a slurry depends uponthe performance of the separation plant

(cyclone split ratio). Any sand remaining inthe slurry directly affects the permeability ofthe cake and its stability on the groundinterface.

4.2.3 - Filterability

Filterability is the ability of a slurry to imper-meabilize the ground interface by forming afilter cake. It depends on the quality andquantity of the components making up theslurry.

Filterability is generally inversely related toslurry density.

4.2.4 – Quality of cake

Filter cake results from deposition of suspen-ded solid materials on the tunnel face andwalls as the slurry penetrates into theground. The intrinsic qualities of a cake aredetermined by its thickness and its ability todeform without breaking. They are represen-tative of its stability of the ground interfaceand of its resistance to mechanical andhydraulic effects induced by excavation.

4.3 – CHEMICAL PROPERTIES

4.3.1 - Presence of electrolytes

The intentional or unintentional addition ofelectrolytes such as Ca++, Mg+, K+, Cl-, andSO4- ions (mix water, ground water, mineralcontent of the ground, grouted areas, etc.)can considerably modify the physical andrehological properties of slurry. Exchange ofions modify the initial ionic environment of

Yield value

Yield point

Velocity gradient

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the clay-layers and can produce coagulationand flocculation of the suspension. Theseions modify ion balances, resulting in phaseseparation due to liberation of free-watermolecules initially attached to clay plateletsor polymer chains: this results in flocculation.Special attention must be paid to the chemi-cal composition of water (lime-bearing water,sea water, etc.) throughout the excavationprocess.

4.3.2 – pH variation

A change to the pH of the slurry also affectsits ion balances and physico-chemical pro-perties. Beyond a pH range from 8 to 10,there is a significant risk of poor slurry perfor-mance in both acid environments (in contactwith organic matter, for instance) and in basicenvironments (in contact with cement, forinstance).

5 - MATERIELS AND PROCEDURES FOR MEASURING SLURRY QUALITY

5.1 – TEST EQUIPMENT

Many types of equipment can be used formeasuring the different physical, chemical,and rehological parameters of slurries. Wehave selected only the following equipmentfor continuous measurement.

❑ Testing rehological properties: measure-ment of AV, PV, YP, gels

➘ Marsh funnel viscometer (not very mea-ningful in this field)

➘ Fann rheometer

➘ Possibly, a rheometer with imposed velo-city gradient (Rhéomat or similar)

- It should be pointed out that many automa-tic viscometers found on tunnelling projectsare derived from the geometry of the Fannrheometer and can thus be used to deter-mine the characteristic rehological parame-ters of slurry on a “quasi-continuous” basisby measuring values at 600 and 300 rpm.

❑ Testing physical properties

➘ Density: Baroid mud balance, densimeter

➘ Sand content kit (sieve analysis apparatus)

➘ Filtration and cake: API fluid-loss-test filterpress and, for a more realistic approach tointerface problems, a large permeameter (cf.Appendix 7).

❑ Testing chemical properties

➘ pH: indicator strips, pH meter

➘ Conductivity: conductivity meter

➘ Specific surface (“activity”): methyleneblue value

➘ electrolyte content

5.2 - PROCEDURES

Most slurry tests required already have stan-dardized test procedures, principally develo-ped by the oil industry (cf. AmericanPetroleum Institute: section 13),("Recommended Practice StandardProcedure for Field Testing Water BasedDrilling Fluids"), and also the ISO 13500 stan-dard. Other test equipment has its own spe-cific test method pertaining to applications inother fields.

The methylene blue value is obtained using astandardized test (French standard NF P18-592).

6 - CHOICE OF MAIN SLURRY CHARACTERISTICS

6.1 - PROPERTIES

When tunnelling using a closed face slurryshield TBM, the main characteristics of theslurry must be determined in accordancewith:

• the type of ground to be excavated,

• the possible presence of a groundwater,together with its chemical properties,

• the risks identified (geological, environ-mental hazards, etc.).

Among the most important criteria for defi-ning a slurry are the following:

➊ Rehological parameters, essentially yieldpoint (YP) and plastic viscosity (PV)

From the yield point and plastic viscosity canbe defined:

• the conditions of impregnation of theground interface (formation of cake),

• the conditions of fluid flow and solid sepa-ration in the slurry circuit and treatmentplant,

Methods for calculating these parametersare presented in Appendix 3.

❷ Filtration properties

Filtration properties govern cake formation.The cake must be sufficiently impermeable,strong and flexible (deformable) to guaran-tee there will be a pressure gradient across

the thickness of the cake. It will sometimeslimit hydration of geological strata contai-ning clay or evaporite.

❸ Density

The density of a tunnelling slurry depends onits components, on physical contaminationby fines from the ground that are recycledthrough the slurry circuit, and on regenera-tion by fresh slurry and additives.

It has an effect on viscosity and therefore onpressure losses in the slurry circuit.

The thickness and permeability of the cakeincrease with greater density, which is there-fore detrimental to the resistance of mem-branes and the impermeability of the tunnelwalls.

❹ Other characteristics:

Other characteristics (pH, polymer or solublesalt types and contents, etc.) have to be iden-tified for better characterization of a slurryand to meet environmental requirements:

• ground/slurry interaction during excavation(contaminated sites and grouted areas)

• recycling in the slurry circuit,

• disposal of liquid waste.

For guidance, Appendix 4 gives a table sho-wing representative values for slurry parame-ters for different ground permeabilities.

6.2 – GUIDING THE CHOICEOF SLURRY TYPE

The number of parameters to be taken intoaccount when choosing a slurry type is sogreat that these Recommendations cannotdefine an exhaustive, “ready-made” proce-dure. Further difficulty arises in the case ofmixed faces (relative to variation in geologyover short distances).

For guidance and training, an elementaryclassification of ground types on the basis oftheir permeability is given below, with sug-gestions for the appropriate slurry types:

➊ Highly permeable ground with granular orintensely fractured structure.

Solution: bentonite slurry with rheology sui-table for high porosity; in particular, appro-priate yield value, 0/10-min. gel strengths,filtrate.

❷ Silty to sandy semi-permeable ground.

Solution: bentonite slurry (pure or with addi-tive) with low filtrate and suitable rheology(as per above).

❸ Impermeable ground with no particularreactivity or sensitivity to water.

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Solution: water or bentonite slurry and/orpolymer.

❹ Impermeable, reactive ground.

To find the right type, it is necessary to:

• have perfect knowledge of the mineralogi-cal composition of the ground,

• analyze the physical and chemical interac-tions of the associated minerals and organicmatter.

This makes it possible to quickly define thebest system for inhibiting hydration of clayand therefore limiting the problems entailedin swelling, dispersion, and sticking.

