slurry or epbm

UNTIL THE INTRODUCTION of closed faceTBMs in the late 1960s, all shield tunnelling insoft ground conditions below the water tablewas carried out with shields combined withcompressed air. Air pressures were not usedabove 3 bar. However the hunt was on for analternative, which would avoid compressed airand it’s associated dangers of the bends andbone necrosis.

Slurry Tunnelling machines (STMs) arosefrom the 1964 patent for the BentoniteTunnelling Machine taken out by John Bartlett.In the UK the use of the patent was first triedduring the New Cross experimental tunnel insouth London, carried out in 1971/72. Prior tothis the Japanese had started trials with the so-called mud shield in 1964 in the very softcoastal alluviums. In Germany trials werecarried out in Hamburg in 1976 using whatthey referred to as the Hydroshield, a type ofSTM still produced today by the TBMmanufacturer Herrenknecht. By the end of the1970s over 1,000 STMs had been usedworldwide.

By the mid 1970s the Japanese wererecognising the limitations of the STMs due tothe narrow range of ground conditions thatthis type of closed face TBM was suitable for.So during the mid 1970s they invented theEPBM. This effectively widened the range ofground conditions suitable for closed faceTBMs into the finer granular materials and alsosilts and cohesive clays. Figure 1 shows theenvelope of ground conditions suitable foreach type. It can be seen that there is a midrange of ground conditionswhich is suitable forboth types.

How an STM worksIn an STM thecutterhead excavatesthe ground in front ofa full bulkhead acrossthe forward end of theTBM. The cutterheadchamber ahead of thebulkhead is filled witha bentonitic slurry,typically a 3% mixture.This slurry iscontinuously pumpedthrough theexcavation chambervia an inbound slurry pipe. As the TBM movesforward during the excavation cycle theexcavated spoil mixes with the slurry and isremoved from the excavation chamber withthe slurry through the outbound slurry pipe.The slurry spoil mix will be typically in therange of 20% to 30% during the excavation

cycle. The excavated quantity is computedusing a magnetic flow meter on both theinbound and outbound pipes together withinfrared density meters on both lines.

The spoil-laden slurry is pumped to thesurface where it passes through a SlurryTreatment Plant (STP). This effectively removes

a large percentage of the spoil from the slurryusing vibrating screens, hydrocyclones and, ifnecessary, belt or hydraulic filter presses for theremoval of very fine material. The use ofcentrifuges is an alternative to the filterpresses. The cleaned slurry is then pumpeddown the tunnel for re-use. For environmentalreasons it is sometimes necessary to furtherclean the spoil using a sand washing plant toremove residual bentonite or polymer.

An extra benefit of the bentonitic slurry is aconsiderable reduction in wear to thecutterhead and tools and also in the pipes andpumps.

Face pressure control is by using a fixed flowinbound pipe and a variable flow outboundpump. However if a Hydroshield is used, themain form of pressure control is the use of alarge air bubble trapped between the mainbulkhead and a semi bulkhead ahead of themain bulkhead. This is a more accurate formof pressure control.

How an EPBM worksAn EPBM is altogether a simper form of closed

32 TUNNELLING JOURNAL

SLURRY OR EPBM?

Independent TBM consultant Alastair Biggarttackles the very important subject of choosing themost suitable type of closed face TBM for a project.The choice will normally be made on issuesconcerning the ground, but also other matterssuch as the environment, or the availability ofspoil disposal areas. However, the over-ridingissue will be the requirement to control settlement

Closed face decSLURRY OR

%

0 0.002 0.06 2.0 60 mm

100

75

50

25

0

Clay Silt Sand Gravel

Range of ground conditions

EPBMs Slurry machines

EPB

or s

lurry

Fig 1: The PSD Curve for grain size distribution

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SLURRY OR EPBM?

face TBM. This means that contractors andowners may be tempted to use an EPBMinstead of an STM against their bettertechnical judgement, purely on the basis ofcost and simplicity. But the wrong choice canturn out to be an expensive mistake.

