Underground Space Use: Analysis of the Past and Lessons for the Future – Erdem & Solak (eds)© 2005 Taylor & Francis Group, London, ISBN 04 1537 452 9

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1 INTRODUCTION

The Botlek Rail Tunnel was the second bored tunnelin The Netherlands and the first that was completedusing an EPB shield. A considerable part of the tunnelwas bored through Pleistocene sand under a highwater table, see Figure 1. The water table was up to 23 mabove the tunnel axis. Foam was used to stabilize thetunnel face.

The support pressure acting at the tunnel face can beseen as a combination of an absolute pressure and thevertical pressure gradient. In an EPB-shield, the aver-age pressure is controlled by the screw conveyer andvalves; the vertical pressure gradient is determined by a

combination of the excavation process and materialproperties and cannot be controlled. Yet this gradient isof importance because it determines the pressure at thecrown of the tunnel for a given pressure at the axis. Thepressure gradient at the tunnel face of a slurry shield isdetermined by density of the slurry at the tunnel face.For an Earth Pressure Balance shield there appear to bemore mechanisms that determine the gradient.

The paper deals with the pressure gradients meas-ured, the densities of the mixture samples taken andpossible mechanism that determine the gradients found.

2 MEASUREMENTS

2.1 Measurements performed

The following measurements where performed at thetunnel face: The density of the muck was measured bysampling the muck through a stopcock in the pressurebulkhead. The total pressure was measured at thebulkhead at 9 locations and the pore water pressure at 3, see Figure 2. The pore pressure gauges could becleaned by a small water jet to avoid blocking of thefilter. The status of the boring process was monitoredas well as the amount of injected foam.

Pressure gradients at the tunnel face of an Earth Pressure Balance shield

A. BezuijenGeoDelft, Delft, Netherlands

J.F.W. JoustraTunnel Engineering Consultants (TEC)/Witteveen � Bos

A.M. TalmonWL | Delft Hydraulics, Delft, Netherlands

B. GroteT & E Consult

ABSTRACT: Measurements are presented that were performed in the pressure chamber of an EPB TBM. TheTBM operated in saturated sand with a high water table. Foam was applied for soil conditioning. Different froma slurry shield there was no direct relation between vertical the pressure gradient and the density of the sandwater foam mixture in the pressure chamber. The pressure measurements differ considerably for different rings.The density of the mixture was measured by taking samples from the pressure chamber during excavation.Densities found indicate a porosity higher than the maximum porosity of the sand. Horizontal pressure gradi-ents were measured during drilling depending on the rotation direction of the cutter head. A possible explana-tion is presented in the paper.

sand sand

peat clay with humusor peat

sand sand

river

318813

30m

bored section 1830 mclay, siltywith sand

middle to coarse sandwith gravel

Figure 1. Geotechnical profile. The numbers indicate theapproximate positions of rings, see further text.

2.2 Density measurements

In order tot determine the exact composition of themuck inside the excavation chamber a sampling devicewas developed. This device simply consisted of a pieceof pipe with some valves which was directly attachedto the pressure bulkhead. By carefully opening thevalves, some muck was allowed to flow out from thepressured excavation chamber through the samplingdevice. By subsequently closing the valves an exactlyknown volume of muck was extracted which wouldbe analysed in a laboratory. In this way, the porosityand water content of the mixture could be determined,which could be converted tot the relative volumes of the different phases of the mixture (solid, liquidand gas).

A total of more than 20 samples of muck have beenextracted from the centre of the excavation chamberat 6 different locations along the tunnel alignment.Although muck with a relatively high consistencyproved problematic to collect because of its inabilityto flow easily, samples taken in a short time span werevery comparable.

2.3 Pressure measurements

2.3.1 Pressures and vertical gradientsPressures were measured during excavation of theBotlek Rail Tunnel at various locations at the pressurebulkhead; see for an example Figure 3. The verticalpressure gradients determined from these pressures areshown in the upper plot of Figure 3. (all figures withmeasurements present the ring number in the captionthat was drilled during the measurement. S is thesouth tunnel, N the north). It shows that the gradient

can be high during excavation but decreases whenexcavation stops (shown with the status, 2 means exca-vation, all other values mean stand still, see Table 1).The values measured for the vertical gradient can beput in perspective realizing that the gradient of thetotal vertical pressure in the not yet excavated soil isapproximately 20 kPa/m, the density of the foam-water-soil mixture is approximately 13 kPa/m and thepore pressure has a gradient of 10 kPa/m. The meas-urements showed that the measured gradients canhigher than 20 kPa/m, but also lower than 10 kPa/m.The vertical gradient decreases during ring building.

