World Tunnel Congress 2013 GenevaUnderground – the way to the future! G. Anagnostou & H. Ehrbar (eds)

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Performance of penetration models for hard rock TBMs in the case of the Gotthard Base Tunnel

J. Cheda(1), R. Schuerch(1), P. Perazzelli(1), F. Mezger(1) (1)ETH Zurich, Switzerland

ABSTRACT: In the last 40 years many models were developed to estimate the field penetration. The models are empirical and (most of them) result from a statistical regression analysis of the observed field penetration, geological conditions, TBM characteristics and operational conditions. Due to the empirical nature of the models, which are largely based on specific site conditions, the estimation of the penetration often diverges from the one measured in the field. The present paper gives an overview of the existing penetration models for hard rock TBMs, identifies the most frequently used input parameters and summarizes the data on which the models are based on. Furthermore, the paper compares the field penetration values achieved during the excavation of a section of the Gotthard Base Tunnel with the penetration estimated with these models. The paper shows, that for the considered case the estimation of the penetration is reasonably accurate when applying models that are based on a database, which is consistent with the project data.

1 Introduction

The gross advance rate of a TBM depends also on the TBM penetration. The penetration is a function of the geological conditions and of the technical characteristics of the TBM. An accurate assessment of the penetration in the tender phase of a project contributes to a more reliable construction schedule. In the last decades many authors developed empirical models for the prediction of the TBM penetration in hard rock.

The aim of this paper is to evaluate the performance of the most widely known penetration models for hard rock TBMs by comparing the model predictions with the penetration achieved during excavation of a section of the Gotthard Base Tunnel. More specifically, the paper focuses on a 578 m tunnel section of the western tube between Faido and Sedrun, which was excavated in October 2008.

The first part of the paper briefly summarizes the geological conditions encountered during excavation, the technical specifications of the TBM and the TBM data recorded during excavation.

The second part of the paper gives a concise overview of the considered penetration models, depicting the main input parameters and the databases upon which the models are based on. Finally, it compares the predicted penetration values with the one measured in the field. The present study is fundamentally different from any pre-construction estimations with respect to the input parameters. In the pre-construction phase the geological conditions and the required thrust force have to be estimated, so that they can considerably diverge from the actual ones. On the contrary, the present study is based on geotechnical data collected during excavation and it considers the thrust force measured during advance of the TBM.

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Figure 1. Part of the geological profile of the Gotthard Base Tunnel between Faido and Sedrun and considered tunnel section (Klose, 2003)

Figure 2. Geological models of the considered tunnel section elaborated during the excavation (AlpTransit Gotthard AG, 2008a)

2 Project data

2.1 Geology

The considered tunnel section is located between Faido and Sedrun. The tunnel was excavated in the Lucomagno gneiss, which is part of the penninic gneiss nappe (Fig. 1).

Figure 2 illustrates the geological conditions encountered during excavation (based upon the daily geological reports). The encountered geology was predominantly characterised by medium to very

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coarse gneiss with variable quartz content (AlpTransit Gotthard AG 2008a). Figure 2 shows, that the tunnel crossed a 3-4 m wide fault breccia approximately at chainage 280.

The rock mass was intact and dry with the exception of the fault zone, where wet material was observed. In some regions minor isolated water inflows could be observed (AlpTransit Gotthard AG 2008a).

In general, the folding was steeply inclined. The strike direction of the folding forms an angle of about 45° with the tunnel axis. Two major joint sets were observed during construction. The joint sets were characterized by a persistency of 2 to 4 m and a spacing of 0.1 to 1.5 m. In the last part of the considered tunnel section the spacing of the joints decreased from 0.1 to 1.0 m. The joints were inclined almost vertically and were oriented in a subparallel or perpendicular manner to the tunnel alignment, respectively.

Table 1 shows the results of the laboratory tests performed during tunnel advance. The uniaxial compressive strength of the rock mass varies between 50 and 100 MPa (average 80 MPa), while the tensile strength varies from 7 to 13 MPa (average 11 MPa).

2.2 TBM technical data

The western tube of the Gotthard Base Tunnel was excavated by a gripper TBM with a diameter of 9.43 m. Table 2 summarizes the technical data of the hard rock TBM.