With an EPBM, as with an STM, thecutterhead excavates the ground in front of afull bulkhead across the forward end of the

TBM. The cutterhead chamber ahead of thebulkhead is filled with the excavated spoil andis removed from the chamber via anarchimedean screw (the screw conveyor). Itcan be seen that depending on the relativeforward progress of the TBM and the speed ofrotation of the screw conveyor, the pressurewithin the excavation chamber can becontrolled and therefore balanced against the

earth and hydrostatic pressures. Hence thename Earth Pressure Balance Machine. Theexcavated spoil has a large amount of friction,which resists the easy flow of the spoiltowards the screw conveyor and also causes alarge amount of torque resistance to thecutterhead as it tries to turn against this highfriction mass. This results in difficulty incontrolling the earth pressure and hence thecontrol of settlement.

When the EPBM was first introduced in themid 1970s the high friction within theexcavation chamber was reduced by injecting

conditioning agents intothe chamber. Thesewere either bentonite,polymer or water or acombination of allthree. In the early 1980sthe Japanese introducedthe use of a surfactantfoam conditioningagent, which is madeup of a combination ofthe surfactant, waterand compressed air. Thishas a dramatic effect byreducing the cutterheadtorque by as much as50% and also allows alarge improvement inthe control of earthpressure, which in turnleads to better

settlement control. The use of foam isstandard today, however EPBMs shouldcontinue to be supplied with the ability toinject bentonite and polymer to assist theworking of the foam. In addition the possibilityof injecting crushed material such aspulverised limestone should also be providedfor areas of ground where the fines content istoo low for the screw conveyor to provide

TUNNELLING JOURNAL 33

ace decisions -RY OR EPBM?

Left: Kawasaki’sCTRL 2 EPBM

Opposite page:The NFM GroeneHart Slurry TBM(courtesy NFMTechnologies)

Below left: Atypical Slurry setup (courtesyHerrenknecht)

Below: A typicalEPBM set up(courtesyHerrenknecht)


adequate resistance to control the earthpressure.

The screw conveyer delivers the conditionedspoil to a belt conveyor system, whichnowadays typically includes a long tunnelconveyor. It is not as easy to measure theexcavated quantity as with an STM. However itis now standard practice to use belt weigherson a short belt within the TBM back-up. Whenused on, say, a 5 ring rolling average this cangive an early indication of over mucking. Inaddition a laser profiler can be used to back upthe information from the belt weigher. Thistechnology is not yet in common use.

Some important technical characteristicsIt is most important to understand the varioustechnical functions of each type of closed faceTBM as this will provide a better backgroundfor making the choice. It will also indicate thevarious things that should be included in theTBM chosen. It is no use choosing the correcttype of TBM and not providing it with thecorrect facilities

Articulation - Closed face TBMs are normallyarticulated. As most tunnels involve somecurves, the articulation is provided to helpnegotiate curves. Even if there are no curvesarticulation is still useful in order to be able tocorrect mis-alignment. There are three basictypes of articulation:• Passive articulation which is normally in the

form of a trailing tailshield connected tothe main shield body by passive hydraulicrams, which in effect reduces the steerablelength of the TBM shield

• Active articulation which is articulationbetween the front and mid body of theTBM shield. This is a more positive form ofarticulation and in effect curves the shieldbody positively in order to force the TBMaround curves

• An articulated cutterhead which ismounted on a spherical bearing and allowsthe cutterhead to be set in such a way thatit can overcut in any quadrant of the circle,thus allowing the TBM to move into theovercut space

• A combination of any of the above for thenegotiation of extremely tight curves.

There is no differentiator regardingarticulation between the two types of closedface TBM

Tailseals - Bydefinition all closedface TBMs have apressure of eitherspoil or slurry in theexcavation chamber.Also by definitionthey are in waterbearing ground witha hydrostatic pressureall around the TBMshield. For bothreasons they require

efficient tailseals around the rear of thetailskin to prevent the ingress of water orslurry under pressure. Todays tailseals aremade up of multiple rows of wire brushseals around the intrados of the tailskin. Theseals and the space between seals iscontinuously filled with a fibrous greasewhenever the TBM is moving forward. Sealscan now be designed to resist up to 13 barexternal pressure. There is no differentiatorin terms of the tailseals between the twotypes of closed face TBMs. See Figure 2 fora diagrammatic view of a tailseal array.