The following cases can arise:

• Ground with high dispersion potential(containing illite or kaolinite, for instance):

Materials dispersing in the slurry rapidly andin large quantities.

Solution: bentonite slurry and/or polymer(phpa, CMC, acrylic plasticizer or phos-phates, etc.)

• Swelling or flowing ground (smectite,fibrous clay, for instance):

Materials very sensitive to hydration by slurry,or of plastic consistency.

Solution: inhibited polymer slurry, KCl, Ca++

type, etc.

• Overconsolidated ground (shale, argillite,for instance):

Materials whose structure is often foliated,making the ground mass somewhat fragile.

Solution: bentonite slurry with little filtrateand appropriate rehological parameters.

• Ground containing evaporites (gypsum oranhydrite, various salts, etc.:)

Solution: water, bentonite slurry and/or poly-mer slurry treated with sodium carbonate; insome cases a Ca++ salt slurry can be used.

7 – COMPONENTS OF SLURRY

7.1 – COMPOSITION

Tunnelling slurry is made up of componentsof four different categories:

➊ Mix water:

• The quality and temperature of the mixwater have a significant effect on the proper-ties of the components.

❷ Active (mostly) bentonite type clays.

❸ Organic and chemical additives:

• Organic additives are divided into twocategories:

- water-soluble polymers (cf. Appendix 2)

- special organic products (defoamingagents, antifriction agents, etc.).

• Chemical additives:

• Various chemical additives such as KCl,sodium carbonate, sodium polyphosphate,etc. for modifying the initial properties ofclayey colloids by ion exchange.

❹ Groundwater and solids from the ground.

Active clays and additives are chosen inaccordance with the effect desired:

• Viscosity improvers and filtrate reducers:bentonites, cellulosic polymers, acrylatepolymers / acrylamides.

• Dispersing agent and plasticizers: low-molecular-weight sodium-acrylate polymers,phosphates.

• Clay stabilizers: cellulosic polymers andacrylate polymers / acrylamides of highmolecular weight, salts.

• pH corrector: sodium carbonate and bicar-bonate

It is important to realize that the way theslurry is prepared (order of addition of com-ponents, thoroughness of mixing, etc.)affects the performance of the componentsin the resulting mix. Any undesirable effectsof the components should also be taken intoaccount. These might include, for example,foaming or micro-bubbling of detergent-type surface-active agents.

Tunnelling slurries are complex products witha range of common properties that are highlysensitive to external factors. Mix-design tes-ting must be carried out, for their propertiesare generally not very “reproducible” anddepend on geological and hydrogeologicalconditions.

In most cases it is recommended that a spe-cial programme for studies into tunnellingslurries be carried out in order to allow themix-design is defined. This programmeshould be validated and, as necessary, cor-rected during the works, in connection withongoing technical management of theslurry. A procedure for this is proposed inChapter 9.

7.2 – CHOICE OF COMPONENTS

Tunnelling slurries are generally colloidal sus-pensions containing two types of colloids:

• Mineral colloids (chiefly bentonite),

• Organic colloids (chiefly polymers).

There are hundreds of branded products

throughout the world described as bento-nites or polymers. This can lead to conside-rable confusion, for although they may besimilar, they often have very different phy-sico-chemical, rehological, or filtration pro-perties.

The choice of the polymers or bentonites tobe used will depend on economic considera-tions, the specific performance of each pro-duct, the circumstances in which it will beused, and the following requisite functions:

• Viscosity level,

• Sensitivity to salts, to pH, to bacterial orenzymatic degradation,

• Capacity to stabilize clays,

• Decrease of filtrate,

• Length of use,

• Ease of use,

• etc.

In addition to the specific test methodolo-gies of each contractor, there is a standardi-zed method for testing the efficacy of certainadditives in different types of slurry:“Recommended Practice StandardProcedure for Laboratory Testing DrillingFluids - API RP 13I”.

It should be remembered that adding bento-nite or polymer to a tunnelling slurry oftenhas a dual effect:

• a principal effect: increased viscosity, redu-ced filtrate, etc.

• one or more secondary effects: inhibition ofswelling or dispersion of clays, increased vis-cosity, etc.

8 – TREATMENT OF EXCESSSLURRY (SEE APPENDIX 6)To be comprehensive, the selection of aslurry system should consider the full life-cycle of the slurry. This will cover its manufac-ture, its maintenance in accordance with theexpected physical or chemical contamina-tion, and its disposal. It is sometimes possibleto give an economic value to in excess slurry:mortars or grouts for ground improvementor tunnel backgrouting, agricultural fertili-zers, ground impermeabilization, etc.

In general terms, disposing of or treating inexcess slurry involves considerable cost for aproject; Attention should therefore paid to:

• Taking account of the difficulties when stu-dying the choice of slurry,

• Minimizing the volume to be wasted

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• Limiting the cost of treatment to the exactneed,

• Making provision for the site footprintsnecessary for treatment plant in the projectplanning stage.

Slurries are stable colloidal suspensionsof particles that do not easily settle outnaturally.

It is generally necessary to use physico-che-mical treatments to flocculate them and cla-rify the water extracted from them.

A variety of standard tests should be perfor-med beforehand to optimize the quality oftreatment required prior to industrial-scaletesting.

9 – SLURRY SCHEDULE

9.1 - OBJECTIVES

The slurry schedule is a document serving toguide the determination of slurry types andof means of testing them to ensure theymeet the technical and economic require-ments of the project. Its purpose is to pro-pose the system that best meets the require-ments.From natural slurry to the mostcomplex chemical formulations, there areenormous numbers of possibilities whichshould, in principle, provide a solution per-fectly suited to every project.

9.2 - PREPARATION

In addition to what is presented in the prece-ding chapters, the slurry schedule shouldinclude a detailed study of what follows.

9.2.1 – Regulatory considerations:environment / health and safety

In France, the use, storage, and disposal ofspoil and slurry are governed by Law No. 92-3 of 3/1/92 on water and waste. The Lawdemands that potential users carefully studytheir project in respect of slurry and spoilstarting at the beginning of the project.Together with equipment for treating waste,the slurry schedule should result in a reduc-tion in the volume of waste to be dumpedand in optimization of the environmentalquality of that waste.

9.2.2 – Site conditions: project geo-metry, geology, geotechnics, andlandtake

The slurry schedule should consider the geo-metry, geology, and environment of the pro-ject, and any ground improvement that may

be required. The geological report shouldspecify the physical, chemical, and mineralo-gical nature of the ground and ground waterthat will be encountered, including any parti-cular or localized features. It is also useful toinclude on a longitudinal geological profilethe prior ground improvement carried out,with indication of the products used. Theprofile should specify and demarcate thesuccession of different formations. This datawill serve to identify the sequence of diffe-rent slurries best adapted to the individualcircumstances.