Table 1. Rock parameters according to laboratory tests (AlpTransit Gotthard AG, 2008b)

Test (chainage)

SKBW-20 (9)

SKBW-21 (85)

SKBW-22 (198)

SKBW-23 (297)

SKBW-24 (399)

SKBW-25 (503)

UCS [MPa] 51.8 78.9 97 70.8 - 66.2

BTS [MPa] 12.3 7.7 11.8 12 - -

Rock types

Gneiss [%] 100 100 100 100 80 60

Amphibolite [%] 20 40

Table 2. TBM technical data (AlpTransit Gotthard AG, 2010)

Model and type Herrenknecht 2 – 210, gripper TBM

Diameter cutterhead [m] 9.43

Torque (max) [MNm] 6

Thrust force (max) [MN] 20

Power (max) [kW] 3500

Revolution (max) [rpm] 6

Cutter specifications

No. Cutter (Center, gauge) [ - ] 66 (54, 12)

Cutter diameter [Inc] 17’’ (432 mm)

Cutter spacing [mm] 100

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2.3 Field data

Figures 3 and 4 present the evolution of the total thrust and of the penetration along the considered tunnel section. The total thrust varies between 7.5 and 21 MN with an average value of 14.6 MN. By considering the number of cutters and by subtracting the frictional forces due to the weight of the TBM, the average thrust force corresponds to a normal disc force per cutter of 237 kN (i.e. boring force per cutter).

Figure 4 shows that the field penetration varies between 7 and 14 mm/revolution and the average penetration amount to 10.5 mm/ revolution.

Since both the disc normal force and the penetration vary alongside the tunnel, their ratio represents a measure of the resistance of the ground to the mechanical diminution. This ratio is the so-called field penetration index FPI. Figure 5 shows the evolution of the FPI over the considered tunnel section. In the first part of the tunnel the boreability can be classified as medium to high and becomes very high in the last part of the section. This observation is in accordance with the geological description of the rock mass (cf. Section 2.1). The smaller spacing between the joints generally leads to a better boreability of the rock mass.

3 Performance of penetration models

3.1 Overview of the penetration models

For the estimation of the TBM penetration in hard rock, 19 prediction models were considered (cf. Figure 6). The older models (up to mid-80s) are semi-empirical and were derived from linear cutting tests. The most recent models were developed based on field data of TBM excavations in different ground conditions. More specifically, they result from a statistical regression analysis of the observed field penetration, geological conditions, TBM operational conditions and characteristics of the TBM.

According to Cheda (2013), the most frequent input parameters used in the considered models are: the uniaxial compressive strength of the intact rock (used by 70% of the models), the distance and the orientation of the discontinuities (used by 50 % of the models), the assumed thrust per cutter (used by 40 % of the models) and the cutter diameter (used by 30 % of the models).

Figure 6 summarizes the conditions on which the analyzed models are based on. More specifically, the figure indicates the range of the uniaxial compressive strength as well as the cutterhead and cutter diameter, which were taken into account in the formulation of the models. The figure shows the models which consider discontinuities of the rock mass.

The figure indicates that the uniaxial compressive strength considered in the models is generally higher than 100 MPa and that the discontinuities of the rock mass are taken into account only by the most recent models. Due to the technological improvements, the diameter of the cutterhead and of the cutter increased over the last decades (c.f. Fig. 6).

3.2 Performance of the penetration models

Figure 7 shows the penetration calculated by applying the considered models. The estimation is based on the average rock mass parameters measured during construction (cf. Section 2.1), the actual TBM characteristics (cf. Section 2.2) and the average measured force per cutter of 237 kN (Section 2.3).

The first column in Figure 7 shows with crosses the predicted penetration for all models. The second column considers only those models, which were developed based upon parameters (uniaxial compressive strength, cutterhead diameter and cutter diameter) deviating less than 100% from the ones of the considered project. The third column considers only models that are based upon conditions which are even closer to those of our project (50% difference).

All prediction models lead penetration values that are relatively close to the average field penetration (dashed line in Fig. 7).

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0

5

10

15

20

25

0 100 200 300 400 500

To

tal

thru

st [

MN

]

Chainage [m]

Average

Figure 3. Total thrust (AlpTranist Gotthard AG, 2008c)

0

2

4

6

8

10

12

14

16

0 100 200 300 400 500

Pe

ne

trat

ion

[mm

/rev

]

Chainage [m]

Average

Figure 4. Field penetration (AlpTranist Gotthard AG, 2008c)

0

10

20

30

40

50

60

0 100 200 300 400 500

FP

I [k

N/m

m/r

ev]

Chainage [m]

Low boreability

Medium

Very high

High

Average

Figure 5. Field Penetration index and boreability index according to Sundin (1994)

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[m] [Mpa][inch]

Cutterhead diameter Cutters diameter DiscontinuitiesUCS

[-]

UC

S r

ange

of t

he G

otth

ard

Bas

e Tu

nnel

Tarkoy (1973)