The Screw Conveyor - This section onlyapplies to EPBMs. So the description isincluded in order to describe the limitationsof the screw. It is important to understandthe way a screw works in an EPBM. It’sability to hold pressure is almost entirelydependant on the length of the screw andthe viscosity of the spoil. There is a simpleformula which can be used to give thepressure holding capacity of the screw, L/l xp where L is the screw length in metres, p isthe pressure holding capacity of a singlescrew flight and is approximately 0.2 barand l is the typical, but variable, length of asingle screw flight and is approximately0.6m. The pressure holding capacity can beimproved when using an EPBM by the use

of a double screw, a very long screw or adouble piston pump attached to the end ofthe screw. As a general rule it is suggestedthat if extremely high pressures are involvedan STM may be more applicable due to it’sability to hold high pressures.

Crushers - This section only applies toSTMs. Due to the configuration in theexcavation chamber of an EPBM it is notpossible to have a crusher in the chamber.The crusher in an STM is situated in theinvert of the excavation chamberimmediately behind the rear of the rotatingcutterhead. Typically the crusher will be astrong jaw crusher set immediately in frontof a grillage that prevents stones or crushedboulder fragments from entering theoutbound slurry pipe. A crusher should beincluded in an STM if large cobbles orboulders are expected in the ground. It ismost important that this part of the groundinvestigation is set up to investigate thepresence of cobbles and boulders. It is alsoimportant that if there are possibleboulders, the gaps between the cutterheadarms are sized to retain the boulders at theface whilst the cutter tools, which should bea combination of picks and discs, breakthem down to a size suitable for the crusherto digest. If boulders are present in groundbeing excavated by an EPBM it is importantthat the cutterhead can break the bouldersdown to a size suitable for digestion by thescrew. As both STMs and EPBMs can digestboulders the presence of boulders wouldnot prevent the use of either type.

Main Bearings and Seals - The heart of aclosed face TBM, or any TBM, is the mainbearing. If this should be damaged duringtunnelling it is a major problem for theproject. It is possible to change a mainbearing within the tunnel but extremelytime consuming and expensive. Nowadaysmain bearings are of the three roller typeand made by specialist companies such asRothe Erde. Protecting the bearing in an

SLURRY OR EPBM?


Emergency seal

Normal build area 130

Shove ram

4 nos. brush seals

1650 1650

Fig 2: A typical Tailseal array

The archimedes screw in an EPBM(courtesy The Robbins Company)

TJ_0211_SLURRY_032_037.qxd:Feature 10/2/11 23:09 4

SLURRY OR EPBM?


open face TBM against the ingress of dirt isrelatively easy, but with a closed face TBM itis extremely difficult. In an open TBM asingle set of seals would normally besufficient. With a closed face TBM it willrequire multiple seals, the number beingdependent on the pressure in the face. Atypical supplier of seals is Merkel, whoproduce seals that will withstand 4 bar ofpressure across the seal. There is nodifferentiator between types regarding themain bearing and its seals. See Figure 3 fora diagram of the seals on a Mixshieldmachine currently in use.

Open Area of the Cutterhead -Cutterheads are designed to suit theconditions and the type of closed face TBM.When using an STM the open area of thecutterhead can be quite small as the slurrycontains a small percentage of spoil and isnot very viscous and can easily pass througha small opening. With an EPBM theconditioned spoil is highly viscous, evenwhen treated with foam. It is difficult for

this material to pass through the cutterheadopenings and it can be difficult for thepressure in the excavation chamber to beproperly transmitted to the face. This cancause difficulty with settlement control. It isrecommended that the open area ofcutterheads in EPBMs is not less than 35%,with an absolute minimum of 30% ifcircumstances dictate. It should be notedthat Japanese manufacturers use nearer40%. There is no differentiator herebetween types as either type can bedesigned with an appropriate open area forthe conditions. The photos above show twoTBMs that were used on the UK’s CTRLSection 2 Tunnels. The top photo is a WirthEPBM which had a low percentage of openarea and had to be modified in the ground.The photo below it shows a Kawasaki TBMwith a larger open area of near 40%.