The schedule should also integrate otherconditions concerning the project, particu-larly the landtake likely to be required for theslurry separation and treatment plant.

9.2.3 - Plant

The correct choice and definition of plant(TBM and treatment plant) are vital for thesuccess of a project.

• The slurry schedule should address thecompatibility of plant, ground, and spoil. Itshould consider the possibility of frequentwater inflow which would modify the initialslurry concentrations. It should contain consi-derations relating to hyperbaric work in thecutterhead chamber.

By considering these factors and the expec-ted TBM advance rates, the slurry prepara-tion and treatment plant capacity should becorrectly sized.

9.2.4 – Quality of components(water, additives)

The specifications drawn up the suppliers ofbentonite and additives should define themeans of delivery, the necessary reserve sup-plies, and the quantities of consumables(water, main additives).

9.2.5 – Consumption estimate

Using the factors described above, it will bepossible to estimate the quantities requiredand the cost price of the different solutionsconsidered (total cost, per cubic metre ofslurry, per cubic metre of excavation).

The quantities of different materials expec-ted to be consumed are estimated by consi-dering the following volumes:

• Volume of purges of the slurry circuit,

• Volume lost to the surrounding ground,

• Volume lost with solid spoil,

• Volume of in excess slurry.

9.3 – TESTING IMPLEMENTA-TION AND FOLLOW-UP

Tests should be carried out to establish a sys-tem for checking the required instructionsand specifications. These tests will concern:

• Slurry types and components,

• Requisite characteristics. The checks arecarried out by means of standardized testsand/or other measuring systems (e.g. real-time logging system).

• Treatment of waste.

9.3.1 – Documentary system andchecks

The checks should be described in proce-dures specific to each project:

• Checks of incoming products delivered tothe site (quantity and quality); the deliverychecks will be associated with sampling, asrequired.

• Ongoing checks should be carried out forat least every lining ring, and should com-pare the values at the start of the circuit andafter desanding in order to quantify the phy-sical and chemical contamination of theslurry.

• Consumption of different products (water,bentonite, additives, etc.) should be monito-red daily.

The results of these checks should be recor-ded on standardized project data sheets(Appendix 8).

For formalization of this data, the documen-tary system will therefore comprise at least:

• a set of procedures,

• a shift report (or daily report) serving todraw up a report on the follow-up of effec-tive parameters and consumptions.

• an executive report at the end of theproject.

9.3.2 – Personnel

Consideration must be given to the technicalqualification of the personnel, and wherenecessary the needs for special training onthe techniques involved should be defined.

The workforce and supervisors must havesufficient knowledge to be at ease with:

• technical aspects of tunnelling slurries andtheir treatment,

• the effect of these aspects on the environ-ment and health and safety (manufacturersissue technical and safety data together withadvisory notices on use of their products).

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10 – USING SLURRY

10.1 – RECEPTION OF BASICPRODUCTS

Bentonite is a natural product. If necessary,its rheological characteristics should be chec-ked when it is delivered to the site.

The quality of the mixing water used shouldbe analyzed at the basic formulation stage.

10.2 – MANUFACTURE

Usually, “mother” slurries with high bento-nite contents (60 to 80 kg/m3) are producedin order to reduce the volumes to be stored.This mother slurry is subsequently dilutedwith water to achieve the required densityjust before it is injected into the slurry circuit.This dilution and injection must be carriedout very carefully, particularly as regardscompliance with the concentrations and qua-lity of the mix. The slurry treatment station

must therefore have equipment specificallyfor these operations.

Bentonite does not hydrate easily and theefficiency of and thoroughness hydrationdepends very much on the mixing energyexpended. It requires a high-turbulencemixer followed by a minimum period ofmaturing (12 to 24 hours).

Incorporation of additives also requires spe-cial equipment for storage, handling, dilution(where applicable), and batching.

• Activity of clay: The activity of clay is repre-sentative of ion charge that gives it the abilityto swell due to adsorption.

• Adsorption: Fixing of water molecules onthe surface of elementary soil particles (clayplatelet for example).

• Apparent viscosity (AV): Ratio of the shearstress to the velocity gradient. The unit ofmeasurement is the pascal second (or milli-pascal second). With the Fann rheometer, itis calculated by testing at 600 rpm and divi-ding the result by 2. The unit of measure-ment is the millipascal second (1 millipascalsecond = 1 centipoise (former unit)).

• Bentonite: Cf. Appendix 2, Chapter 3.

• Bingham fluid: Fluid requiring exertion of aminimum stress to set it in movement. TheYield Value determines the value of thatstress.

• Cake: In laboratory testing, the thickness ofa layer of solid materials deposited on filterpaper in the filtrate measurement test (filter-cake); the unit is the millimetre. For the mea-ning in the context of excavation, refer to thedefinitions in Chapters 3.1.2 and 3.1.4 .

• Coagulants: Products that neutralize orinvert surface charges of suspended matter:ferric chloride, lime, aluminium sulphate, etc.

• Density: Ratio of the mass of a givenvolume of one body to the mass of the samevolume of a reference body, under condi-tions which must be specified for bothbodies (water at 4°C for liquids). It is expres-sed as a dimensionless number.

• Fann rheometer: A.P.I standardized appa-ratus for applying shear to a fluid by meansof an immersed cylinder rotating at a control-led speed and for determining the responseof the fluid by measuring the stress appliedto a concentric cylinder left free to turn. Thepreset speed of rotation are 3, 6, 100, 200,300, and 600 rpm. Stress is read off inlbs/100 Square feet. Simple calculationmethods enable the data obtained to be

used to determine the values of differentrehological parameters (apparent viscosity,plastic viscosity, yield point)

• Filter press: A filter press consists of aseries of hollow trays clamped between amobile plate and a fixed plate by a hydrauliccylinder. The edges of the chambers thuscreated are covered with filter cloth. Slurry ispumped into the chambers, generally fromthe centre of the trays, and the filtrate is col-lected in a channel at the side.

• Filtrate test: Laboratory test characterizingthe ability of a slurry to retain water. Filtratemeasurement is expressed as the quantity offluid collected in a test tube after 30 minutesat a pressure of 7 bars exerted by an appara-tus known as the filter press. The unit of mea-surement is the millilitre. All things beingequal, the volume of filtrate is proportionalto the area of filtration, to the pressure, andto the square root of the test duration.

• Flocculants: Products with inter-particleaction due to bridging. They include high-molecular-weight polymers, bentonite, etc.

• Fluid: See Appendix 5, Chapter 3.

• Gradient or velocity (or shear velocity):See Appendix 5, Chapter 3.