Another way to present the pressure measurementsfor the same ring is shown in Figure 4. It shows thepressures from gauges E1 until E5 for different timesteps. It can be seen that there is a more or less linearincrease with pressure apart form the lowest measure-ment position (E5) with various gradients. The pres-sure decreases at the position of E5, because thisgauge is close to the screw conveyer, where the mixture is removed from the pressure chamber.

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Figure 2. Position of instruments in the TBM looking fromthe tunnel to the TBM and definition of rotation.

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Figure 3. Ring 318 N. Pressures measured at the bulkhead,status of the TBM and the gradient determined from thepressures. Only a part of the pressure readings is shown, butall readings (E1 – E9) are used to calculate the gradient.

Table 1. Meaning of the status bit.

Status Meaning

1 Temparary stop during excavation2 Excavation3 End of excavation phase4 Start of ring building phase5 Actual ring erection6 Pause after ring erection

2.3.2 Effective stressPore pressure gauges were installed near some totalpressure gauges, see Figure 2 to determine the effec-tive stress in the soil water foam mixture. Results ofboth total pressure gauges and pore pressure gaugesare shown in Figure 5. It is clear that both pressuresare comparable, which means that the effective stressin the mixture is negligible. W3 did not function andtherefore could not be compared with E5.

Figure 5 shows the pressures measured for onlyone ring. The conclusion that there is hardly anyeffective stress in the mixture is a more general onefor this boring (Joustra, 2002). Although there aresome indications that there is an effective stress neartransducers E4 and E6 during the drilling of somerings, as will be dealt with later in this paper. The con-clusion that there is hardly any effective stress is alsoconfirmed by the density measurements performed. Itwas found that in most cases the porosity of the sam-ples was above the maximum porosity. A porositylower than the maximum porosity is necessary tohave a grain skeleton and effective stresses.

2.3.3 Horizontal pressure differencesThe measurements show for some rings remarkablehorizontal pressure differences depending on thedirection of rotation of the cutter head, see Figure 6 andFigure 7. Pressures varied 1 bar and more dependingon the direction of rotation. The pressure variations

are most significant for pressure gauges E4 and E6,less for the other gauges and more or less disappearedat the top of the tunnel, see Figure 7.

Pressure variations differ for different rings. Mostrings have only small variations, see Figure 8. Also herepressures varied depending on the rotation direction,but the variation was so small that it could not benoticed in the original data, see Figure 3. By taking amoving average over 7 measurements to reduce thenoise it is possible to see a variation of approximately0.25 bar.

2.3.4 Variations in pressureComparing the results of Ring 318 N with those of Ring813 S, it appeared that there is a considerable differencein the results of these rings. Compare Figure 6 withFigure 8 and Figure 4 with Figure 7. Measurements

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Figure 4. Ring 318 N. Pressure distribution along thegauges E1 until E5 for various time steps.

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Figure 5. Ring 318 N. Total pressures and pore pressurescompared.

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Figure 6. Ring 813 S. Pressures measured right (E4) andleft (E6) of the TBM axis compared with the rotation of the cutter head, see also definition in Figure 2.

from other rings showed also such differences(Joustra, 2002); although for quite a number of ringsthe soil conditions are comparable. The excavationprocess showed al lot of variability, which leads to theassumption that there is still a considerable margin ofoptimisation left with respect to the control of thisprocess. The process is sensitive to pressure variations,because by the air involved these variations influencethe volume in the mixture chamber and the perform-ance of the screw conveyer. This was also found inlaboratory experiments simulating the EPB process

(Bezuijen & Schaminée, 2001). A more constant pres-sure may lead to a smoother drilling process with lessstops. Now it can be seen from Figure 3, the status,that there are sometimes a lot of stops during drilling.

3 DISCUSSION

3.1 Vertical gradient

In a slurry shield TBM the vertical gradient found inthe pressure chamber was equal to the gradient thatcorresponds with the density of the slurry (Bakker et al., 2003). In the predictions made for these measure-ments it was assumed that this was also the case for anEPB shield. However, this was not found for this tun-nel. Pressure gradients varied during drilling and thisvariation is much larger than can be expected fromvariations in densities. Measured vertical gradientsare sometimes larger than 20 kPa/m. Assuming thatsuch a gradient is a good indication for the densitywould mean that there would be rather dense saturatedsand in the pressure chamber, which is quite unlikely.

From the results it was concluded that also theyield stress of the soil has an influence on the pres-sure gradient. This can also be seen from the pressuredrop that is present at E5 compared to the pressuresE4 and E6, see Figure 4 and Figure 7. At the lower endof the tunnel, the screw conveyer removes the mixturefrom the pressure chamber and due to the yieldstrength of this mixture this leads to a pressure drop.