Excavated Spoil Measurement - In anSTM the excavated quantity is measured bymagnetic flow meters and infra red densitymeters on both the inbound and outbound

slurry lines. This does not give an absolutemeasurement as it requires correction forground density and water content. It willalso be inaccurate if there is leakage ofslurry into the ground or leakage of groundwater into the excavation chamber – bothbeing dependant on accurate pressurecontrol to balance the hydrostatic groundpressure. Generally this form of spoilmeasurement is reasonably accurate, but itshould be considered as a comparativemeasurement rather than an absolutemeasurement and therefore be used on a 5ring rolling average basis to give an early

indication of over-excavation.

In an EPBM, upuntil quite recentlythe crude method ofcounting muck carswas used, which washighly unsatisfactory.Today, with the

almost universal use of long tunnelconveyors there is no opportunity anywayto use muck cars. Today’s technology is touse belt weighers on a fixed belt within theTBM back-up. Preferably this should be ashort, constant tension, horizontal belt foraccuracy. These weighers have a declaredaccuracy of 10%. They should therefore beconsidered as giving a comparativemeasurement rather than an absolutemeasurement and therefore be used on a 5ring rolling average basis to give an earlyindication of over-excavation. The use oflaser profilers is gradually coming in tosupplement the use of the belt weigher, butthis technology is not yet proven. The beltweigher results also need to be correctedfor ground density and ground watercontent. As the methods for both the STMsand the EPBMs give in the order of 10%accuracy and are used on a 5 ring rollingaverage, the measurement of excavatedspoil is not a differentiator.

Why use closed face TBMsBefore going on to describe the criteria forchoosing between the two types, it isimportant to think about why closed faceTBMs have been introduced to TBMtunnelling.

The main motivation for the introductionof closed face TBMs was to enable TBMs toexcavate in water bearing ground ingranular soils without the aid ofcompressed air to hold back the water.

Secondly there was a wish to widen therange of ground conditions that can besafely tunnelled using TBMs. In effecttunnels can now be driven with closed faceTBMs in any soft ground conditions, wherebefore the introduction of closed facemachines it would not have been possibleto tunnel at all.

The over-riding motivation is to enable

Left top: The WirthCTRL EPBM

Left: The KawasakiCTRL EPBM


SLURRY OR EPBM?


TBMs to control surface settlement withinacceptable limits when tunnelling in unstablesoft ground conditions.

Additional benefits are the reduced impacton the environment and the increase in thesafety of the workforce.

Criteria for Making the Choice BetweenSTMs and EPBMsIn examining the various criteria for choosingwhich type of closed machine to use it isnecessary to recognise that the primerequirement is to control settlement at thesurface and to cause minimum environmentalimpact. Contractors and Owners will also wishto consider the cost and risk implications oftheir decision.

Ground Conditions In soft ground tunnelling the groundconditions are well illustrated by the ParticleSize Distribution (PSD) curve. These should beplotted for the full length of the tunnelalignment from boreholes at realistic centres asnear to the tunnel alignment as possible. ThePSD curves can be drawn with envelopes ofground which are suitable for each type ofclosed face TBM.

Generally the STM is more suitable for theground with a higher granular content,whereas the EPBM is more suitable for groundwith a higher percentage of fines. However,there is a mid point between the granular andthe finer material which is suitable for bothtypes (figure 1).

