3D simulation of TBM excavation in brittle rzones: The Brenner Exploratory Tunnel case

Kai Zhao a,, Michele Janutolo b, Giovanni Barla b, Guoxina College of Transportation Science & Engineering, Nanjing University of Technology 210009, Nanji

Keywords:Brenner Base TunnelGranitic rocksFault zoneBrittle failure3D simulator

t a mhe funssociated with a sub-vertical fault zone, parallel to the tunnel axis. The segmental

tered in

rization of the tunnel,age 6 + 151.se geological conditionpt failure. During TBM

Engineering Geology 181 (2014) 93111

Contents lists available at ScienceDirect

Engineering

j ourna l homepage: www.e lsadvancements, except for the primary deformation induced by theexcavation, some unexpected deformation was observed at areasremote from the face, and lead to large-scale failures. Based on the

cept constant (HoekBrown); t, time; T, tensile strength; u, displacement; uf, displacementat the face; un, nal displacement; UCS, uniaxial compressive strength of intact rock;UCSrm, uniaxial compressive strength of rock mass; p, hardening/softening parameter;nel and, particularly of a geomechanical charactewith main concern of the situation around chain

At the principal issues to overcome this adveris to understand and correctly anticipate this abru

exponent (HoekBrown); c, cohesion (MohrCoulomb); CI, crack initiation threshold; E,Young's modulus; Kn, elastic normal stiffness of the interface element; Ks, elastic shearstiffness of the interface element; Kt, elastic shear stiffness of the interface element;mpeak, peak HoekBrown slope constant; mres, residual HoekBrown slope constant;RMR, rock mass rating; speak, peak intercept constant (HoekBrown); sres, residual inter-the YacambuQuibor tunnel in Venezuela (Hoek and Guevara, 2009),the Gotthard base tunnel and the Lotschberg base tunnel beneath the

curred at the sidewall during TBM drive in the granitic rocks associatedwith a sub-vertical fault zone parallel to the tunnel axis, leading to a sig-nicant damage of the shields and grippers of the machine and to astoppage of the excavation in almost 4 months. The present papergives a brief historical reviewof the construction of this exploratory tun-

Abbreviations: apeak, peak curvature exponent (HoekBrown); ares, residual curvaturepi, principal plastic strains; pl, equivalent plastic strPoisson's ratio; , stress; 1, maximum principal stress; shear stress; , friction angle (MohrCoulomb); , dilatio Corresponding author at: No.200, Zhongshan North R

China. Tel./fax: +86 2558139212.E-mail address: zhaokaiseu@yahoo.cn (K. Zhao).

http://dx.doi.org/10.1016/j.enggeo.2014.07.0020013-7952/ 2014 Elsevier B.V. All rights reserved.a great variety of under-literature, inter alia from003; Aydin et al., 2004),

tunnelling in deep Alpine tunnels, particularly when the faults arenear parallel to or cross the tunnel axis at a low angle. This is the caseof the Brenner Exploratory Tunnel in Italy. Serious local instabilities oc-ground projects. Extended reports exist in theseveral tunnel cases in Turkey (Dalgic, 2000, 2Thin rock pillar

1. Introduction

Brittle fault zones have been encounapproximately 60 m in the longitudinal direction, leading to a subsequent damage of the shields and grippers ofthe machine and to a stoppage of the excavation in almost 4 months.To dealwith these severe geotechnical problems encounteredwhen tunnelling through a fault zone, a realistic 3Dnumerical simulation based on a site investigation and characterisation of the fault zone, can provide a helpfuldecision aid as they give a quantitative assessment of the potential mode of failure. In the case of the BrennerExploratory Tunnel, the behaviour of the rockmass is neither ductile nor brittle, but governedby the combinationdue to the presence of the brittle fault zone. This paper focuses on the 3D simulation of such complex failureevolution. Special emphasis is placed on the modelling of the fault zone and of the TBM excavation process.The results demonstrate the role that the local rock mass condition and the complex interaction between therock mass, the TBM components, and the tunnel support play on the characterization of this instabilityphenomenon.

2014 Elsevier B.V. All rights reserved.

Swiss Alps (Kovari and Fechtig, 2000; Loew et al., 2010). However, itstill represents a major challenging geological environment for TBMmonitoring convand Stiros (2005this additional dsome commonreliable predictio

ain; , friction coefcient; ,3, minimum principal stress; ,n angle (MohrCoulomb).oad, Nanjing, Jiangsu, 210009,ively to nearby, previously stabilized sectionswith a length ofAvailable online 12 July 2014liT

ning was collapsed at a distance of more than 2D (D is tunnel diameter) behind the face, without any evidence.he deformation and failure then propagated intensAccepted 3 July 2014 drive in the granitic rocks ab Department of Structural and Geotechnical Engineering, Politecnico di Torino, Italy

a b s t r a c ta r t i c l e i n f o

Article history:Received 18 March 2013Received in revised form 27 June 2014

Brittle fault zones representunnels, particularly when tof the Brenner Exploratory Tock associated with fault

g Chen a

ng, China

ajor challenging geological environment for TBM tunnelling in deep Alpineaults are near parallel to or cross the tunnel axis at a low angle. This is the casenel in Italy. Serious local instabilities occurred at the left side wall during TBM

Geology

ev ie r .com/ locate /enggeoergence data, Kontogianni et al. (2004), Kontogianni, 2006), and Kontogianni et al. (2008) revealed thateformation has a systematic, clear pattern and sharescharacteristics. Yet, difculties are met in makingns at the design stage. In the present paper, we try to

The geology in the project area is characterised by Brixner Granitetectonic unit of the Southern Alps (Figure 3). It is a non-metamorphicunit which underwent the main deformation during the Alpine oro-geny. The most commonly encountered rocks are biotitic granites,biotiticamphibolitic granodiorites and local leucocratic varieties.Different sets of joints and faults (mainly subvertical) cross-cut thishomogeneous lithology, which are mainly responsible for the differentrockmass qualities encountered during the tunnel excavation, associat-ed with water inows. Brittle deformations including these joints andfaults occurred along the tunnel alignment. Signicant deformationwas observed in the fault zones, determined by the unfavourable orien-tation and position. In particular, the most serious instability events oc-curred in the sector around chainage 6+ 151, where the fault zones areparallel and tangent (almost touching) to the left tunnel wall along thetunnel axis (discussed in detail in Section 3).

The behaviour of the rock mass was, in general, stable outside thefault zones. Only slight gravity-driven instability (i.e., spalling and

Fig. 1. New stretches along the existing railway corridor MunichVerona (from www.bbtinfo.eu).

94 K. Zhao et al. / Engineering Geology 181 (2014) 93111investigate this deformation mechanism and provide a quantitativeassessment tool, by means of completely 3D numerical modelling.