Assume a layer of sand-water-foam mixture betweenthe cutter head and the pressure bulkhead of the pres-sure chamber at L metre apart. The adhesion betweenthe cutter head or the pressure bulkhead and the mix-ture is �a and the density of the mixture is �m. In caseof vertical flow, equilibrium of forces leads to the following equation:

(1)

Depending on the flow direction the pressure gradientcan be 2�a /L higher or lower than the pressure gradi-ent corresponding to the density of the mixture. Incase of a flow with a horizontal component as can beexpected in the pressure chamber between E6 and E5as well as between E4 and E5, the influence of theadhesion becomes even bigger.

There is no direct field data from the shear strengthof the mixture in the pressure chamber, but frommeasurements in the laboratory it was found that thisshear strength is one to a few kPa. This means thatwith an average density of the mixture of approxi-mately 1500 kg/m3, the pressure gradient can vary aswas measured depending on the direction of the flow.Lower values of the shear stress of 0.2 to 0.6 kPa were

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Figure 7. Ring 813 S. Distribution of pressures when thecutter head is rotating to the left (negative values) at 0:26:02and to the right at 0:40:03.

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Figure 8. Ring 318 N. Pressures measured right (E4) andleft (E6) of the TBM axis compared with the rotation of thecutter head, see also definition in Figure 2 (lines smoothed,see text).

found from back calculation of the performance ofthe screw conveyer (Talmon & Bezuijen 2002).However these values are likely to be lower than in thepressure chamber due to the pressure relief in thescrew conveyer that leads to a higher porosity.

From a physical point of view the concept of adhesion as described here is probalby too simple.Adhesion on flat iron surfaces can be less and theroughness, combined with the conhesion results in anapparent adhesion. However, also this apparent adhe-sion will result in the pressure gradient variationsdescribed here.

After the actual drilling, during ring building, thevertical pressure gradient decreases. Foam injectionsexecuted during ring building are necessary to keep astable boring face, but also lead to a decrease in theaverage density of the mixture. The pressure loss iscaused by ground water flow from the tunnel face tothe ground water caused by the excess pore pressurein the chamber (see also Bezuijen et al., 2002 andBezuijen, 2002). The water that flows out is compen-sated with foam that contains only about 10% waterand 90% air.

Consequence of the relatively low measured verticalgradient during most of the drilling cycle, comparedto the gradient that corresponds with the density ofthe soil, is that the pressure gradient is lower than thesoil pressure gradient. It is sometimes mentioned asan advantage of the EPB shield that there is a better‘match’ with the pressure gradient in the soil whencompared with a slurry shield. For the ground condi-tions encountered here this is not the case. The rela-tively high permeability sand (k is 5.8 10�6m/s whenRing 318 N was drilled 3.10�4m/s during drilling ofRing 813 S) leads to expelling of water from the mix-ture. This reduces the density of the mixture. Thiswith the influence of the yield stress that leads to afurther reduction of the vertical gradient results inpressure gradients that are significantly lower thanmeasured in a slurry shield (Bakker et al., 2003).

3.2 Horizontal gradient

Some horizontal gradient can also be explained by theyield stress of the air-sand-water mixture, assumingthat the pressure chamber is not completely filled.The cutter head pushes the mixture in one directionand to get a stable situation it has to flow back.However, it can only flow back if there is a pressuredifference large enough to overcome the yield stress.This pressure difference is created because there ismore mixture on one side of the TBM than on theother side, see Figure 9.

The figure shows a possible pressure distributionon both sides of the axis (the centre part with a radiusof 5 m) assuming that there is more mixture on theright side due to the rotation of the cutter head.

Assuming a stable situation, the pressure has to be thesame on both sides at 180 degrees (the lowest point ofthe mixture, but is higher on the right hand side forother positions, because the mixture is higher on thatside. The difference between both pressures is higheston the top level of the mixture on the left hand side.

A pressure distribution as shown in Figure 9implicitly assumes that the pressure chamber is onlypartly filled with sand water foam mixture. This wasmeasured for some rings (the pressure gauges on topof the TBM showed the same pressures) but not for alland not for Ring 813 s, see Figure 7.

The mechanism sketched above can explain somehorizontal gradient, but not the approximately 100 kPathat was measured during the drilling of Ring 813 S.Assuming again a density of the mixture of 1500 kg/m3,such a pressure difference corresponds to a differentin height of at least 6.6 m, which is quite unlikelybecause that is more than halve of the height of thepressure chamber. Looking at the pressure gradientmeasurements, shown in Figure 7, there is no indica-tion that there is an empty pressure chamber, overseveral metres from the top of the TBM. An emptypressure chamber would mean that the pressure remainsconstant, which is not the case.