There are, nowadays, a number of factorsthat widen the range of ground conditionssuitable for each type of closed face machineand also narrow the differences between thetwo types.• For STMs the slurry treatment plants (STPs)

are becoming much more sophisticated withuse of improved vibrating screens,hydrocyclones and hydraulic filter presses orcentrifuges, thus allowing the slurry to bemore thoroughly cleaned and therefore theground range to extend further into themore silty/clayey materials

• For EPBMs the use of long screws, doublescrews, double piston pumps and the use ofmore sophisticated conditioners, includingthe re-injection of fines, such as pulverisedlimestone, enables a better ability for thescrew conveyor to control the pressurewhere the natural fines are limited, thusallowing; a) the ground range to extendfurther into the more sandy, gravellymaterials and; b) the TBM to work at muchhigher pressures

The above factors mean that generally the twotypes of closed faced TBMs are; a) moreversatile and; b) it is more difficult to decidewhich type to use.

So the first thing to look at in making adecision on type is the PSD curve. This will givean early indication of the correct type, but willnot give the final answer.

Fines Content of the GroundThe cut point for fines content is 60µ. If thereare less than say 10% fines below this sizethen it is more difficult to use an EPBM.However, as mentioned above it is possible toinject fines such as pulverised limestone toincrease the natural fines content andtherefore give the screw conveyor a betterpressure holding capability.

There is also a point at which too many fineswill inhibit the use of an STM by blocking upthe slurry treatment plant. It must beremembered that when examining the finescontent it is necessary to look at the ground

content for the full vertical span of the tunneland along the whole tunnel length.

Ground PermeabilityThe permeability of the ground must beexamined and it will therefore be necessary toinclude pumping tests to give a generalindication of permeability along the tunnelalignment. A rough guide to the level ofpermeability suitable to each type is that anSTM is suitable for ground with a higherpermeability than 1 x 10-5m/s and an EPBMwith permeability below this figure. Howeveran EPBM can be used in ground with a higherpermeability, which implies fewer fines, by

injecting re-constituted fines into theexcavation chamber, which increases thecapability of the screw conveyor in terms ofholding pressure.

Hydrostatic HeadAs a general rule an STM is more capable ofresisting a high hydrostatic head than anEPBM. This is because the pressurised slurry ispumped around a closed circuit. Whereas withan EPBM there are limits to the capability ofthe screw conveyor in holding pressure. Aspreviously mentioned this capability can beimproved by re-injecting fines into the

excavation chamber. Themost difficultcombination wouldbe to have a highpermeability togetherwith a highhydrostatic head.Permeability cantherefore be a strongfactor in deciding thetype of closed faceTBM. This would beespecially so if sayhigh permeabilityexisted for the wholelength of a tunnel,which would indicatethe use of an STM.

Settlement Control One of the primereasons for using aclosed face TBM is tocontrol groundmovement. Nowadays

both types of closed face TBM are excellent atcontrolling surface settlement. Before theintroduction of foam conditioners to EPBMs,the STM was better in terms of settlementcontrol. However with the advent of foamconditioners both types of closed face TBMcan be used to control settlement to anacceptable degree. If the ground is verygranular with few fines below 60µ then it maybe necessary to re-inject re-constituted fines ifusing an EPBM. Table 1 show volume lossesachieved on closed face machine projects

Presence of BouldersBoth types of closed face TBM can be used in

R2800

10

FA1-

12

PA1-

4

OA

1-4

LA1-

4

Fig 3: The Bearing and seal system on TBM

Because closed face TBMs havepressurised material in front of themain bulkhead there is always anadditional backward force to beovercome in addition to the normalforces of friction and the force to burythe cutters and tow the back up sledges


ground containing cobbles and boulders. Inboth types the cutterhead tool configurationand size of openings must be designed tobreak up boulders at the face before they passthrough the cutterhead. In an STM they mustbe broken down to a size suitable for thecrusher and in an EPBM they must be brokendown to a size that can be digested by thescrew conveyor.

Thrust and TorqueIt is most important that with all closed faceTBMs they are not limited by insufficienttorque or thrust.

Because closed face TBMs have pressurisedmaterial in front of the main bulkhead there isalways an additional backward force to beovercome in addition to the normal forces offriction and the force to bury the cutters andtow the back up sledges.