A 3D nite element model for TBM tunnelling in squeezing andspalling rock conditions, which takes into account all relevant compo-nents and realistically models the step-by-step construction process, isproposed in Zhao et al. (2012). The present paper extends this 3Dmodel, with potential to account for the discontinuous behaviour ofthe rock mass (with fault zones) and also for the relevant interactionprocess between the ground, the TBM and the support systems, withreference to the Brenner Exploratory Tunnel case.

2. Project description

2.1. The Brenner Base Tunnel

The Brenner Base Tunnel is the main infrastructure to be built alongthe railway corridor between Munich (Germany) and Verona (Italy),along the NorthSouth axis of the European high-speed (for passen-gers)/high-capacity (for freights) rail link HelsinkiValletta (EuropeanCommission, 2011). The construction of the Brenner Base Tunnel willencourage the development of intermodal transport in the Alps, byshifting freight and passengers to the railway.

With a circular internal cross-section of 4.05 m radius, the BrennerBase Tunnel will be 55 km long. It is a twin-tube system consisting oftwo single track tunnels, connected together by cross passages at 333 mintervals and built 40 to 70 m to each other according to the geolog-ical conditions of the rock mass. TBM excavation is likely to be adoptedfor 70% of the total length. A specic feature of this tunnel is the plannedconstruction of a pilot tunnel, generally 10 m below the two main tun-nels to be used later for drainage and emergency access. A 6.3 m diam-eter exploratory tunnel excavated on the Italian side betweenAicha andMules has been completed in 2010. In Austria, a pilot tunnel betweenInnsbruck and Ahrental, started in 2009, has also been completed(Figure 1).

2.2. The Aicha exploratory tunnel

2.2.1. General featuresThe Aicha exploratory tunnel, as shown in Fig. 2, has been excavated

from April 2008 to November 2010 by a 6.3 m diameter Double ShieldUniversal (DSU) TBM. This exploratory tunnel does not follow inthe rst part the main tunnel alignment, as the portal of Aicha islocated further south with respect to the main tunnel portal. Untilchainage 7+ 835 it is aligned with the main tunnel, lying in themiddleof the two tubes 10 m below.

Simultaneously with the excavation, a 20 cm thick segmental lininghas been installed. It was assembled by ve precast concrete segmentswith L= 1.5 m, D= 6m (L= length, D= diameter), plus the closureoor segment containing a canal for water drainage. As soon as thesegment ring was positioned, the void between the lining and therock mass was lled by pea-gravel (without mortar injection).

In order to update the knowledge of the rock mass conditions andto monitor ground-TBM interaction during excavation, a systematicinvestigation programmewas necessary for the TBM,with the followingequipment:

Several windows for inspection/entry at the face, for the observationand classication of the encountered rock mass

Windows in the shield for the direct systematic observation of thewalls

Probe drilling ahead of the face A measure and registration system of the excavation operativeparameters.

Furthermore, geo-seismic investigations and geo-electrical surveyswere conducted continuously, as well as other monitoring activities

(Grandori et al., 2011). rockburst) were encountered along the stretch (Grandori et al., 2011).

d M

95K. Zhao et al. / Engineering Geology 181 (2014) 931112.2.2. Construction historyAfter the rst 2 km in very hard, massive and abrasive rock mass,

the exploratory tunnel entered the stretch (with a length of 3.5 km),characterised by alternating zones with intact rocks and zones withfractured rocks (with minor length) (Grandori et al., 2011). In the rst5.5 km stretch, the uniaxial compressive strength (UCS) of the intactrock was around 154 MPa and the rock mass was mainly categorizedas class RMR II, except in some fault/fractured zones (with limited ef-fects). In the following section, between chainage 5+500 and chainage6 + 200, the granite exhibits highly fractured and altered. An exampleof the detailed RMR rock mass classication, on the basis of measure-

Fig. 2. Aicha exploratory tunnel anments and observations conducted on the representative rock masscondition of this fractured granite, is given in Table 1. In particular, thefaults causing serious instability phenomenon are located in this sec-tion, which are the main concern in this paper. Since then, the explor-atory tunnel arrived at the zone of massive or slightly fractured rockmass, extending to the end of this tunnel. The UCS was lower thanthat in the rst part, around 120 MPa, probably as the Periadriatic Linewas approaching.

3. Stability problem

3.1. Instability phenomenon at chainage 6 + 151

The instability phenomenon at chainage 6 + 151 started on August7th, 2009 when damage was observed at the left-hand sidewall liningduring TBM advancement. In the following days the damage extendedtowards the face, reaching the shield, and also backwards, progressivelydecreasing to a fracturing state up to approximately 60 m behind theface. After 710 days, this sudden phenomenon stabilized.

Prior to the failure, as well as during and after the rehabilitationworks (Section 3.3), the tunnel was systematically monitored on thebasis of the extensometers. The instruments were installed with vibrat-ing wire in the reinforcing bars at the extrados and intrados of the seg-ments at certain positions. In particular, among them, the section atchainage 6 + 099 was installed on August 5th, 2009 (two days beforethe failure occurred), which was 52 m behind the breakthrough pointof the tunnel (chainage 6 + 151). Recordings at this section revealeda sudden increment of the deformations on the day the failure occurredat chainage 6+151. It is an entirely extraordinary eventwith respect tothat observed in the previously installed monitoring station, and obvi-ously, not as a result of accumulating deformation due to the face ef-fect. Total strain at the monitoring sites reached 900 in theextrados (i.e., zone of compression) and 600 in the intrados (i.e.,zone of tension) at the left-hand sidewall lining (9 'o clock position)(Figure 4). The corresponding stress was 35 MPa in compression and20MPa in tension, inducing permanent damage of the lining in this sec-tion. Since then, it is seen that the stress redistribution occurswithin thedamage area. The maximum stresses decrease to the values of 30 MPa

auls adit (from www.bbt-se.com).and 15 MPa, respectively, which indicate a tendency of crack propaga-tion along the longitudinal direction (Figure 5).

The results of systematicmeasurements at this section for the 3 day-long subsequent period (August 7th to August 10th) indicated a ten-dency for stabilization. With TBM advancement and the consequentincreasing distance from the face, the deformation continued to accu-mulate till August 10th, 2009 when the excavation was arrested.

The following conditions were encountered after the TBM stopped(Barla et al., 2010):

The rock mass at the face was scarcely fractured and composed of anon-altered granite; the only exception was a very fractured zonelocalised at the bottom left.

The shield was in contact with the rock mass along the left sidealmost up to the face, while it was completely detached from thebored prole on the right side.

The gap between the lining rings and the bored prole was correctlylledwith pea gravel on the right side, while it was almost completelyabsorbed by the deformations on the left side for the entire stretchaffected by the phenomenon.

A few segments of the left-hand lining sidewall were wrecked so thatthe rock mass was visible at the back: cloritized cataclastic granite,with angular decimetric clasts (Figure 6a).

Breaking and cracking of the lining were mainly sub-horizontal andwere concentrated in the portion of the lining around the horizontaldiametral plane (Figure 6b). Cracks affected directly the segments.

