Geological Engineering Problems Associated With Tunnel Construction in Karst Rock MassesThe Case of Gavarres Tunnel (Spain)

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  Geological engineering problems associated with tunnel construction in karst rockmasses: The case of Gavarres tunnel (Spain) S. Alija  a,1 , F.J. Torrijo  b, ⁎ , M. Quinta-Ferreira  c,1 a Geosciences Center, University of Coimbra, 3000-272 Coimbra, Portugal b Department of Earth Engineering, Universidad Politécnica de Valencia, 46022 Valencia, Spain c Department of Earth Sciences, Geosciences Center, University of Coimbra, 3000-272 Coimbra, Portugal a b s t r a c ta r t i c l e i n f o  Article history: Received 24 April 2012Received in revised form 2 February 2013Accepted 16 February 2013Available online 4 March 2013 Keywords: Tunnelling in karstKarsti 󿬁 ed rock massesRetrospective analysisGavarres tunnelNATM Arepresentativeexampleoftheproblemsassociatedwiththeexcavationandsupportoftunnelsinkarstgroundispresented. Itisa peculiarcaseinterms ofheterogeneityand spatial distributionof zones of poor geotechnicalquality, requiring the need to de 󿬁 ne, preferably in the study phases, adequate site investigation, suitable designprocedures,ef  󿬁 cientconstructiontechniquesandappropriategroundtreatment.Thedif  󿬁 cultiesassociatedwiththe instability of the karsti 󿬁 ed ground, and the presence of cavities, wholly or partially  󿬁 lled with soils of lowcohesion, are discussed via retrospective analysis. The solutions adopted to solve the problems encounteredduringthetunnelconstructionenabledasystematicapproach,usefulfornewconstructionprojectsinlimestoneterrains of medium to high karsti 󿬁 cation.© 2013 Elsevier B.V. All rights reserved. 1. Introduction The design and construction of tunnels in karst terrains is fraughtwith problems associated with the unexpected location, irregulargeometry and unpredictable dimensions of the karst structures.In a karsti 󿬁 ed terrain, prospection and regular testing campaignsshould be supplemented with other techniques adapted to locateand anticipate the problematic zones. It must be taken into accountthat no site investigation technique is one hundred percent accurate,and therefore several techniques should be used, adapted to eachspeci 󿬁 c situation, taking into consideration the budget for the workand the risks that can be assumed in the project.Arealcaseofatunnelconstructedinakarsti 󿬁 edlimestonegroundispresented, the problems encountered are described and the proposedsolutions are discussed. A systematic approach, as a knowledge toolfor future work in similar situations, is presented. 2. Geological framework  From the geological point of view, the study area is located in theLes Gavarres region, which is included within the Catalan TransverseSystem, directly related with the Neogene depression of the Empordà(Agustí et al., 1994).Les Gavarres region consists of a fringe of Palaeogene materials(mainlyEocene),arrangedaroundaHercynianrockmassif,outcroppingsouth of thestudy area. Theage of these materials is prior to theAlpineOrogeny, as they have suffered deformation and fracturing during thistectonic phase. The series is dislocated in blocks, separated by fracturesthatlead to theuplifting of the massif. The general structure is a mono-clinearrangement,dippingmainlytoNortheast(IGME,1983,1995).Thegeological formations affecting the tunnel are (Figure 1) the following: ã  Barcons SandstoneFormation ( E   A ).Itiscomposedbyglauconitic sand-stone, mediumto coarsegrained,locallyconglomeratic.The predom-inant colour is grey-yellowish or ochre. The grains are mainly of quartzandfeldsparwithascarceclaymatrix.Ithascalcareouscementandabundantbioclasts.Atthebaseandtopoftheseries,thelayersaredecimetric to metric, presenting a more massive appearance in themiddle of the formation. The average sedimentation corresponds toa deposit in the frontal area of the delta, which is rather thick, but of limited extent. The age of the series is Eocene. ã  Banyolas Limestone Formation  ( E  M  ). This formation is composed of layers of limestone and marl, whose relative proportion variesthroughout the series. They are of grey and bluish grey colours, andthe layers have decimetric thickness. The carbonate content rangesfrom marly clay to limestone, affecting materials' strength, weather-ability and the stability behaviour of the rockmass. Some spans of the series are mainly composed of hard clay and marls. The age of  Engineering Geology 157 (2013) 103 – 111 ⁎  Corresponding author. Tel.: +34 963877582. E-mail addresses:  santiagoalija@gmail.com (S. Alija), fratorec@trr.upv.es(F.J. Torrijo), mqf@dct.uc.pt (M. Quinta-Ferreira). 