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Conference Paper

Some remarks concerning EPB ans slurry shields

Author(s): Anagnostou, Georg

Publication Date: 2008

Permanent Link: https://doi.org/10.3929/ethz-a-010819279

Rights / License: In Copyright - Non-Commercial Use Permitted

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1. INTRODUCTION

Traffic congestion and environmental con-straints intensify the utilization of undergroundspaces in urban areas. Construction projects arebeing carried out more and more frequentlyunder difficult conditions: several constraintsand potential conflicts associated with nearbyinfrastructure or other uses of space; highpotential third imparty impact; unfavourablegeology involving soft ground or mixed faceconditions.

The increasing demand for urbanunderground space has promoted technologicalprogress in the last decade, particularly in thefield of mechanised tunneling. The constructionmethods used in shallow soft ground tunnelingbeneath the water table must ensure control ofthe ground at the tunnel heading and,additionally, prevent seepage flow towards theworking face. Slurry and EPB shields fulfillthese goals for a wide range of geological andhydrogeological conditions by providingsupport to the tunnel face continuously duringexcavation. Modern closed shields limit theneed for costly and time-consuming groundimprovement measures such as grouting orartificial ground freezing.

Technological developments have decreasedconstruction times and costs and this, in turn,

has raised demand. It is interesting to note,furthermore, that the development of largediameter closed shields (today's largestmachines exceed 15 m in diameter) does notchange only the way structures are constructedbut it also affects the layout of the structuresitself. For example, the rail tunnel in Figure 1incorporates the platform level functionsthereby reducing construction interferencesbetween tunnels and stations.

As shown by recent successful projects,improvements in soil conditioning methods andin the technology of additives have extended thetraditional range of applicability for closedshields (typically, fine-grained soils for EPBshields, coarse-grained soils for slurry shields).This has led to the impression that the ground isin general not the key parameter for machine

Figure 1. Platform level (Barcelona Metro)

ABSTRACT: The paper discusses the issue of shield tunneling under suboptimal geological conditions from ageotechnical and a contractual perspective. The increasing demand for urban underground spaces has promotedconsiderable technological progress in the field of mechanised tunneling. On the other hand, closed shields areapplied more and more frequently under geological conditions that are beyond their optimum operational range.In order to reduce the risk to an acceptable level, additional measures such as ground improvement are oftennecessary. For ensuring tender price comparability, the cost of these measures should be estimated in the pre-tender design and be included - separately for the different shield types - in the bill of quantities.

Some remarks concerning EPB and slurry shields

G. AnagnostouETH Zurich, Switzerland

Development of Urban Areas and Geotechnical Engineering16-19 June, 2008, St. Petersburg

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Development of Urban Areas and Geotechnical Engineering16-19 June, 2008, St. Petersburg

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Figure 2. Face support: (a) EPB shield; (b) slurryshield

type selection - an attitude cultivated sometimesby the TBM industry itself. On the other hand,the set-backs experienced in some large projectsunderline the need for a careful evaluation ofthe pros and cons of the various face supporttypes for a given geotechnical environment.

2. GEOTECHNICAL CONSIDERATIONS

The importance of the geology is clearly re-flected in the recommendations from majorprofessional organisations (e.g., AFTES 2000,BTS 2005, DAUB et al 1997 & 2000). Theevaluation of the different closed shield typestypes is particularly demanding when thegeology changes frequently along the alignment(thereby necessitating frequent changes ofoperation mode) or when the ground conditionsare beyond the optimum operational ranges bothof EPB and slurry shields over long portions ofthe alignment. Closed-shield operation undersub-optimal ground conditions involves a higherprobability of failure or of inadmissible settle-ment. In many cases additional measures suchas ground improvement will be necessary inorder to reduce the risk to an acceptable level.Urban environments are particularly adverse inthis respect, as the lack of free spaces at thesurface may limit the feasibility of any addi-tional measures or increase their cost considera-bly. For a concise discussion of the importanceof systematic risk management in the context ofurban tunneling, the reader is referred to Kováriand Ramoni (2006).

A detailed discussion of the mechanics offace support with closed shields is given inAnagnostou and Kovári (1994, 1996). Thestability of the tunnel face and the deformationsof the ground in EPB tunneling depend forgiven ground conditions on the face supportpressure. In a fine-grained, low-permeabilityground (the typical soil for EPB application),the face support pressure is equal to the totalstress prevailing in the muck within the workingchamber. In the case of a coarse-grained muck(e.g. consisting of the chips generated whentunnelling through weak rock), the groundresponse is controlled by the joint effect of theeffective stress s' acting on the tunnel face andof the pore pressure p in the working chamber(Fig. 2a). A high pore pressure p reduces themagnitude of the destabilizing seepage forcesacting within the ground towards the face and isfavourable also regarding surface settlementbecause it limits pore pressure relief andconsolidation of the ground. Consequently, thehigher the pore pressure p , the lower thenecessary effective support pressure s', and viceversa. However, these two parameters aredifficult to control in practice as they depend onthe characteristics of the excavated ground, theway the ground is mixed in the work chamber,the rotational speed of the screw conveyor andthe excavation advance rate. So, the groundresponse to tunneling by an EPB depends to alarge degree on the complex interplay ofgeotechnical and operational factors.