The right-hand sidewall experienced neither damage nor importantdisplacements; a windowed segment showed a hard and unaltered

Fig.3.Geologicalsection

alongtunn

elalignm

ent.

96 K. Zhao et al. / Engineering Geology 181 (2014) 93111

Fig. 4.Monitored strain in the lining at chainage 6 + 099.

Table 1RMR classication parameters and their rating parameters for the representative fractured granite.

Classication parameters and their ratings

Parameter Range of values

Strength of intact rock material Point load test N10 MPa 410 MPa 24 MPa 12 MPaUCS N250 MPa 100250 MPa 50100 MPa 2550 MPa 525 15 b1

15 12 7 4 2 1 0RQD 90100% 7590% 5075% 2550% b25%

20 17 13 8 3Spacing of discontinuities N2 m 0.62 m 200600 mm 60200 mm b60 mm

20 15 10 8 5Condition of discontinuities Discontinuity length (Persistence) b1 m 13 m 310 m 1020 m N20 m

Rating 6 4 2 1 0Separation (Aperture) None b0.1 mm 0.11.0 mm 15 mm N5 mmRating 6 5 4 1 0Roughness Very rough Rough Slightly rough Smooth SlickensidedRating 6 5 3 1 0Inlling (gouge) None Hard lling b 5 mm Hard lling N 5 mm Soft lling b 5 mm Soft

lling N 5 mmRating 6 4 2 2 0Weathering Unweathered Slightly weathered Moderately weathered Highly weathered DecomposedRating 6 5 3 1 0

Groundwater General conditions Completely dry Damp Wet Dripping FlowingInow per 10 m tunnel length None b10 l/min 1025 l/min 25125 l/min N125 l/minRating 15 10 7 4 0

Discontinuity orientations Very favourable Favourable Fair Unfavourable Veryunfavourable

0 2 5 10 12RMR index: 45Rock mass class: III

97K. Zhao et al. / Engineering Geology 181 (2014) 93111

the presence of a fault dipping at high angle towards NE (N 5060),

Fig. 5. Stress in the lining at chainage 6 + 099.

98 K. Zhao et al. / Engineering Geology 181 (2014) 93111granite. The collapse of the lining on the left side blocked the segment carrierand deformed the back-up, drifting the shield with respect to thelining (Figure 7).

Furthermore, it is interesting to note that another failure occurred atchainage 6 + 105 and chainage 6 + 112 on August 10th (i.e., after thestabilization of the serious failure events at chainage 6 + 151). It wascaused by the execution of a probe borehole at the left sidewall. Thefracture extension and propagation induced serious damage of 4segments. The borehole was intercepted by a pressure water inow at20 m from the left sidewall, i.e., at 13 m from the fault zone. It reectedthat no water was contained in the fault zone, which was also veriedFig. 6. Lining breakage at chainage 6 + 151: (a) Disjointed segment immediately behind the scracks in the lining.running parallel and tangent (almost touching) to the left tunnel wallbetween chainage 6 + 020 and chainage 6 + 151.by the observation at chainage 6 + 123 that the breccia at the end ofthe lining was dry.

3.2. Geological model

The original site investigations involved walk-over surveys and coredrilling. In total, 12 boreholes were drilled and allowed one to re-construct the geological model, as shown in the plan-view of Fig. 8and in a cross-section of Fig. 9 at the tunnel level. The boreholes denotedhield with outcrop of cloritized cataclastic granite (Barla et al., 2010); (b) Sub-horizontal

The overall thickness of the fault zone ranged from 7 to 10 m. Itconsisted of a core of non-cemented cataclastic rocks with an averagethickness of 5 m and of an external damage zone with highly fracturedgranite (Figure 10). The fault was crossed/dislocated by minor bundles

the excavation as observed, except the heavy water inowsencountered.

Furthermore, the geo-structural kinematic analysis has been per-formed in the outcrops on the eld. The fault was interpreted as a

Fig. 7. Effects of the deformations of the segments on the back-up: (a) carpentry injured; (b) segment detached from the shield; (c) segments' belonging.

99K. Zhao et al. / Engineering Geology 181 (2014) 93111of faults.In addition to this core, otherminor faults were detected at 1430m

to the left sidewall. In the other stretches the boreholes exhibit few tomeanly fractured, with an alteration along the discontinuity surfaces.According to the observations at proximity of the cracked lining and inthe injection holes for the pea gravel, the fault breccia could originallybe very compact and dry with a thin silt component. This fault materialhad a residual cohesion before the deconnement due to the exca-vation, as a result of compaction for the charge and crystallisation ofmineral sin-kinematic phases during the fault movement (chlorite,quartz, epidote). Clay gouges were detected only in some short bore-holes with a maximum thickness of 5 cm.

From the geological model, it is seen that the size and orienta-tion of another fault zoneRio Bianco dominates the morphology,as it determined the deep incision of the homonymous stream. It iscomposed of a core zone and a damage zone, as it can be observedin surface (Figure 11). Nevertheless, this fault has been crossedby a more favourable angle and lower topographical cover. Forthis reason, it did not cause signicant stability problems duringFig. 8. Geological reconstruction (pparticular element strictly related to the Rio Bianco fault zone: asschematically illustrated in Fig. 12, it is a lateral strikeslip fault con-jugate to the Rio Bianco fault. The two faults are related by the struc-tural elements of the Brixner Granite which developed in the frameof a stress eld, with a NWSE principal compression axis. Therefore,the fault was considered as a local disturbance and it was expectedthat similar features could not be met in the other parts of the rockmass, as actually observed. After chainage 6 + 151, the fault zonegradually diverged from the tunnel axis and then disappeared,going towards North in the direction of excavation.

This geological model was in perfect agreement with the ob-servations when the excavation re-started.

3.3. Rehabilitation works

First of all, several activities (i.e., bolting, installation of steel ribs,etc.) in order to ensure the safety of the tunnel were undertaken.Then, the following works were carried out to restore the damagedtunnel:lan-view) (Barla et al., 2010).

Injections of lling and stabilizing resins Cutting, demolition and replacement of reinforced concrete segmentsshowing excessive shifts and/or breakage, for subsequent smaller

permanent deformation on the entire left side with considerabledenting on the lower zone

Filling of the tunnel with pea gravel.

3.4. Resume of the excavation

OnDecember 4, 2009, reasonable progresswasmade after introduc-ing a special segmental ring. It was formed with the same segments ofthe previously installed rings, but with additional reinforcement andsteel crown segments.

For the continuation of the advancement outside the fault zone,several options for the support design were suggested to increase theorder of load capacity, listed as follows (Barla et al., 2010):

a) Installation of standard concrete rings reinforced with 10 mm-diameter steel bars

b) Installation of concrete rings reinforced with UPN 120 structuralsteels

c) Installation of concrete rings reinforced with structural steels andwith guide bars, in order to sustain heavily asymmetric loads

d) Installation of concrete rings with guide bars and reinforced withstructural steels also between two adjacent rings.