1 Tel.: +351 239860500.0013-7952/$  –  see front matter © 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.enggeo.2013.02.010 Contents lists available at SciVerse ScienceDirect Engineering Geology  journal homepage: www.elsevier.com/locate/enggeo  the series is Eocene. It is important to note that the Banyolas Lime-stoneFormationis inconcordancewith the underlying GironaFossil-iferous Limestone Formation. ã  Girona Fossiliferous Limestone Formation  ( E  C  ). It is a fossiliferous lime-stone, presentingoolitic terms atthebase. The predominantcolourisochre.Itisratherrecrystallizedandarrangedinlayersofawiderange Fig. 1.  Location map and geology pro 󿬁 le along the Gavarres tunnel.104  S. Alija et al. / Engineering Geology 157 (2013) 103 – 111  ofthickness,fromdecimetrictometric.Theenvironmentofsedimen-tation corresponds to proximal marine environments of carbonateplatform. The age of the series is Eocene. ã  Pontils Group Conglomerates  ( E  CG ). This formation is constituted byconglomerates and red sandstones with clay layers. These depositshave  󿬂 uvial srcin. The age of the series is Lower Eocene, but mayalso include part of Palaeocene.The boundary between the Les Gavarres region and the SW marginoftheAmpurdándepressionismarkedbyafracturealignmentorientedNW – SE, calledBanyolas Fault orCamós – Celrá.This alignment is part of a system of fractures orientated predominantly NW – SE. They arenormal faults related to quaternary volcanism and recent seismicity.This important regional fault intersects the line of the tunnel, corre-sponding to intense fracturing of the rock material. 3. Geotechnical characteristics Accordingtothegeologicalcrosssectionde 󿬁 nedinthedesign,mostof the tunnel would be excavated in the materials of the Banyolas MarlFormation (Figure 1) while the northern part is affected by the faultsystem associated with the Banyolas Fault or Camós – Celrá.The two fundamental geotechnical units are described below. Theresults of the laboratory tests, from samples collected in the tunnelboreholes, are shown in Tables 1 and 2: ã  Limestone and Marl Geotechnical Unit  . This unit is entirely constitutedby calcareous rocks of the Banyolas Limestone Formation (E M ). Therock samples tested generally present medium to low strength, witha weathering grade, in the vicinity of the tunnel, ranging from III toV (according to ISRM, 1981). The seismic pro 󿬁 les carried out in thetunnelcon 󿬁 rmed this data.The watertabledetectedintheboreholeswas located below the invert of the tunnel. The average densities(Table 1) and uniaxial compressive strength (Table 2) gave very scattered values, depending on the degree of weathering of thesample(Bartonetal.,1974).Duringthesiteinvestigationprogramme,permeability tests revealed medium – low permeability terrains(González de Vallejo et al., 2002), around 1 × 10 − 7 m/s. Consideringthe RQD values obtained in the borehole samples and the uniaxialcompressive strength, a representative RMRvalue of 30 wasestimat-ed (class IV or Bad, Bieniawski, 1989). ã  Fault ZoneGeotechnical Unit  ( E  M  veryfractured ).Itisahighlyfracturedzone,whereargillite, calcareous myloniteand marl have been identi- 󿬁 ed. The rock weathering ranges from grade II to V (according toISRM, 1981). Water levels were found at different heights, associatedwithfractureplanes.Althoughmostoftheunitconsistsofhighlyfrac-tured limestone and marl, from the Banyolas Limestone Formation,the presence of a small thickness of Girona Fossiliferous Limestone(E C ) was also observed in boreholes as well as conglomerates andred sandstones of the Pontils Group (E CG ). Both formations presentweatheringgradesofIV  – V(accordingtoISRM,1981).Thepermeabil-ity tests showed low to medium values, similar to those usuallypresented by fractured rock masses of limestone and dolomite(1 × 10 − 6 m/s). Mainly based on the RQD values of the rock coresand on the uniaxial compressive strength values, an RMR value of 20 was estimated (class V or Very Poor, Bieniawski, 1989). 