With the exception of extremely coarse andpoorly graded gravel soils (Fig. 3), slurryshields provide a support pressure which isindependent on the nature of the ground.Contrary to the situation with EPB shields, facesupport is determined by only one parameter(the slurry-pressure, Fig. 2b) and, moreover,this parameter can be regulated directly. AsEPB shields support the face with the excavatedmuck itself, the geology is decisive not only forthe shear strength and stiffness of the groundahead of the face but also for the spatialdistribution and the fluctuation over time of thepressure within the working chamber. EPBshields are thus more sensitive to deviationsfrom the optimum ground conditions than slurryshields and will likely require more additionalmeasures than slurry shields in the case of avariable geology. In addition, EPB shields are

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more prone to wear, thereby necessitating morefrequent maintenance in the working chamber.Safe access to the cutter head may also requireadditional measures depending on the lengths ofthe drives and the spacing of shafts.

On the other hand, a comparativeassessment should take into account, that theconsequences of a face failure in the case of aslurry shield will likely be more serious thanwith an EPB shield, because of the largervolume of the ground that can enteruncontrollably into the working chamberthrough displacement of the slurry. Particularattention in this respect deserves theunderground of old cities as it involves a higherprobability to encounter unidentified cavitiessuch as aqueducts or ancient wells duringexcavation (Fig. 4). When encountering suchcavities, the slurry pressure drops to that of theground water and, if the ground is lackssufficient cohesion, the face collapses.

It should be noted, however, that a slurrypressure in excess to the ground water pressureis required for maintaining stability only fortunneling through granular or very low cohesionground. If the ground has some strength (e.g.,weathered but not completely decomposedweak rock), hydraulic equilibrium betweenchamber and ground suffices for stability (Fig.5). An excess pressure may be, nevertheless,still necessary in order to reduce settlementdepending on the stiffness of the ground and onthe sensitivity of the buildings. Else the slurryshield might be operated even withoutbentonite, i.e. only with pressurized water.

Figure 4. Collapse of an old well filled by pottery(Athens Metro)

Figure 3. De-stabilization due to deep slurry infiltra-tion into an extremely coarse ground (Anagnostouand Kovari 1994)

Figure 5. Necessary slurry excess pressure !p at limitequilibrium as a function of the cohesion c ("' = 11kN/m3, "slurry = "water, computational method afterAnagnostou and Kovári, 1994)

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3. CONTRACTUAL CONSIDERATIONS

In many geotechnical situations, both EPB andslurry-shield tunneling may be feasible but incombination with additional measures (Fig. 6).The locations and the quantities of theadditional measures should be evaluatedseparately for each type of face support, as oneand the same ground may be favourable for theone mode of operation but unfavourable for theother.

A rigorous and unbiased evaluation of theadditional measures is very important,particularly when considering that, for non-geotechnical, but nevertheless importantreasons, EPB shields are often, almost bydefault, the first choice: Slurry-shield tunnelingin urban environments is generally problematic,due to the limited space available at the surfacefor separation plants or muck disposal. This, incombination with the lower capital costs and thesimpler operation of EPB shields creates a biasin favour of the latter, although slurry shieldsallow for a better control of the pressure in theworking chamber under mixed face conditionsor when tunneling through weak rock.

The machine selection should take intoaccount, besides the capital and operationalcosts, also the costs and the time needed for theadditional measures. Estimates of theappropriate quantities for complex geologicalformations are fraught with uncertainties.Procurement methods such as design-build with

Figure 6. Execution of grouting (Kovári and Ramoni,2006)

a lump sum do shift a considerable portion ofthe geological risk to the contractors, andexperience from a number of projects showsthem to be unsuitable: they may lead to highcontingencies in the bids or, as happens moreoften, to project delays or poor quality workincluding unacceptable settlements or evencollapses. The limited time available for bidpreparation, in combination with the quiteunderstandable calculated optimism of thetender phase, do not allow for a cautious as-sessment of the necessary additional measures.

4. CLOSING REMARKS

Leaving the decision about the machine type tothe market is undoubtedly advantageous.Nevertheless, the quantities of additionalmeasures should be estimated by the owner orhis engineers in the pre-tender design and beincluded - separately for the different types offace support or modes of operation - in thetender documents or in the bill of quantities.Failing this, tender price comparability will belost.

5. REFERENCES

AFTES, French Tunnelling Society, 2000. Choosingmechanized tunnelling techniques. Revue Tunnels

& Ouvrages Souterrains, 157.Anagnostou, G. & Kovári, K. 1994. The face stability

of slurry-shield driven tunnels. Tunnelling and

Underground Space Technology, 9 (2), 165-174.Anagnostou, G. & Kovári, K. 1996. Face stability

conditions with Earth Pressure Balanced shields.Tunnelling and Underground Space Technology,11 (2), 165-173.

BTS, British Tunnelling Society, 2005. Closed-Face

Tunnelling Machines and Ground Stability.Thomas Telford Ltd.

DAUB, ÖGG & FGU 1997. Recommendations forselecting and evaluating tunnel boring machines.Tunnel, 5/97, 20-35.

DAUB, ÖGG & FGU 2000. Recommendations forDesign and Operation of Shield Machines. Tun-

nel, 6/00, 54-76.Kovári K. & Ramoni, M. 2006. Urban tunnelling in

soft ground using TBMs. Int. Conf. & exhibition

on tunnelling and trenchless technology, SubangJaya – Selangor Darul Ehsan; 17-31; The Institu-tion of Engineers, Malaysia.

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