3.5. Risk management for the remaining stretch

After the worst hazard scenario occurred in chainage 6 + 151, it isrecognized that the problems encountered in tunnelling through

Fig. 9. Reconstruction of the geological section at chainage 6 + 100 (Barla et al., 2010).

100 K. Zhao et al. / Engineering Geology 181 (2014) 93111portions, with steel panels, subsequent rear lling of panels withcement and installation of radial nails with 3 m long self drilling bars

Dismounting of the portion of the segment carrier which was notblocked by the shifts in the lining

Excavation of an unlocking tunnel for the TBM (Figure 13) Restoration of the gripper: the grippers showed a signicant deforma-tion of both shoes, a puncture of the front part of the shield and thecollapse of a supporting ank

Restoration of the damaged shield: the shield showed widespreadFig. 10. Boring at chainage 6 + 117 (05 m)the brittle fault zone are so diverse and complex that monitoring canbe employed meaningfully, as well as their subsequent evaluation.Therefore, the Geotechnical RiskManagement Procedurewas suggestedwithin the framework of the observational method. Monitoring wasapplied in different ways. In particular, the load history on the liningwas continuously recorded during TBM advancement, and also thefollowing data were considered to improve the decision making onselecting appropriate support system and to establish early warningsystems against incipient collapse:and 6 + 060 (05 m): tectonic breccias.

Rock mass classication (weighted RMR on the excavation face) Important asymmetries in the rock mass structure at the face Data of the machine (Grandori et al., 2011) Analysis of the muck Water inow measures.

In addition, the monitoring of the machinerock mass interactionwas performed based on the analysis of the TBM operation data. In agood to excellent rockmass condition, the thrust value and the penetra-tion rate were around 10 MN and 20 mm/min, respectively. The strongvariations of these monitoring data could denote a worsening of therockmass quality, which imposed a further facemapping in order to de-

Fig. 11. Outcrop of the fault i

101K. Zhao et al. / Engineering Geology 181 (2014) 93111cide either tomaintain or to change the support type (Barla et al., 2010).In reality, as the excavationwas carried out by a Double Shield TBM, thetunnel faces could not be documented continuously using rock massclassication systems, but only with a frequency.Fig. 12. Structural scheme which describes the relationship between the Rio Bianco faultand the causive fault encountered in the tunnel; the bigger arrows show the palaeo-axisof tectonic deformation which determined the movements (little arrows) along the faultnetwork of the zone (Barla et al., 2010).Furthermore, the following parameters were systematically intro-duced for the Geotechnical Risk Evaluation (Barla et al., 2010):

the re-gripping pressure: an anomalous value can denote an anom-alous load on the tail shield and therefore on the lining

Volume of pea gravel injected instead of the theoretical value: it candenote the rock mass deformations

Values of the induced deformations/stresses in the lining. It is im-portant to dene the threshold values for the strain in the lining(attention value, alarm value).

With the aid of these measurements, one is in the position to keepthe ground pressure under control and the lining resistance can bechosen accordingly, until an optimum value is achieved.

4. Computational model

4.1. Foreword

In this paper, a simulator of TBM excavation of deep tunnels hasbeen developed to this end, by using 3D FEM modelling and the midasGTS (Geotechnical and Tunnel Analysis System) computer code (TNODIANABV) (Midas, 2010), to reproduce and to analyse the instabilityphenomenon as described before. The presence of water pressure andconsolidation problems is not taken into account. Emphasis is placedon the discontinuous behaviour of the rockmass and on the interactionbetween the rock mass and the TBM and the support componentsassociated with the progressive advance of the working face.

n the Rio Bianco Valley.4.2. Model construction

The geological conditions have been simplied by considering only 2rock types: the fault and the granitic rock mass. The fault is assumed tobe parallel to the excavation, 7 m thick and approximately 0.2 m farfrom the excavation boundary. The dip of the fault has been assumedto be 73 N towards NE. Due to the highly heterogeneous behaviour ofthe rock mass, the entire domain is modelled. A cylindrical domainis used in order to acquire high mesh quality. 8-node hexahedronsolid elements are chosen as appropriate. In particular, a high level ofmesh renement is desirable to capture high strain gradients of thepillar rock between the side wall and the fault zone. The transitionelements are used in order to assure a better shape at the interface be-tween the fault zone and thehost rockmass. Fig. 14 shows themesh lay-out of the model.

Themesh size should be large enough so that the external boundarycan represent an innitely extended medium. Otherwise, the presenceof the articial boundaries will induce a signicant inuence on thestressstrain eld around the tunnel. The external boundary is set to a

102 K. Zhao et al. / Engineering Geology 181 (2014) 93111distance of 11D in the transversal plane tominimize boundary effects. Inthe longitudinal direction, a total length of 20D is applied in order tomaintain a sufcient distance between the rear boundary and the lastexcavation face. The mesh discretization is equal to the excavationlength (1 m); then, after the last excavation face, it is graduallyincreased. In all cases, parametric studies are necessary to evaluate theappropriate size for every specic case.

As far as mesh grading in the transversal plane, the accuracy of theresults depends on the ability of the mesh layout to capture the high

Fig. 13. Unlocking tunnel for the TBM: (a) phot

Fig. 14. 3D rock mass mstrain gradients in the overstressed zone of the rock mass (Diederichs,2007). In this failure localization cases, a very rened region aroundthe tunnel contour with an element size of 20 20 cm at the tunnelboundary on the left side and a coarser zone away from it are used.

Once the 3Dmodel is created, the following boundary conditions areimposed:

In the circular outer boundary, displacements along the vertical(Y) and the horizontal direction (X) are prevented;

o; (b) cross-section (Grandori et al., 2011).

FaultRock mass

esh (initial stage).

In the front and in the rear outer faces, displacements along theexcavation advance direction (Z) are prevented.

As the model refers to a deep tunnel with a signicant fault close tothe boundary, the in situ state of stress is applied as a hydrostatic initialstress without consideration of the free ground surface and of the stressgradient due to the gravity. In reality, in the Brenner tunnel case, thediscontinuities, i.e., fault zones, act to signicantly perturb the stresseld (i.e. local effect) and thus the eld measurements become anessential component of the overall design process. The in situ stressmeasurements in the boreholes showed that the range of k0 (ratio ofhorizontal stress/vertical stress) is comprised from 0.8 to 1.2. Also bytaking into account the presence of the pre-sheared fault zone (Hoekand Marinos, 2010), it is assumed that the horizontal and verticalstresses are equal in the 3D model. The in situ state of stress is thereby,assumed to bewith a vertical and horizontal state of stress of 12.72MPa,evaluated at 480m depth, by introducing a rockmass unit weight equalto 26.5 kN/m3.