• Marsh viscosity (MV): Laboratory testwhere the time in seconds for 946 ml of slurryto flow through a 4.75 mm nozzle (water = 26s). The reading obtained dependson YP, PV, thixotropy, and density.Consquently this measurement is hardlyrepresentative of the characteristics of aslurry.

• Newtonian fluid: Fluid for which the shearstress is directly proportional to the shearrate.

• pH: Expresses the acidity or alkalinity of anaqueous solution. The pH value of purewater is 7. The products used in tunnellingslurry achieve their optimum yield when thecolloidal state of the slurry is in the pH rangefrom 8 to 10.

• Plastic viscosity (PV): See Article 4.1.3.With the Fann rheometer, plastic viscosity iscalculated by testing at 600 rpm and redu-cing from the reading the reading taken at300 rpm. , The unit of measurement is themillipascal second (1 millipascal second = 1 centipoise (former unit)).

• Polymers: See Appendix 2, Chapter 2.

• Rheogram: Graphic representation of thebehaviour of a fluid subject to shear. Shearstress is plotted against the velocity gradient.

• Rheology: Branch of mechanics which stu-dies the deformation of materials or theirflow under stress (see Appendix 5).

• Shear stress: Force exerted on a surface bya fluid and causing shear (refer also toAppendix 5, Chapter 3).

• Slurry: Also known as drilling mud/fluid.Colloidal clay suspension, with or withoutwater-soluble polymers and/or salts toenhance its original characteristics, that hasdesirable rehological and filtration propertiesand possibly the ability to stabilize ground tobe excavated.

• Stiffness: See Article 4.1.4.

• Thixotropy: Reversible phenomenon ofcertain colloidal suspensions whereby theyfluidify when agitated and gel when left torest. Thixotropy can be estimated from theevolution of gelling over time. With the Fannrheometer, gels are measured by testing atno more than 3 rpm after a waiting period of10 sec. (gel 0) and 10 min. (gel 10). The unitof measurement is lbs / 100ft

• Yield point: See Article 4.1.4.

• Yield value (YV): An extrapolated yield-point value (see Article 4.1.4) derived from aFann rheometer test. Calculated by testingat 300 rpm and deducting from the readingthe reading for the plastic viscosity, PV. Theunit of measurement is the lb/100 Squarefeet. To convert to Pa, divide by 2.

• Yield point (YP): See Yield value

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1- GENERAL Polymer: chemical compound obtained bypolymerizing molecules of a given com-pound known as a monomer.

Polymers often have high molecular weights(high polymers) and are formed by chainsrepeating:

➬ the same unit:

-A-A-A-A-A

➬ or several units:

(-A-B-A-B-A-B)n

➬ or linear units with branches:

B-B

|

(-A-B-A-B-A-A-B-A-B)n

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The degree of polymerization ( n ) refers tothe number of repetitions of the basic unit.The molecular weight - which depends onthe degree of polymerisation, the degree ofsubstitution, and the distribution of substitu-tion govern the physical and chemical pro-perties of a polymer, Organic polymers,which are the most common products on themarket, can be pure or mixed with eachother or with inorganic products such as clay,salts, carbonates, etc.

2 - TYPES OF POLYMERSUSED IN TUNNELLING SLURRYThere are four types of water-soluble poly-

mers:

• Bacterial polymers: xanthan, scleroglucane

• Natural polymers: starches, guars

• Synthetic polymers: acrylamides, acrylates

• Semi-synthetic polymers: cellulose ethers,modified starches and guars

2.1 – STARCHES AND AMYLASEDERIVATIVES

There are various natural starches: potato,corn, wheat, rice starch, etc. and they havecharacteristics as follows:

• They consist of amylose and amylopectin.

• They are hydrolyzed by heat to increasetheir water solubility.

• They are sensitive to fermentation and arenot greatly used other than as slurry filtratereducers.

• Etherification gives them a better level ofviscosity.

2.2 - GUAR GUM

Guar gum is an extract of the guar seed. It isoften referred to as a natural polymer orpolysaccharide and is used as a viscosityimprover. Being non-ionic, it exhibits goodtolerance to mono-valent salts such as NaCl,KCl, etc. or divalent salts like CaCl2, etc.However, it is sensitive to bacterial attack, toenzymes, to oxidation, and to high pHvalues. It is difficult to screen. It can be cross-linked to form a stiff jelly by creating bridgesbetween the different polymer chains.

2.3 - XANTHAN

Xanthan is obtained from bacterial fermenta-tion of sugar by-products. It is often referredto as a bipolymer, and is used as a viscosityimprover. Its pseudo-plastic nature—high vis-cosity at low shear rates and low viscosity athigh shear rates—is sought after in slurrieswith low bentonite contents. Despite its anio-nic nature, it exhibits good resistance to salts.

2.4 – CELLULOSE ETHERS

Cellulose ethers are extracted from cellulose.Depending on the derivation process, theymay be anionic (CMC, PAC) or non-ionic(HEC). CMCs are widely used as viscosityimprovers and filtrate reducers.

CMCs are characterized by:

• Their molecular weight: The higher theirmolecular weight, the more viscous theaqueous solutions made with them will be.

• Their degree of substitution (or the numberof carboxyl groups attached to the molecule(DS= 0.7 – 1.2 ): This characteristic givesCMCs solubility and resistance to salts. Ifsubstitution is more regular, as in the case ofPACs, clay stabilization is increased by redu-cing the clay hydration and, consequently,their dispersion and swelling.

• Their degree of purity, i.e. their active-mat-ter content: the other products they containare salts, which are by-products of the reac-tion (degree of purity = 50 to 99%).

Cellulose ethers are not stable under bacte-rial or enzyme attack.

2.5 - POLYACRYLAMIDES

Polyacrylamides are derivative of acrylamide.A whole series of derivatives has been prepa-red from the basic structure by copolymerisa-tion:

• Either with acid compounds like acrylicacid, giving anionic copolymers: phpa.

• Or with amine-type compounds, givingcationic copolymers.

They are anionic, non-ionic, or cationic long-chain, purely synthetic high polymers. Theyremain stable under bacterial or enzymeattack but are sensitive to high shear rates.They are used as viscosity improvers and asflocculants, possibly in association with acoagulant to stabilize clay. The basic unit isrepeated a very large number of times andhas undergone more or less thorough hydro-lysis, which results in a broad variety of pro-ducts with varied characteristics regarding:

• Viscosity,

• Adsorption on clay,

• Flow resistance,

• Sensitivity to salts.

2.6 - POLYACRYLATES

Anionic low-molecular-weight synthetic poly-mers used as plasticizers and dispersingagents.

There are other varieties with higher molecu-lar weights which are used as filtrate reducersor bentonite dopes.