4. Construction project The tunnel is part of the Madrid – French border high-speed railwayline,andislocatedwithintheprovinceofGirona(Figure1).Itisadoubletrack tunnel having a total length of 758 m with a maximum overbur-denof31 m.Theaveragealtitudeofthetunnelis93.5 mabovesealevel.Thefreesectionofthetunnel,de 󿬁 nedintermsofhealthandcomfortcriteria,was110 m 2 .Thegeometriccharacteristicsofthetunnel'scrosssection were designed using a circular vault extending into the  󿬂 oor,without differentiating the gables (López, 1996).Havinginmindthecharacteristicsoftheinsitumaterialsandthedi-mensions of the tunnel, it was considered that the mechanical excava-tion was the most suitable procedure and that blasting could be usedin the unweathered limestone zones (Díaz, 1997).  Table 1 Sieve analysis and consistency limits (Atterberg limits) of the soil materials of the Gavarres tunnel.Geotechnical unit Characteristics Values Speci 󿬁 c gravity(g/cm 3 )Unit weight(g/cm 3 )Atterberg limits Sieve analysisLiquid limit(%)Plastic limit(%)Plasticity index(%)Gravel(%)Sand(%)Silt or clay(%)Limestone and foam Test number 4 11 30 30 30 29 29 29Bedrock altered rock Max 2.67 2.06 38.00 21.00 18.00 64.00 38.00 98.00Min 1.76 1.54 23.40 12.40 7.90 0.00 2.00 25.00Mean 2.12 1.79 30.09 16.34 13.75 14.11 15.74 70.26Fault zone Test number 15 10 10 10 10 10 10 10Bedrock altered rock Max 2.55 40.30 18.70 21.60 21.00 49.00 98.00Min 1.71 26.80 13.00 11.90 0.00 1.00 30.00Mean 2.03 33.31 16.34 16.97 3.60 10.80 85.60  Table 2 Strength parameters of the Gavarres tunnel obtained over rock cores tested in the laboratory.Geotechnical unit Characteristics Values UCS(kg/cm 2 )Triaxial test (CU) Direct shear test (CD)Effective cohesion(kg/cm 2 )Effective friction angle(°)Effective cohesion(kg/cm 2 )Effective friction angle(°)Limestone and foam Test number 18 4 4 1 1Bedrock altered rock Max 183.6 0.42 35.83 0.10 44.00Min 0.3 0.14 24.95 0.10 44.00Mean 26.03 0.24 31.12 0.10 44.00Fault Zone Test number 1 1Max 0.3 28Min 0.3 28Mean 0.3 28105 S. Alija et al. / Engineering Geology 157 (2013) 103 – 111  The design recommended the use of the New Austrian TunnellingMethod (NATM), since it could allow pre-support during tunnel ad-vance, through mechanical pre-cutting.Theexcavationphasesusedinthetunnelwereasfollows:oneexca-vation phase in full section in the top heading, two excavation sub-phases in the bench and one excavation phase in the inverted vault.Inthedesignofthetunnelsupport,threesectiontypeswerede 󿬁 ned(Figure 1), ranging from the better quality terrains to the weakest(Hoek and Brown, 1980; Hoek et al., 1995): ã  S-II: this section type applies to the weathered calcareous rocks of the Banyolas Limestone Formation. The excavation should beperformed in advances of 1.0 m in the top heading, with primarysupport based on a 5 cm sealing of shotcrete with steel  󿬁 bre, lightsteel ribs type TH-29 and shotcrete with steel  󿬁 bre, 25 cm thick intotal (excluding the 5 cm of sealing). The two sub-phases of thebench were implemented in 2.0 m spans extending the support of the top heading. ã  S-III:wasusedforthefaultzoneunit.Inthissectiontype,theexcava-tionwouldbedoneinadvancesof0.5to1.0 mwithsupportbasedona 5 cm sealing of shotcrete with steel  󿬁 bre, heavy steel ribs of type HEB-160 and shotcrete with steel  󿬁 bre, 30 cm total thickness(excluding the 5 cm of sealing). The drilling of the bench would bedone in two sub-phases, with advances from 1.0 to 2.0 m extendingthe support of the top heading. ã  S-E:wasthesectiontypeforthetunnelportals.Itwascharacterisedastype “ heavy ” asthesezoneswereexpectedtobemoreweathered,de-compressed due to the previous excavation of the portal slopes andpresentingaratherthinoverburdenabovethetunnel.TheS-Esectionconsisted of a heavy micropile umbrella, 20 m long and 150 mm indrilling diameter, spaced 0.5 m between axes and  󿬁 tted with steelpipes,110 mmofexternaldiameterand8 mmthick, 󿬁 lledwithmor-tar. The excavation and support sequence for this section would besimilar to S-III, with the difference that the steel ribs used below theumbrella would be type HEB-180.All sections should have a concrete inverted vault with weldedwire mesh 150 × 150 × 6 mm.A summary table with the support structures de 󿬁 ned for the tun-nel is presented in Table 3. 