4.3. Rock mass model

The stability problems with respect to the discontinuous nature ofground are not only a result of brittle fracture of the hard rock at greatdepth, but also of shear failure along pre-existingdiscontinuities. The lo-cation andproperties of the fault zone are of paramount importance andtherefore, respected in the 3D model in particular to identify thepotential failure mode. Furthermore, two constitutive models for re-producing the discontinuous behaviour of the rock mass in a consistentand simple way are illustrated in the following.

Aica exploratory tunnel, the low connement caused by both the pres-ence of the excavation boundary on one side and the fault a non-cohesive material on the other side, generates a brittle failure typicalof hard rocks resulting in progressive slabbing and spalling processes.

Due to the fact that brittle failure involves a tensile fracturing

Fig. 16. Granitic rock mass, Diederichs model (based on the generalised HoekBrown cri-terion) and equivalent one (based on the traditional HoekBrown criterion).

103K. Zhao et al. / Engineering Geology 181 (2014) 931114.3.1. Granitic rock mass (brittle failure)Brittle failure, both in the form of major spalling and potentially

strain burst, often dominates rock damage and failure processes in crys-talline rocks (like granitic rockmass) near excavation boundaries underhigh in situ stress environment (Diederichs, 2003). In the case of theFig. 15. Composite strength envelope foprocess, the conventional yielding criteria for continuum medium asthe MohrCoulomb and the HoekBrown cannot properly describethe actual behaviour of the rock (Kaiser et al., 2000). In this respect,Diederichs (2007) proposed a specic criterion for susceptibility to brit-tle spalling (as opposed to plastic shear), based on the generalisedHoekBrowncriterion andanelasticperfectly-brittleplastic constitutiver brittle rocks (Diederichs, 2007).

sure is applied, given by the maximum cutterhead thrust divided bythe area;

104 K. Zhao et al. / Engineering Geology 181 (2014) 93111law. Peak and residual yield functions are dened by damagethreshold and spalling limit, respectively (Figure 15).

A model based on the traditional HoekBrown criterion equivalentto the generalised one proposed by Diederichs (2007) is adopted, asshown in Fig. 16. The procedure for determining the input parametersfor the generalised HoekBrown criterion is the following:

Determine the crack initiation threshold CI from uniaxial compressiontests;

Set apeak to 0.25; Obtain a reliable estimate of tensile strength, T (from laboratorytests);

Calculate the appropriate s andm from:

speak CI=UCS 1

apeak 1

mpeak speak UCS= Tj j 2

Set ares= 0.75, sres=0 or 106 (for numerical stability) andmres=5to 9 in order to model the transition envelope to high connementshear (spalling limit).

In particular, CI is empirically estimated as 0.45 UCS according to Cai

Table 2Model parameters for Granitic rock mass, fault zone and the interface between the faultzone and the granitic rock mass.

Granite

General parameters Criterion ofDiederichs

Equivalent parameters

E 25 GPa apeak 0.25 apeak 0.5 0.25 speak 0.041 mpeak 1.4UCS 140 MPa mpeak 0.656 sres 0.2T 8.1 MPa ares 0.75 ares 0.5CI 63 MPa mres 9 mres 6 sres 106 sres 106

Fault zone InterfaceE 250 MPa Kn 10 GPa/m 0.25 Ks 1 GPa/mUCS 7 MPa c 0 c 0GSI 25 5 38 38mi 19 0 0et al. (2004), who suggest that the ratio between CI andUCS is generallywithin the range of 0.30.5. The other parameters are obtained accord-ing to Diederichs (2007).

The assumed rock mass parameters are shown in Table 2. Itshould be noted that the model cannot consider shear at high conne-ment correctly and thus it is only limited to the near-excavationanalysis.

4.3.2. Fault zoneAlpine fault zones are complex structures, exhibiting highly hetero-

geneous rock mass conditions. The simulation of the behaviour of thefault zone must take into account both (i) shear across a cataclasticcrush zone in the fault zone, and (ii) sliding on existing planes betweenthe fault zones and the granitic rock mass.

For the shear behaviour of the fault zone, we adopted the standardlinearly elastic, perfectly plastic material model with theMohrCoulombyield criterion and non-associated ow rule. Theweak cataclastic rockmasses in the Brenner Exploratory Tunnel fail in shear through themassrather than along individual discontinuities,which can be traced back tothe fault zones down to depths at which the frictional resistance to slip Shields (front, telescopic, gripper and tail shield), which are modelledwith plate elements applied at the excavation boundary and with thestiffness properties of steel;

Pea gravel, which is modelledwith solid elements and is activated 2mbehind the shield;

Grippers, which are modelled indirectly, as a portion of the shields inwhich a given pressure is applied;

Lining, which is modelled with plate elements (with the stiffnessproperties of concrete); the segmentation of the tunnel lining isdisregarded (the lining is assumed as continuous), since the model isnot especially designed to investigate in detail the behaviour of thetunnel lining.

All these components have a linearly elastic isotropic law. The TBMlayout for the simulation is shown in Fig. 17. The size of the gap betweenshield and ground, as well as between segmental lining and ground, istaken as constant around the circumference. In reality, the shield slidesalong the tunnel oor, whichmeans that the gap around the shield andsegmental lining is not uniform and wider above the centre than in thelower portion of the tunnel cross-section. Given the aim of this paper(to simulate the instability which occurs mainly at the sidewall withthe gap of average size), however, it is sufcient to only consider thecontacts between the shield and sidewall rock mass.

The simulation parameters are given in Table 3. The elastic modulusof the lining is the composite modulus of the reinforced concrete: theconcrete has class C45/55 (E= 36 GPa) and the reinforcement consistsin 8 mm-diameter bars with a 10 cm spacing. The self-weight is alsoexceeds the ductile yield strength of the rock mass. However, it shouldbe kept in mind that fault zones represent complex geological struc-tures, which are composed of various rockswith very different materialproperties. The geomechanical properties of the faulted rock mass ishard to characterise because of many difculties arising in gettingrepresentative samples during eld investigations, in specimen pre-paration, and in performing appropriate laboratory testing. Therefore,in addition to the laboratory testing on the core samples, the key param-eters were empirically estimated in the eld on the basis of the descrip-tive classication (GSI) of Hoek et al. (1998). In particular, the GSI chartfor heterogeneous rock masses is used to account for the lling mate-rials in the fault zones. Additionally, a weighted average of the intactstrength properties of the strong and weak materials is considered asthe reasonable approximation of the equivalent material propertiesUCS and mi for the fault zones, as proposed for highly heterogeneousrock mass by Marinos et al. (2006). Based on the estimated UCS, thematerial constant (mi) value and GSI value attributed to this faultedrock mass, the cohesive strength and friction angle were also evaluatedfor design and analysis purposes (Barla et al., 2010). The parameters areshown in Table 2.

On the other hand, zero-thickness interface elements are usedand the criterion for Coulomb frictional law is assumed to repre-sent the potential sliding mechanism between the fault and therock mass. The elastic and plastic parameters (according to theCoulomb friction law) are listed in Table 2. It is assumed that theinterface has the same friction coefcient as the fault material aswell as no cohesion.