3 - BENTONITEThe gross percentage breakdown ofMontmorillonite is the same as that of alumi-nium silicate and hydrated magnesia, likemany natural silicates, but its exceptionalproperties are due to a special molecularstructure that is characterized as follows.

This appendix presents the main types of polymers used for underground works.

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• 175 •

3.1 - STRUCTURE OF BENTONITE

The elementary Montmorillonite plateletconsists of two tetrahedral layers of Si4+ oneither side of a third octahedral layer whosecentre is occupied by a cation such at Al3+.The three layers comprise an elementary pla-telet about 10 Å thick. The smallest volumewith all the characteristics of crystal is a sand-wich of two layers of these elementary plate-lets. In practical terms, bentonite alwaystakes the form of primary particles of aboutten platelets or micro-aggregate or agglo-merations of several hundred platelets.These are the finest particles of a bentonitewell dispersed in water. They are microscopicin dimension (0.1 to 2 ùm).

During the alteration of igneous rocks (peg-matites, diorites, rhyolites, ash), a series ofcation migrations have resulted in substitu-tion of Si by Al, for example, in the tetrahe-dral layers and of Al by Mg-Fe-Li cations,etc., in the octrahedral layers. Substitution ofcations of lower valence creates excess nega-tive charges in the platelet. This excess iscompensated by cations between platelets(Na+, Ca2+ for instance). These cations are thesource of the capability for cation exchange,an essential characteristic of bentonites (CEexpressed in milliequivalents of cation per100 g of bentonite calcined at 1000° C).Since these substitutions, the numbers ofwhich are highly variable, occur in veryhaphazard fashion and allow a great manypossible combinations, it is easy to imaginethe very large variety of bentonites that canexist.

The exchangeable cations attached to theavailable negative charges are located in thespaced between the layers and in breaks in

the platelets. In an aqueous medium, watermolecules attach between the plates, cau-sing them to move apart, causing intra-struc-tural swelling of the bentonite. The spacebetween layers becomes the seat of cationexchange. Some important properties ofbentonites depend closely on the type andnumber of cations exchanged: swelling, vis-cosity of suspensions, plasticity, Atterberglimits, and some other characteristics ofinterest for foundry work. The type ofexchangeable cations provides a basis forclassifying natural bentonites into two cate-gories:

• Natural calcium bentonites:

In which the exchangeable cations are gene-rally Ca++ and Mg++ cations in varying propor-tions.

• Natural sodium bentonites:

In which the exchangeable cations are mostlyNa+ (70-80%) cations, with approximately 20to 30% Ca++ and Mg++ cations.

3.2 - BENTONITE PROPERTIES

The properties of a bentonite depend on allthe mineralogical characteristics referred toabove, in short:

• the type and variety of Montmorillonite,

• the purity of the raw bentonite(% Montmorillonite),

• the size of colloidal hydrated particles (1-2 µm),

• the specific surface of the particles (expres-sed in m2/g),

• the cation-exchange capacity,

• the type of exchangeable cations (alkaline –alkaline earth),

• behaviour under heat (up to 1000°C),

• possible presence of electrolytes (stabili-zing or flocculating action).

The properties of a bentonite also dependon the physical state it is in. It can be solid,semi-solid, plastic, semi-liquid, or liquid,depending on the proportions of bentoniteand water. The essential properties of bento-nite as a slurry component in tunnelling areas follows:

• adsorbant properties,

• binding, agglomerating, and adhesive pro-perties,

• rhelogical properties (colloidal suspen-sions),

• clogging properties (colloidal suspensions).

3.3 - BENTONITE FORUNDERGROUND WORKS

Depending on the intended use (deep orshallow drilling, special foundations, bento-nite/cement grout, tunnelling, or groundimpermeabilization), bentonite producerspropose a variety of different products adap-ted to each application. The performance ofthese products depends on their origin andthe treatment the raw material may havebeen given in the plant: activation (sodiumcarbonate or other salts for calcium bento-nites), drying, crushing, addition of mineralor organic dopes (CMC, acrylate/acrylamide,etc.). The requisite tests should be carriedout in order to select the products that bestmeet the characteristics required by theslurry schedule. In this respect, particularlyclose attention should be paid to bento-nite/polymer interaction, stability over time,etc.

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• 176 •

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The principle of ground support using slurrypressure relies on the possibility of mobili-zing a pressure in a liquid against a soil sub-ject to a state of stress, be it saturated or not.

Soil is a porous medium through, which aliquid whose rheological characteristicsmatch the Bingham model, can flow.

Slurry penetration into soil has been studiedexperimentally in the laboratory by a numberof TBM manufacturers, using large permea-ters (see Appendix 7). The test set-ups usedcomprise a cylinder filled with the soil to bestudied, into which the slurry is made topenetrate by a hydrostatic pressure diffe-rence. The pressure gradient, the depth ofpenetration, and the flow rate can also bemeasured.

In the extreme case of a ground mass with avery high in-situ permeability, and with a veryfluid slurry, flow occurs under the effect ofthe “hydrodynamic” (over)pressure, untilsuch time as this pressure ceases to existwhen the slurry stops penetrating into theground because of head losses or the yieldvalue of the slurry.

In these experiments, it has been seen thatthe volume of ground to be penetrated canbe insufficient to stop slurry penetration, andthat as a result there is permanent flowthrough the ground, with a certain flow rate.

This extreme case corresponds to creation ofa hydraulic flow regime in a soil as a result ofa hydrodynamic pressure. The pressure canbe maintained only if there is the requiredflow rate (generally high), which, in themedium term, is not good for the supportfunction required.

On the other hand, as soon as a “mem-brane” (polyethylene film, for instance) pre-vents “outward” flow, hydrostatic thrust ismobilized. This is the mechanism wanted inslurry-pressure support system: ideally, for-mation of a film or membrane, or more com-monly of a clogged zone called the “cake”that will mobilize all or part of the potentialhydrostatic thrust.

Such a cake can be the “membrane” or“impregnation” type (see section 3.1.4).