A possible explanation for the high horizontalpressure gradient is the effective stress that can occurin the mixture chamber, see Figure 10. This Figure isthe same as Figure 6, but the reading of pressure gaugeE5 and pore pressure gauge W3 are added. FromFigure 10 it is clear that the variation in the pore pres-sure, measured with W3, is much less than the varia-tion in the total stress transducers E4 and E6 (thepressure fluctuations on the pore pressure gauge afterdrilling (rotation is zero), are caused by the cleaning

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Figure 9. Sketch of possible pressure distribution due to rota-tion of cutter head without effective stress, see further text.

system on the pore pressure transducer). E4 is moreor less equal to the pore pressure transducer for nega-tive rotation (turning to the left) and E6 is equal to thepore pressure transducer when the cutter head is turn-ing in the opposite direction. In both cases the otherpressure transducer is approximately 1 bar (100 kPa)above the pore water pressure.

When the cutter head is rotating in a way that themixture is transported from screw conveyer, the mix-ture becomes more or less squeezed between the cut-ter head and the bulk head. This resulted locally in ahigh pressure. The pressures are lower at higher levelsin the pressure chamber and close to the screw con-veyer (see the reading of E5 in Figure 10). No effec-tive stress was measured at these locations.

4 CONCLUSIONS

From the study described in this paper we came to thefollowing conclusions:

– While boring in sand with an EPB shield an impor-tant function of the foam is to increase the porosityof the sand to such a value that deformation is pos-sible with no or only limited grain stresses. This isdifferent from boring in clay where the lubricatingis more important (Mair et al., 2003).

– Significant differences between tunnel rings incomparable geology of the pressure measurementsand other process parameters suggest that the reg-ulation of the drilling process has some potentialfor optimisation. A better regulation can possiblyimprove the drilling process.

– The vertical pressure gradient is only to a certainextend influenced by the density of the mixture.Yield stress of the mixture also seems to have aninfluence.

– The vertical pressure gradient can be lower for aEPB using foam than for a slurry shield.

– Horizontal pressure gradients can be quite consid-erable over the tunnel face. These may be causedby compression of the sand grains in the mixtureleading to effective stresses in some parts of thepressure chamber.

ACKNOWLEDGEMENTS

The laboratory tests were commissioned and supervisedby BTL (Research foundation: Horizontal Drilling &Tunnelling) and COB (Centre for UndergroundConstruction). The authors wish to thank these foun-dations for their permission to publish the results. Thedata on the Botlek Rail Tunnel presented here arederived from the COB-pilot project F300 in which theauthors participate. The authors are very grateful toboth COB and the builder of the tunnel BTC Botlekvof, with the granted opportunity to publish.

REFERENCES

Bakker K.J., Teunissen E.A.H., Berg P. van den, Smits M. Th. J.H., 2003, K100 research at the secondHeinenoord tunnel. Proc. ITA 2003, Amsterdam,

Bezuijen A., Pruiksma J.P., Meerten H.H. van 2001. Porepressures in front of tunnel, measurements, calculationsand consequences for stability of tunnel face. Proc. Int.Symp. on Modern Tunneling Science and Techn. Kyoto.

Bezuijen, A., & P.E.L. Schaminée, 2001, Simulation of theEPB-shield TBM in model tests with foam as additive,Proc. Int. Symp. on Modern Tunneling Science and Techn.Kyoto.

Bezuijen A. Influence of soil permeability on the propertiesof a foam mixture in a TBM. 3rd Int. Symp. on Geotch.Aspects of Underground Construction in Soft Ground, IS-Toulouse.

Joustra, J.F.W., 2002, Additional data analysis independentresearch project Botlek Rail Tunnel, the ‘EPB-process’(in Dutch), Delft University of Technology report.2001.BT.5566.

Mair, R.J., Merrit, A.S., Borghi, X., Yamazaki, H., andMinami T., 2003. Soil conditioning of Clay Soils.Tunnels and Tunnelling International, April.

Talmon A.M. & A. Bezuijen, 2002, Muck discharge by thescrew conveyor of an EPB Tunnel Boring Machine, 3rd Int.Symp. on Geotch. Aspects of Underground Constructionin Soft Ground, IS-Toulouse.

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Figure 10. Ring 813 s. Pore pressure measured at W3 compared with the pressures measured at E4, E6 and E5 andthe rotation of the cutter head. Variations in W3 after 1:00are caused by the cleaning mechanism on the gauge.