4.4. Main TBM components

The following TBM components are considered:

Cutterhead, which is modelled with plate elements (with the stiffnessproperties of steel) at the current excavation face and where a pres-applied to all these components.

cons

105K. Zhao et al. / Engineering Geology 181 (2014) 931114.5. TBM advancement and simulation procedure

A step-by-stepmethod is adopted to simulate the TBM advancement.The excavation length is taken to be equal to 1 m in order to:

Fig. 17. TBM layout reproduce as well as possible the continuous process of TBMexcavation (Vlachopoulos and Diederichs, 2009);

improve the convergence of the elasto-plastic solution; trade-off accuracy and computational time.

The construction stages are dened as follows:

in the rst step, initialisation takes place accounting for the in situstress eld;

in the second step, the TBM enters the model (the cutterheadelements are activated) and the rst slice is excavated;

in the further steps, the rst slice of the front shield is activated,according to the following paragraph;

in each step thereafter the TBM (front and rear shield) progresses intothe model;

in the fteenth step, the lining is activated as well as the pea gravel; the stages proceed until a steady-state condition is reached.

Fig. 18 illustrates the simulation process on the left half of thedomain.

Table 3Parameters for the components of TBM excavation.

Cutterhead Shield Lining

Diameter 6.3 m Diameter 6.3 m Outer dThickness 3 cm Thickness 3 cm Thickn

Length front shield 5 mLength rear shield 7 m

Young's modulus 200 GPa Young's modulus 200 GPa Young'Poisson ratio 0.3 Poisson ratio 0.3 PoissonPressure at the face 0.4 MPa

Friction coefcient rock-shield skin 0.3 Friction4.6. Interaction between the machine components and the rock mass

The simulation of the interaction problems requires a specialattention to the discontinuous behaviour at the following interfaces:(i) rock mass-shield and (ii) pea gravel-lining. This behaviour involves

idered (plan-view).frictional sliding and the possible closure of the gap between the shieldand the rock mass.

Zero thickness interface elements are used to represent the frictionalshearing mechanism. High normal and shear stiffnesses have been set.For plastic slip, a Coulomb friction law with null-cohesion and a valueof the skin friction coefcient = 0.3 have been adopted for both theinterfaces. Since the numerical formulation used is based on the small-strain/displacement assumption, the shields have been modelled righton the tunnel boundary even if there is a gap in-between. In order tosimulate the gap between the rock mass and the shields, special inter-face elements have been used in which the elastic stiffnesses are set tozero, i.e., Kn = Ks = Kt = 0, so that no stress can transfer from therock mass elements to the shield elements.

In order to know the regions along the shields in which the contactwith the rock mass takes place (on the left sidewall in this case), theeld of the radial displacements has to be carefully monitored. Whenthese displacements exceed the radial gap, the properties of the inter-faces in the relevant step are modied. As a slice of the rock core ahead

Grippers Pea gravel

iameter 6 m Base 2 m Thickness 15 cmess 20 cm Height 4.5 m Thickness 15 cm

s modulus 38.1 GPa Young's modulus 1 GParatio 0.2 Poisson ratio 0.3

Pressure 7 MPacoefcient rock-lining 0.3

106 K. Zhao et al. / Engineering Geology 181 (2014) 93111of the face is excavated, the displacement uf at the face is removed.To consider the geometry update, the nal horizontal displacementun to be monitored is un = u uf, where u is the horizontaldisplacement.

Furthermore, the gripper shield is activated as it is in contact withthe rock mass through the grippers: the properties of the interfacesallowing the contact are therefore activated for the size of the grippershoes.

(a) 13th

(b) finalFig. 18. Illustration of the 3Dmodel on the left side at two excavation steps. The two gradationsthe parts which are not in contact (light-blue).5. Results and discussions

Evidence from the Brenner Exploratory Tunnel indicates that, ex-cept for the primary deformation (which is a consequence of the spa-tial stress redistribution associated with progressive excavation),under certain circumstances additional, unexpected deformation isobserved at areas far from the working face, and can even lead tolarge-scale failures. This deformation and also the induced collapse

step

step

of blue differentiate the parts of the shield in contact with the rockmass (dark-blue)with

of tunnel segments are explained with the numerical results in thissection. It starts with a discussion of the local instabilities along thetunnel (Section 5.1), and illustrates how the interaction betweenthe shield, the ground and the tunnel support inuence the degreeof overstressing of the ground (Section 5.2). Specically, this sectionalso shows the load transfer along the longitudinal direction, whichshed some light on this extensive deformation remote from the tun-nel face (Section 5.3).

5.1. Local instabilities

First of all, the failure zones denoting the collapse of the thin rockpillar between the tunnel wall and the fault are analysed. The degreeof failure is expressed through the equivalent plastic strain, dened as:

pl 23

2p1 2p2 2p3 r

3

with p1, p2, and p3 the principal plastic strains.This denition comes from the most commonly used expression for

the softening/hardening parameter (based on incremental plasticstrain) which is (Vermeer and De Borst, 1984):

p pt

23p1p1 p2p2 p3p3 r

: 4

pillar between the tunnel wall and the fault. This discontinuous stressdistribution in the vicinity of the boundary between the weak and stiffrock can be traced back to the strong stiffness contrasts between theadjacent host rock and the fault zone. This stress concentration leadsto the brittle failure of the more competent pillar rock, associated withthe complex interaction between the ground, TBM and support systemat the excavation side (Section 5.2). Furthermore, a stress redistributioncan be checked in the fault zone, which is responsible for the yielding ofthe weak fault material.

5.2. Interaction between TBM, ground and tunnel support

Fig. 22 provides a complete picture of the horizontal displacementson the left sidewall of the tunnel along the longitudinal direction. Thenal displacements, as already described in Section 4.4, are obtainedby removing the displacement at the face uf (the so-called pre-deformation). The nal displacements along the line corresponding tothe minimum thickness of the rock pillar between the tunnel and thefault, are plotted in Fig. 23, as well as the ground pressure developedon the shield and the lining. It is noticed that: (i) the displacementsclose the gap at the front shield (equal to 3.5 cm) 2 m behind the face,(ii) the contact zone of the rear shield through the grippers, togetherwith the applied pressure, blocks the displacement in these areas, (iii)the displacements close the gap at the tail of the rear shield (equal to9.5 cm), but for a really short distance (around 50 cm), so that the lastslice of the rear shield has not been activated (since the discretization

107K. Zhao et al. / Engineering Geology 181 (2014) 93111It is noted that this softening parameter has not been introduced inthe constitutive law, and the equivalent plastic strain is thus only avariable describing the degree of damage in the rock mass.