The stability of the ground supporteddepends on the stability of the soil particlesat the ground interface. When the stability ofan infinitesimal volume of a non-cohesive soilat the edge of the free surface is studied, it isdemonstrated [1] that the pressure gradient

required to ensure stability can be be calcula-ted from the relationship:

f = (1 – n ) (γg – γs ) sin (α – ϕ)

sinϕ

where

n = porosity of the soilγg = density of solid grainsγs = density of slurryϕ = angle of internal friction of the soilα = angle of the free surface to the hori-

zontal

Soil saturated with slurry γ - γs = (1 - n ) (γg - γs )

for α = 90°

The slurry is considered to act like a perfectBingham fluid, establishing a gradient knownas the “stagnation gradient” because of itsthixotropic properties. This stagnation gra-dient, fso, depends on the permeability andporosity (pore diameters) of the ground. Ithas been studied both theoretically andexperimentally:

1. Theoretically: a study of the mobilizationof the flow of a Bingham fluid with a yieldpoint τf in a circular tube of radius R showsthat flow in the tube ceases if the hydraulicgradient δp /L proves the relation R = 2τf x[L/δp], yielding a relationship of thetype:

fso = (τf in Pa)

2. Experimental studies [2,3] have demonstra-ted a relationship between R and the d10 ofthe in situ soil (d10 in mm):

fso = a. τf / d10, with a varying from 0.25 to 2.a = dimensionless coefficient

3. More recently, theoretical and experimen-tal studies [4] have led to a relationship of thefollowing type being established:

fso =

where K is the horizontal permeability of thein situ soil, and A is an adimensional coeffi-cient between 5 and 10 which in theoryincreases with the degree of scatter of soilpore diameters and decreases with gradualclogging of the ground by slurry.

Comparison of the two gradients, one requi-red by the ground and the other provided bythe slurry penetrating the ground, using thesafety factor inherent to uncertainties overthe values of the parameters (F ≥ 2), indicatesthe yield point τf required for the slurry, inaccordance with ground conditions.

When the pores are too big, the value of τfrequired may be high. Rather than adjust thecharacteristics of the bentonite solution(higher concentration, additives, etc.), it maybe possible to reduce the pore diameter byadding clay or sand fines, for example.

The particles added form a granular mix withmaximum dimension dslurry for which the fol-lowing characterization can be proposed:

d15ground ≤ 5dslurry (filter condition)

According to some authors [2], experienceshows that relations of the type d15ground ≤ 8dslurry or d20ground = 3dslurry

are acceptable.

∆ρtgϕ.dS > (γ - γs ). dv or fg s

2.τf

R

REFERENCESREFERENCES[1] Les boucliers à pression de boue, by J.Ferrand and J. Pera – Annales de l’ITBTP n°420 (December 1983)

[2] Stability of slurry trenches –Müller–Kirchenbauer – 5th International SoilMechanics Congress, Madrid 1972, IV-12.

[3] Nachweis des Sicherheit gegen denSchlitz gefährende Gleitflägen im Boden,nach DIN 4126 Vornorm. Kilcher (M) andKarstedt (J).

[4] La prévention des fontis dans les travauxau tunnelier : des études en temps réel.P. Aristaghès, F. Berbet, P. Michelon –Tunnels et Ouvrages Souterrains No. 128,March/April 1995.

[5] Imperméabilisation du front de taille parinjection de boue bentonitiques préalable-ment à des interventions hyperbares (BPNL)– B. Demay, L. Nicolas – Tunnels etOuvrages Souterrains No. 150 – Nov./Dec.1998.

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• 177 •

1 - DEFINITION OF RHEOLOGYRheology can be defined as the science ofthe flow and deformation of matter. It studiesthe reactions of all solid or liquid substancesto deforming forces and classifies them usingprecise terms, and in the least complexcases, serves to draw up empirical equationstaking account of deformation. It thus servesto accurately define properties that we referto with vague terms such as “thick”, “thin”,“sticky”, “viscous”, etc, but it calls into ques-tion concepts that we feel we know about,such as solid and liquid: when subjected tolong-term stress, most “solids” flow anddeform. The stained-glass windows of cathe-drals, which are thicker at the bottom than atthe top, are one of the most spectacularexamples proving the fluid behaviour of glass.

2 – RHEOLOGY OF TUNNELLING SLURRIES The rheology of tunnelling slurries can bedefined by a number of quantities (viscosity,gel strengths, etc.) whose testing and measu-rement are of great practical importancesince they govern::• Pump ratings• Cleaning of the excavation and suspensionof spoil• Correct flow velocities.

3 - DEFINITIONS AND TERMINOLOGYFluid:A fluid is a deformable continuous sub-stance. It has no specific shape. It can flowand undergo substantial deformation underthe effect of even very weak forces. Liquidsand gases are both fluids, but whereas a

liquid occupies a determined volume whichvaries little with external conditions (tempe-rature, pressure, etc.), a gas always takes upthe greatest volume available to it. In whatfollows, the term is used loosely, and anyfluid is taken to be a liquid.

Laminar flow:Laminar flow is a special kind of hydraulicflow characterized by conservation of theform of liquid layers during their movement.Reynolds defined a quantity which bears hisname and which determines the kind of flowin a pipe (laminar, unstable, or turbulent).

Viscosity:Viscosity is the expression of frictional forcesbetween the particles of a fluid; it demons-trates the “imperfection” of its fluidity. Itmust be remembered that viscosity existsonly in relation to the movement of a fluid.

Velocity gradient (shear gradient):

A liquid flowing under a laminar regime canbe assimilated to a stack of layers sliding

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The table below gives orders of magnitude in the form of bracket values for slurry parameters against different permeabilities in homogeneousground. These values are merely orders of magnitude for the purposes of illustration.

SOLS TYPESParameters Unit Type 1 Type 2 Type 3 Type 4

K>1*10-4 m/s 10-4<K<10-7 m/s K<1*10-7 m/s K<1*10-7 m/spermeable semi-permable impermable neutral impermeable reactive

(alluvium ETC.) (sandy soils, etc.) (chalk, etc.) (clay, etc.)

AV cp 15 - 40 10 - 20 8 - 20PV cp 5 - 20 5 - 10 5 - 10YP Pa 8 - 5 5 - 10 < 150/10 gel strengths lbs/100ft 10 - 30 3 - 20 Not applicableFiltrate ml 20 - 50 15 - 30 Not applicableCake (API test) mm 2 - 6 1 - 5 1 - 5Marsh viscosity s/946 ml 40 - 120 35 - 50 30 - 50Density 1.02 - 1.15 1.02 - 1.20 1.02 - 1.60 1.02 - 1.30pH 7 - 10 7 - 10 7 - 10Sand % 1 – 5 1 - 5 1 - 5

(standardized test methods “Recommended Practice Standard Procedure for Field Testing Water-Based Drilling Fluids API RP 13 B-1, 1 June 90)


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• 178 •

over each other, each driven at its ownspeed. The velocity gradient between anytwo layers of a fluid is therefore the quotientof the velocity difference between thoselayers divided by the distance betweenthem. The dimensional measurement of thevelocity gradient (sometimes called the sheargradient or the shear rate) is the reciprocal ofa time; it is measured in s-1.