In a 6m diameter TBM with 3 bar in the facethis force will be 848 tonnes. In a 16mdiameter TBM with 3 bar pressure the forcewill be 6,034 tonnes. A 10m diametermachine with 13 bar in the face will have abackward force of 10,214 tonnes. If an STM isbeing used with a slurry in the face it ispossible to have rapid changes in this forcedue, for instance, to a rapid loss of slurry. AnEPBM with a much more viscous material willnot suffer the same rapid changes. So this is adifferentiator between the two types. Thesevery large forces must be considered carefullyin the design of the pre-cast lining.

Torque is a factor where there isconsiderable difference between an EPBM andan STM. With an STM there is a very lowviscosity slurry in the excavation chamber.agent. Even with foam there is still a severeresistance for the cutterhead to overcome. Anapproximate method for calculating therequired torque in tonne metres for each typeis obtained from the simple empirical formulaαD3 - where α is a coefficient relating to themachine type and D is the TBM diameter inmetres. For an EPBM α is between 2 and 3and for an STM α is between 0.75 and 2. Thismeans that an EPBM of 6m diameter wouldrequire a torque between 432 and 648 tonnemetres, whereas an STM of the same diameterwould only require between 162 and 432tonne metres. For an EPBM the abovecalculations take account of the use of foamwith its ability to reduce torque by up to 50%.This is a considerable differentiator betweenthe two types. However either type can be

designed with sufficient torque.

Environmental ConsiderationsThe main environmental consideration is theeffect of conditioners or bentonite slurry onthe spoil disposal areas.

STMs, since they were first introduced in the1960s, have always used a bentonitic slurry asa transportation medium, sometimessupplemented by the use of polymers. AnEPBM today uses a surfactant foam usuallysupplemented by polymers or bentonite.These are occasionally supplemented by a re-constituted material such as pulverisedlimestone in areas of very few fines in theground. These different materials are a definitedifferentiator between the two types. For an

STM it is possible, if itis necessary, to cleanout any residualcontaminants using asand washing plant.For an EPBM this isnot so easy. However,most of theconditioners are bio-degradable and at theworst it may be

necessary to lay out thematerials in thin layers for aweek or two to aid the bio-degrading process. Insummary it is necessary totake into account the effecton the environment of thevarious materials used witheach type.

LogisticsTunnel logistics are greatlyimproved by using an STM,due to the spoil beingremoved with the slurry in theoutbound pipeline. Apartfrom reducing total traffic thisimproves the quality of air inthe tunnel by the consequentreduction in diesel fumes.There is also a consequentimprovement in personnelsafety due to the reduction intraffic.

The same logisticalimprovement does not applyto EPBMs. However,nowadays it has becomecommon practice to use longtunnel conveyors whichprovide the sameimprovement in logisticstogether with the otherconsequential effects.

Logistics would not be afactor in deciding which typeof closed face TBM to use.However the logistical effectsmust be born in mind when

taking the decision.

SummaryThe main criteria in deciding between an STMand an EPBM are ground related. None of thefactors described above can be examined inisolation. As stated the main criteria is theinformation contained in the PSD curves. Butthis information must be examined togetherwith the ground permeability and thehydrostatic head and the distribution of fineswithin the whole length of the tunnel. It isessential to consider the ability of the chosenTBM to control settlement within acceptablelimits.

In coming to a decision the other factors toconsider are the presence of boulders, therequirements for adequate torque and thrust,the effect on the environment and also thetunnel logistics. But none of these additionalfactors will have a strong influence on thechoice.

Having made the decision, thought mustthen be given to the various facilities thatshould be built into the TBM to ensure that itis capable of dealing with the knownconditions and also possible unknownconditions that may arise.

SLURRY OR EPBM?


Tunnel logistics with a slurry system

Table 1: Percentage Volume Loss (PVL)

CTRL EPBMs: 30.4km –average 0.6%St Clair River EPBM: 1.87km <1%SMART Slurry: 9.7km <0.75%Dockland Light Rail Slurry: <1.0%Toronto Subway EPBM: 8.8km <0.8%Kowloon Southern Link EPBM: <0.5%Warrington Sewer Slurry:1.4km 1.37%Heathrow T5 EPBM/CA: average 0.5%Seattle CSO EPBM: 1.89km <0.5%


slurry or epbm

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