Fig. 19 shows such plastic strain contours in a 3Dviewof the left side,while Fig. 20 in a cross-section at the steady-state. As in this sectiondominated by the weak fault zone on the left sidewall, the failure islocalised, in the manner of the combination of the brittle failure of thepillar rock between the tunnel and the fault, and the shear yielding ofthe gouge. Furthermore, as the breakage of this pillar, the yieldingfault zone is loosened and thus the weak gouge material washed out.This coincides very well with the in situ observations.

Fig. 21 depicts the maximum principal stress contours around theexcavation. It is seen that a signicant stress is channelling into thisFig. 19. Plastic strain contours along the tunnelis 1 m), (iv) the displacements ceased when the support system isinstalled. The maximum nal horizontal displacement at the steady-state on the left side is equal to 11.3 cm, while on the right side is onlyequal to 0.5 mm.

In reality, not only the nal magnitude of the displacements is ofimportance, but also the development with TBM advancement, as thiscontrols the shield and lining load development. Fig. 23b shows thedistribution of the ground pressure acting on the shield and the lining,as soon as the converging ground closes the gap. Unexpectedly, a sig-nicant load concentration develops at the end of the front shield. Thesegmental lining also experiences a dramatic increase of the groundpressure with the distance from the tunnel face even as the face effectbecomes less pronounced.in a 3D view on the left half of the model.

a c

108 K. Zhao et al. / Engineering Geology 181 (2014) 93111It should be noted that the prefabricated segments are installedunder the protection of the shield, leaving a radial gap remains behindthe rear shield. Meanwhile, backlling with pea gravel followed bygrouting is performed at a certain distance behind the shield with a

Fig. 20. Plastic strain contours inconsequence that an unsupported span exists. As shown in Fig. 23b,the rock mass experiences three unloading processes during the ex-cavation. Firstly, as the tunnel remains unsupported at 2 m behind theface (Figure 17), the tunnel boundary experiences the rst unloadingprocess. Then, as soon as the gap is closed, loading of the shieldtakes place. Secondly, the entire tunnel boundary is unloaded at the

Fig. 21.Maximum principal stress crear shield due to the conicity of the machine. Until the pea gravel isbacklled at certain distance behind the shield, the last unloadingprocess occurs.

Consequently, as depicted in Fig. 24, a signicant stress concentra-

ross-section at the steady-state.tion develops at the end of the front shield where contact with theground occurs, causing permanent damage to the TBM.

Moreover, the maximum stresses in tension in the lining canalso be checked in Fig. 25. This value (16 MPa) is in good agreementwith the monitoring data (15 MPa) of the extensometers installedin the lining.

ontours around the excavation.

Fig. 22. Horizontal displacements on the left side of the tunnel.

Fig. 23. Results of the model: (a) Longitudinal displacement prole (LDP); (b) contact pressure developing on the shield and on the lining.

109K. Zhao et al. / Engineering Geology 181 (2014) 93111

Fig. 24.Maximum stress in the shield and cutterhead.

110 K. Zhao et al. / Engineering Geology 181 (2014) 931115.3. Load transfer along the longitudinal direction

The key to understand this special failuremode is that it represents acomplex load transfer mechanism along the longitudinal direction,resulting from the discontinuous behaviour of the rock mass (with afault zone) and the interaction between the ground, the TBM and thesupport system. Indeed, the similar mechanism has been reported andwell explained through the evidence from geodetic monitoring at theMessochora tunnel and the Tymfristos tunnel, Greece (Kontogianniet al., 2004, 2008; Stiros and Kontogianni, 2009).

Combining the support of the core ahead of the face, the shield (afterclosing the gap) and the backlled segmental lining, three archingactions typically develop between each other, which determine theload transfer in the longitudinal direction. However, the fault zonedestroys these arching effects due to its near-parallel orientation tothe tunnel axis. In that, the load transfer along the longitudinal directionFig. 25.Maximum strdeteriorates, signicantly. High convergence, as a result, occurred at thecertain area associated with re-acceleration in the strain accumulationin the neighbouring sections.

As the convergence closes the gap progressively, the shield starts tosupport the ground and undertakes the role of the core after excavationto stabilize the tunnel. The spatial stress redistribution due to the exca-vation front advance has minor effect on the previously excavated, de-formed and stabilized tunnel segmental linings at large distances fromthe front. Nevertheless, as the TBM advances leaving an unsupportedspan behind the machine, a signicant secondary stress re-distributiondeveloped and transferred at bi-lateral directions. It would lead to aheavy load concentration on the shield and also induce a further distur-bance to trigger failure of the pillar rock between the wall and the fault.As soon as the failure of the stiffer pillar rocks occurs, the stresses haveto be redistributed, causing a load increase in the weak material, andalso in previously less loaded stiff blocks. It transferred at distance ofess in the lining.

12m (more than 2D) backwards along the tunnel, in areas remote fromthe working face and presumably free from the face effect.

Through the results, it suggests that despite the extremelyweak rockconditions and rock yielding in the failure zone, ground improvement(i.e., grouting and rock bolts) contributed to the development of thephysical bearing capacity of the rock around the excavation and alsoto the improvement of the groundsupport interactionwhich sustainedstability around the opening.

Furthermore, the arching action between the core and segmental

China (no. 2011CB013605). The authors would like to thank for theprecious nancial and moral support.

References

Aydin, A., Ozbek, A., Cobanoglu, I., 2004. Tunneling in difcult ground: a case study fromDranaz tunnel, Sinop, Turkey. Eng. Geol. 74, 293301.

Barla, G., Ceriani, S., Fasanella, M., Lombardi, A., Malucelli, G., Martinotti, G., Oliva, F.,Perello, P., Pizzarotti, E.M., Polazzo, F., Rabagliati, U., Skuk, S., Zurlo, R., 2010. Problemidi stabilit al fronte durante lo scavo del cunicolo esplorativo AicaMules della Galle-ria di Base del Brennero. MIR 2010, Torino.

Cai, M., Kaiser, P.K., Tasaka, Y., Maejima, T., Morioka, H., Minami, M., 2004. Generalized

111K. Zhao et al. / Engineering Geology 181 (2014) 93111ther increases the ground pressure acting on the shield. Therefore, thestiffer the lining and the shorter its distance from the face, themore pro-nounced will be the arching effect and the less will be the ground pres-sure (Ramoni and Anagnostou, 2011). In this respect, backlling withcontinuous grouting is more advantageous than pea gravel and mortarwith an unsupported span behind the shield (for the machine with agiven length). On the other hand, a stiff support with embedmentright behind the machine is favourable for the shield but, inevitably, at-tracts a higher ground load.

6. Conclusions

The brittle fault zone sub-parallel to the tunnel axis presentedmanychallenges to the engineers and contractors during the construction ofthe Aicha exploratory tunnel (along the Brenner Base Tunnel). Seriouslocal instabilities occurred at the side wall (close to the fault zone)during TBM drive. The segmental lining was collapsed at a distance ofmore than 2D (D is tunnel diameter) behind the face, without anyevidence. The correct anticipation of the fault zones as the tunneladvances and a timely increase both in reinforcement (grouted bolts)and holding elements (strong anchored bolts and heavy mesh ormesh over steel sets) are the key to success in this case.