Shear stress:

Shear stress is the result of forces of internalfriction opposing the flow of liquid. Itdepends on the velocity gradient of the fluid,and its evolution in accordance with the velo-city gradient characterizes the response ofthe material to deforming forces, which the-refore defines its components performance.Shear stress is sometimes referred to asshear tension; it is always compared to a unitsurface area, making it similar to a pressure,and it is therefore expressed in Pa in theSystem International (S.I).

4 – COMPONENTS MODELSIn a laminar regime, the flow of a liquid ischaracterized by a relationship between ashear stress and a deformation velocity.Depending on the type of fluid, this relation-ship can take on different forms, the mostcommon of which are as follows:

Newton model: τ = η x G where τ: shear stress

G: velocity gradientη: viscosity

Bingham model: τ = YP + PV x Gwhere YP: yield point

PV: plastic viscosity

Ostwald model: τ = K x G N

where K: consistency indexN: rehological performance index

Representative rheograms of these threemodels are as shown below:

In practical terms, these very simple theoreti-cal models do not fit tunnelling slurries per-fectly. Since they can define a fluid only overa limited range of velocity gradients, theycannot take account of the plastic or visco-plastic behaviour of tunnelling slurries.Consequently other models have been deve-loped. Most of them are based on the basicmodels above.

Herschel-Bulkley model: τ = YP + K x G N

This model is a synthesis of the above threemodels, for, depending on the values of YP,K, and N, it can represent them all:• YP = 0 , N = 1: Newton model (K = η)• YP = 0 , N ≠ 1: Ostwald model• YP > 0 , N = 1: Bingham model (K = PV)

This model gives a better representation ofthe behaviour of tunnelling slurries andallows for subsequent introduction of notionsof gelling in time (YP = f (t)), which is charac-teristic of destructuring/restructuring pheno-mena.

Steiger-Ory model: G = aτ 3 + cτThe Steiger-Ory model was developed fromassumptions of symmetry and continuity(curve through the origin, symmetry of repre-sentation). The formula used to draw therepresentative rheogram isa mathematical function, G= f (t ), whose terms do nothave any real physicalsignificance, but the equa-tion has the advantage ofillustrating the behaviourof many fluid types:Newtonian, plastic,pseudo-plastic. We shallrestrict ourselves here togiving the empiricalSteiger-Ory formula and topresenting differentvariants for different valuesfor coefficients a and c.

If:

a = 0, the plot is a straight line passingthrough the origin; the fluid is Newtonian,with viscosity h = 1/c (1)

a < 0, is meaningless

a > 0, the liquid is pseudo-plastic or plastic,depending on whether c is positive or negative

c< 0, in addition to the origin, the plot hastwo further intersections with the verticalscale. There is a yield point. The liquid is plas-tic and close to being a Bingham fluid (2)

c ≥ 0, the liquid is pseudo-plastiqc (3), c = 0being a special case where the liquid can beconsidered to be a plastic material whoseyield point has been moved to the origin.

Shea

r st

ress

Shea

r st

ress

Shea

r st

ress

Velocity gradient Velocity gradient Velocity gradient

NEWTON BINGHAM OSTWALD

Shear stress t

Velocity gradient

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• 179 •


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6.1 - SEPARATION OF SOLIDSPOIL FROM SLURRY: (inreal time)

6.1.1. Scalping: Separation oflarge particles (> 6 mm approx.):gravel and pebbles, but alsolumps and flakes of earth thathave not been broken down.

a) Scalping with vibrating screen: For sandyand stoney ground that does not stick

b) Scalping with drum screen: Especiallygood for sticky, clogging clayey ground.

6.1.2 - Desanding: Separation ofsand from slurry

Sand is separated out from the slurry in 1, 2 or3 series stages of hydrocyclones. Centrifugesmay also be used to separate out silt.

How a hydrocyclone works: (from Frenchdocumentation: Mines et Carrières /L’épuration physico-chimique des eaux)

6.1.3 - Dewatering:

• Vibrating dewatering units are fitted withslot-type grilles on a slight slope. They areused to dewater underflow material fromhydrocyclones. After dewatering, sand andsilt has residual humidities of between 10 and20%, depending on the characteristics of themucking slurry.

• Unlike screens, there is a “self desatura-tion” phenomenon in a dewatering unit: witha given circulating load, an underlying layerof filter material is formed.

• Because of the diversity of the groundtypes encountered and the broad range ofviscosities of mucking slurries, a desandingunit with 2 separation stages plus dewateringis generally necessary for most tunnellingprojects. The second separation stage refinesthe relatively coarse split of the first.

6.2 - SEPARATION OFEXCESS SLURRY FROMRECYCLED SLURRY: (in realtime)

After desanding and desilting, most of theslurry is fed back into the slurry circuit, aftercompensating with fresh slurry to maintainthe required rehological characteristics. Theexcess is disposed of, to maintain the correctweight/volume balance (density and levels).

6.3 - TREATMENT OF EXCESSSLURRY: (in real time, part-ly as a background task)

The treatment of excess slurry is closely lin-ked to the required spoil consistency (liquid,paste, or solid) and the quality of the groundwater absorbed and of the water dischargedfrom the plant (pH, SS, hydrocarbons, etc.).

Cycloning is a process for separating liquids from solids that uses centrifugal force. Slurry is injected tangentially into the cylindrical part of thecyclone. The vortex created causes the heavy, i.e. coarse particles to be thrown against the walls of the cyclone and to be carried down the coni-

cal section with downward flow while the finest particles, which are less affected by centrifugal force, are carried bythe upward flow. The fineness of separation increases with increasing inlet velocity, and a small-diameter hydrocy-clone also separates out finer particles.

The split size, or d50, is the particle dimension which has equal chances of being in the overflow or underflow. It isthe grain size that is split exactly into two equal amounts by the hydrocyclone. The empirical Yoshioka and Hottaformula can be used to calculate the d50 integrating viscosity:

0,2 x 106.Dc0,1 x D0

0,8 x Di0,6 x √η

d50 (µm) =√Q x √(ρs - ρp)

d50: split size in µmDi: equivalent diameter of cycline

inlet in metresQ: inlet flowrate in m3/s

Dc: cyclone diameter in metresη : apparent viscosity of slurry

≈ 1 + 2,5 x ((ρp –1) / (ρs –1) ) en mPa.sρs: real density of solids in kg/ m3

D0: overflow diameter in metresρp: pulp density in kg/m3

A division curve can be drawn up, with on the horizontal scale the grain size and on the vertical scale the percentage by mass of the underflow gra-ding. The plot is generally linear between points d25 and d75, from which the split size can be characterized by the imperfection coefficient I:

d75 – d25I = ;

the smaller I is, the better the split (≈ 0.3-0.4 for cyclones)

entrée – inlet asurverse – overflow sous-verse – underflow

2 x d50

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• 180 •

Treatment Appearance Density of Detailsof discharge discharge

No treatment (direct discharge) Liquid 1,15 – 1,25 Slurry is remoed at its optimum density for separation conditions