Roxborough and Phillips (1975)

Graham (1976)

Farmer and Glossop (1980)

Snow don et al. (1982)

Nelson and O'Rourke (1983)

Nelson et al. (1983, 1985)

Sanio (1985)

Hughes (1986)

Boyd (1986)

Innaurato et al. (1991)

Gehring (1995)

Rostami et al. - CSM (1993, 1997)

Barton - QTBM (2000)

Burland - NTNU (2000)

Yagiz - MCSM (2002, 2006)

Ribacchi and Lembo Fazio (2004)

Khademi Hemidi et al. (2010)

Farrokh et al. (2012)

0 6 12 10 16 22 0 200 400

Figure 6. Overview of the considered models and corresponding geological database (for references see Cheda, 2013)

The average of the estimated penetration given by all the models is equal to 11.2mm/rev (Table 3) and the corresponding deviation from the field penetration amounts to 7%. The average of the penetration based on the selected models is equal to 10.5 mm/rev (both for 100% and 50% restrictions) and the deviation amounts to 0.02–0.04%.

Figure 8 compares the field penetration measured during the excavation of the analyzed tunnel section with the average of the penetration estimated with the penetration models. The reliability of the penetration models increases for models, which are based on a database in (or close to) the range of the project data.

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0 5 10 15 0 5 10 15 0 5 10 15

100%

Penetration [mm/rev]

All active

Penetration [mm/rev]

50%

Penetration [mm/rev]

Tarkoy (1973)

Roxborough and Phillips (1975)

Graham (1976)

Farmer and Glossop (1980)

Snow don et al. (1982)

Nelson and O'Rourke (1983)

Nelson et al. (1983, 1985)

Sanio (1985)

Hughes (1986)

Boyd (1986)

Innaurato et al. (1991)

Gehring (1995)

Rostami et al. - CSM (1993, 1997)

Barton - QTBM (2000)

Burland - NTNU (2000)

Yagiz - MCSM (2002, 2006)

Ribacchi and Lembo Fazio (2004)

Khademi Hemidi et al. (2010)

Farrokh et al. (2012)

Figure 7. Predictions of the models (black crosses) grouped according to the difference between the data underlying each prediction model and the actual project data (red band and dashed line: range and

average of the actual penetration, respectively)

Table 3. Estimated penetration and corresponding deviation from the actual penetration

Case All active 100% 50%

No. active models [-] 19 14 10

Average P [mm/rev] 11.2 10.5 10.5

[%] +7.0 +0.2 +0.04

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Av. all

Av. 50%Av. 100%

0

2

4

6

8

10

12

14

16

0 100 200 300 400 500

Pe

ne

trat

ion

[mm

/rev

]

Chainage [m]

Av. GBT

Figure 8. Comparison between field penetration (marked points) and estimated penetration considering the average of the predictions of all models or of a subset of the models developed for more or less

similar conditions (UCS, diameter cutterhead and diameter of the cutters) as the specific project

4 Conclusion

The paper compares the field penetration achieved during the TBM excavation of a section of the Gotthard Base Tunnel with the penetration estimated using several models from the literature. All considered models give penetration values that are relatively close to the observed averaged field penetration. The estimation of the penetration becomes more accurate, when considering the average of those prediction models derived for conditions (i.e. uniaxial compressive strength, cutterhead diameter and cutter diameter) which are relatively close to those of the project.

5 Acknowledgements

The authors wish to thank the AlpTransit Gotthard AG, Switzerland for the permission to use the data from the tunnel construction between Faido and Sedrun for this research.

6 References

AlpTransit Gotthard AG. 2008a. Geologisches Baujournal - Obekt: 452-EST W-TBM.

AlpTransit Gotthard AG. 2008b. Felsmechanische Laborversuche: Weströre Tkm 227.590–238.678.

AlpTranist Gotthard AG. 2008c. TBM raw data - Week 41–43. Faido.

AlpTransit Gotthard AG. 2010. Project data – raw construction Gotthard Base Tunnel. LZ01-223480-v1.

Cheda, J. 2013. Performance of penetration models and cutter wear for hard rock TBM. Master Thesis ETH Zurich.

Klose, C. 2003. Engineering geological rock mass characterisation of granitic gneisses based on seismic in-situ measurements. ETH Zurich, DISS. ETH No. 15265.

Sundin, N., Wanstedt, S., 1994. A Boreability Model for TBM’s. Proc. 1 st North American Rock Mechanics Symposium, University of Texas at Austin, A.A. Balkema, 311–319.