A 3D numerical model is presented and applied in this paper, tosimulate this instability problem and in particular the specic failuremode, i.e., the combination of the brittle failure of the pillar rock be-tween the tunnel wall and the fault, and the shear yielding of thegouge. It is seen thatmodel is highly effective in reproducing the discon-tinuous behaviour of the rock mass (with a fault zone) and the inter-action between the ground, the TBM and the support system.

With this further extension, the 3D numerical model developedby the authors is capable to deal with the main mechanical adverseconditions for TBM tunnelling inmountain terrain (as for example illus-trated by Verman et al., 2012): brittle failure (spalling), squeezing andfault zones. Nevertheless, the main problem which still remains isthe geological fault zones with the so-called swimming gouge (i.e.,practically cohesionless material under high water pressure). It isexpected that a coupledhydraulic andmechanical analysiswill be intro-duced in the near future.

Acknowledgment

Thework of the present articlewas developedwithin the frameworkof the Youth Fund of the Natural Science Foundation of Jiangsu Province(no. BK 20130933) and the National Key Basic Research Program ofcrack initiation and crack damage stress thresholds of brittle rockmasses near under-ground excavations. Int. J. Rock Mech. Min. Sci. 41, 833847.

Dalgic, S., 2000. The inuence of weak rocks on excavation and support of the BeykozTunnel, Turkey. Eng. Geol. 58, 137148.

Dalgic, S., 2003. Tunneling in squeezing rock, the Bolu tunnel, Anatolian Motorway,Turkey. Eng. Geol. 67, 7396.

Diederichs, M.S., 2003. Manuel Rocha Medal Recipient, rock fracture and collapse underlow connement condition. Rock Mech. Rock. Eng. 36 (5), 339381.

Diederichs, M.S., 2007. The 2003 Canadian Geotechnical Colloquium: mechanistic inter-pretation and practical application of damage and spalling prediction criteria fordeep tunneling. Can. Geotech. J. 44, 10821116.

European Commission, Mobility & Transport, TEN-T/Transport infrastructures, 2011. Listof the pre-identied projects on the core network in the eld of transport. http://ec.europa.eu/transport/infrastructure/connecting/doc/revision/list-of-projects-cef.pdf.

Grandori, R., Beniawski, Z.T., Vizzino, D., Lizzadro, L., Romualdi, P., Busillo, A., 2011. Hardrock extreme conditions in the rst 10 km of TBM driven Brenner ExploratoryTunnel. Proc. of Rapid excavation & Tunnelling Conference 2011, San Francisco, USA.

Hoek, E., Guevara, R., 2009. Overcoming squeezing in the YacambQuibor Tunnel,Venezuela. Rock Mech. Rock. Eng. 42 (2), 389418.

Hoek, E., Marinos, P., 2010. Tunnelling in overstressed rocks. Rock engineering in difcultground conditions soft rocks and karst. Taylor & Francis Group, London.

Hoek, E., Marinos, P., Benissi, M., 1998. Applicability of the Geological Strength Index (GSI)classication for very weak and sheared rock masses. The case of the Athens SchistFormation. Bull. Eng. Geol. Environ. 57, 151160.

Kaiser, P.K., Diederichs, M.S., Martin, C.D., Sharp, J., Steiner, W., 2000. Invited keynote:underground works in hard rock tunneling and mining. Geotech. Eng.TechnomicPublishing, Melbourne, Pennsylvania (USA), pp. 841937.

Kontogianni, V., Stiros, S., 2005. Induced deformation during tunnel monitoring: evidencefrom geodetic monitoring. Eng. Geol. 79, 115126.

Kontogianni, V., Stiros, S., 2006. Convergence in Weak Rock Tunnels. 2. Tunnels andTunneling International, pp. 3639.

Kontogianni, V., Tzortzis, A., Stiros, S., 2004. Deformation and failure of the Tymfristostunnel, Greece. J. Geotech. Geoenviron. Eng. ASCE 130, 10041013.

Kontogianni, V., Papantonopoulos, Stiros, S., 2008. Delayed failure at the Messochoratunnel, Greece. Tunn. Undergr. Space Technol. 23, 232240.

Kovari, K., Fechtig, R., 2000. Historical Tunnels in the Swiss Alps. Staubli AG, Zrich.Loew, S., Barla, G., Diederichs, M., 2010. Engineering Geology of Alpine tunnels: past,

present and future. 11th IAEG Congress, Auckland, New Zealand.Marinos, V., Fortsakis, P., Prountzopoulos, G., 2006. Estimation of rock mass properties of

heavily sheared ysch using data from tunneling construction. IAEG2006, Notting-ham, United Kingdom.

Midas, 2010. Geotechnical and Tunnel Analysis System, User's Guide.Ramoni, M., Anagnostou, G., 2011. The interaction between shield, ground and tunnel

support in TBM tunnelling through squeezing conditions. Rock Mech. Rock. Eng. 44,3761.

Stiros, S.C., Kontogianni, V.A., 2009. Coulomb stress changes: from earthquakes tounderground excavation failures. Int. J. Rock Mech. Min. Sci. 46, 182187.

Verman, M., Carter, T.G., Babenderde, L., 2012. TBM vs D&B a difcult choice in moun-tain terrain some geotechnical guidelines. In: Harmonising Rock Engineering andthe Environment Qian & Zhou (Ed.), Taylor & Francis Group, London.

Vermeer, P.A., De Borst, R., 1984. Non-associated plasticity for soils, concrete and rock.Heron 29, 164.

Vlachopoulos, N., Diederichs, M.S., 2009. Improved longitudinal displacements proles forconvergence connement analysis of deep tunnel. Rock Mech. Rock. Eng. 42,131146.

Zhao, K., Janutolo, M., Barla, G., 2012. A completely 3D model for the simulation ofmechanized tunnel excavation. Rock Mech. Rock. Eng. 45 (4), 475497.lining leads to an additional ground deformations in this area and fur-

3D simulation of TBM excavation in brittle rock associated with fault zones: The Brenner Exploratory Tunnel case1. Introduction2. Project description2.1. The Brenner Base Tunnel2.2. The Aicha exploratory tunnel2.2.1. General features2.2.2. Construction history

3. Stability problem3.1. Instability phenomenon at chainage 6+1513.2. Geological model3.3. Rehabilitation works3.4. Resume of the excavation3.5. Risk management for the remaining stretch

4. Computational model4.1. Foreword4.2. Model construction4.3. Rock mass model4.3.1. Granitic rock mass (brittle failure)4.3.2. Fault zone

4.4. Main TBM components4.5. TBM advancement and simulation procedure4.6. Interaction between the machine components and the rock mass

5. Results and discussions5.1. Local instabilities5.2. Interaction between TBM, ground and tunnel support5.3. Load transfer along the longitudinal direction

6. ConclusionsAcknowledgmentReferences