Thickening by flocculation-settlement Paste 1,20 – 1,40 Coagulant and flocculant required

Epaississement par centrifugation Paste 1,25 – 1,45 Flocculant required

Treatment with cement, and lagooning Solid 1,30 – 1,50 Addition of 20 to 40% cement by weight of drysolids + 12 to 24-hours holding time

Belt-filter treatment Solid 1,60 – 1,75 Flocculant added to slurry

Filter-press treatment Solid 1,70 – 2,00 0 to 10% slaked lime added to slurry (by weight ofdry solids)

6.3.1 - Thickening by flocculation-settlement of thick slurry paste

• Flocculation requires effluent concentra-tions of about 20 to 30 g per litre of drysolids, which generally implies addition ofconsiderable quantities of water for dilutionbefore treatment.

• Clay particles consist of a stack of elemen-tary platelets of variable specific surface(≈ 20 m2/g for kaolinite / ≈ 100-150 m2/g forconstruction bentonite): this structureexplains the high swellability of bentonite.There is a layer of adsorbed water on the sur-face of the particle (ionic double layer) whichis electrically charged with easily exchan-geable cations (+). Its potential increaseswith the specific surface of the particles. Thepresence of more or less bentonite usuallyrequires there to be a phase of coagulationby adsorption of cations to reduce thepotential of the ionic double layer, followedby addition of a flocculant (generally a syn-thetic polymer such as polyacrylamide) toform inter-particle bridges (coagulantconsumption ≈ 100 - 200 ml/T of dry solids –flocculant consumption ≈ 100 - 200 g/T ofdry solids).

6.3.2 - Additional treatment bypressing to obtain solid bulkwaste that can be handled easilty:

The filterability of bentonite slurries is gene-rally poor ( r0.5 = 1014 to 1015 m.kg-1 )

• If a belt filter is used, the slurry must beflocculated beforehand, and belt filters canonly be used if the slurry has a low specific

resistance (r0.5 < 1011 m.kg-1) and is compres-sible (s>1), for the filter pressures that can beapplied are relatively low (0.3 to 1 bar).

• In most cases a filter-press produces hard,solid cakes from slurrys with high specificresistances (r0.5 = 1015 m.kg-1), using the pos-sible filtration pressures of 0 to 20 bars. It isgenerally necessary to treat the slurry before-hand by adding lime or some other filtrationaid to dewater a slurry with a specific resis-tance of r0.5 = 1012 m.kg-1.

6.3.3 - Slurry filter index: (fromDegrémont technical report onwater)

6.3.3.1 - Specific resistance to filtration

• Use a filtration cell of the Baroid filter-presstype using compressed air and a paper filter.

• Pour 100 to 150 ml of slurry into the cell.

• Allow the filtrate to flow off and record thevolume V0

Apply the required pressure (0.5 to 16 bars)and record the volume of filtrate every 10,15, 20, 30, or 60 seconds, depending on thefiltrate flow velocity.

• Filtrate volumes V0, V1, V2… correspondto times T0, T1, T2…

• Plot points against Vx horizontally andTx / ( Vx – V0) vertically, and measure slope aof the linear part of the curve.

Specific resistance r in m.kg-1 = ( 2 A P S2 ) / ( η C )

Where

P: filter pressure in Pa A: slope of straight plot for filterability test, in

s.m-6

S: filtration surface in m2

η: dynamic viscosity of filtrate in Pa.s(approximately 1.1 x 10-3 Pa.s at 20°C)

C: dry solids at 105°C divided by the volumeof slurry in kg.m-3

➬ Different slurries are generally comparedin terms of their r0.5 values (0.5 bar pressure):

➬ If r0.5 = 1014 to 1015 m.kg-1: slurry not easilypressed, requiring an admixture such as limeto assist filtration

➬ If r0.5 = 1x1011 to 5x1011 m.kg-1: slurry easilypressed, requiring no filtration admixture.

6.3.3.2 - Compressibility factor

• Measure the specific resistance to filtration,r, at several pressures, P.

• Plot curve log r = f (log P)

The value of s is determined from the slopeof the straight line = (log r1 – log r) / (log P1 – log P)

➬ If s < 1: the filteration flow rate increasesas P increases => the slurry is not very com-pressible and high pressures are recommen-ded (15-16 bars).

➬ If s > 1: the filtration flow rate decreases asP increases => the slurry is compressible andit is recommended that the pressure be rai-sed gradually to facilitate drainage.

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• 181 •


AAAAPPPPPPPPEEEENNNNDDDDIIIIXXXX 7777

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PressionValve + pressure regu-lator with aadjustment from 0

to 0.15 MPa ajustable

Compressed air in

Levels

Translucent cylinder Service pressure up to 0.15 MPa

Soil - (20 litres)

Attachment

4 legs allowing permeameter handling

ManomPressure Entonfunnelnoir

Valve

Tube

TBoue - (15 litres)

Overflow

gracylindergg

NB It would be useful to have the permeater To a tilting base so that filter cake Performance under compressed air can beTested and to facilitate emptying

Additionnal equipmentScales for soil weightScales for water weight

PERMEAMETER - PRINCIPLE GUIDE

All

repr

oduc

tion,

tran

slat

ion

and

adap

tatio

n of

arti

cles

(par

tly o

r tot

ally

) are

sub

ject

to c

opyr

igth

.



• 182 •


SSSSTTTTAAAANNNNDDDDAAAARRRRDDDD SSSSLLLLUUUURRRRRRRRYYYY----MMMMOOOONNNNIIIITTTTOOOORRRRIIIINNNNGGGG DDDDOOOOCCCCUUUUMMMMEEEENNNNTTTTAAAATTTTIIIIOOOONNNN

The following forms are given to illustrate standard documentation for monitoring tunnelling slurry:

• Daily monitoring report

• Test report

All

repr

oduc

tion,

tran

slat

ion

and

adap

tatio

n of

arti

cles

(par

tly o

r tot

ally

) are

sub

ject

to c

opyr

igth

.



• 183 •

test

on

par

ent

slur

ryP

aren

t sl

urry

ad

diti

on

Co

ncen

trat

ion

Ad

diti

ve t

ype

Ad

diti

ve a

dd

itio

nC

onc

entr

atio

n

Slu

rry

pH

Co

men

tari

es

All

repr

oduc

tion,

tran

slat

ion

and

adap

tatio

n of

arti

cles

(par

tly o

r tot

ally

) are

sub

ject

to c

opyr

igth

.

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