5. Construction of the tunnel The construction of the Gavarres tunnel began by its south portalin limestone materials (Figure 1). First, the excavation and supportof the portal slopes was carried out. The excavation was done usingmechanical heavy duty rotating machines. During this early stage of excavation, the heterogeneity of the limestone rock mass wasdetected. The working face presented very weathered areas whichare easy to excavate and, alternating with limestone, are very dif  󿬁 cultto break mechanically.Once at the tunnel crown level, a micropile umbrella was carriedout, for the S-E section type (35 micropiles in total). During theimplementation of these micropiles, the heterogeneity of the groundcontinued to be revealed, since the implementation speed rangedfrom 1 to 4 micropiles per day. The drilling residues changed drasti-cally from limestone fragments to a clay-like material.According to the geotechnical characteristics of the ground duringexcavation and support, mainly associated with the karsti 󿬁 cation pro-cesses, different zones were considered along the tunnel (Alija, 2010): ã  Portal Zone  ( sections 0 –  22 ). The excavation of the tunnel startedwith mechanical equipment, reaching an average progress speedof 4.7 m/day. In this zone, four sections of convergence wereinstalled and eight engineering geology front maps were prepared.The ground materials were characterised as blocks of limestone andmarl, sometimes broken, embedded in a clay matrix. The strati 󿬁 ca-tion was as follows:   S  0 : oriented between 200/15 and 200/30 (dip direction/dip angle),withsomecontinuityandsomeroughness.Betweenlayers,openingsof 5 to 10 mm were observed,  󿬁 lled with clay or even calcite.Two families of joints were identi 󿬁 ed (Figure 2):   J  1 : with an average orientation of 213/71, spaced about 30 cm,with some continuity and, when  󿬁 lled, it is with clay material.   J   2 : with an average orientation of 124/70, spaced from 20 to60 cm, very rough and usually closed.These two families of joints and the strati 󿬁 cation maintained theirorientation all along the tunnel, but due to the heterogeneity of the rock mass, they were not found or distinguished on all of thefronts mapped.According to the front reports, the average RMR value obtained forthis area was 36, corresponding to a rock mass of class IV (poorgrade). During the excavation and support operations, small fallsof rock and clay occurred. In sections 2 and 3, the instabilities inthe roof of the tunnel achieved 12 m 3 . Detachments were also  Table 3 Summary of the support structures proposed for the three section types of the Gavarres tunnel.Section type Geotechnical unit RMR Excavation/pass Shotcrete with steel 󿬁 bre (cm)Trusses Special systemsS-II Limestone and loam 30 – 45 Top heading (1 phase) and bench(2 phases) 2 m bench5 (sealing) + 25HM-35TH-29//1 – 1.5 mElephant footInverted footS-III Fault zone 20 – 29 Top heading (1 phase) and bench(2 phases) 2 m bench5 (sealing) + 30HM-35HEB-160//0.5 – 1 mElephant footInverted footSE OutletsPortals b 19 Top heading (1 phase) and bench(2 phases) 2 m bench5 (sealing) + 30HM-35HEB-180//0.5 – 1 mHeavy micropile umbrellas ϕ  = 150 mmElephant footInverted vault S 0 J 1 J 2 Fig. 2.  Tunnel-working face view of section 19 with details of the strati 󿬁 cation (S 0 ) and joints (J 1 , J 2 ). Marl and limestone blocks, some broken, in a clay matrix.106  S. Alija et al. / Engineering Geology 157 (2013) 103 – 111
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