切内里基线隧道——穿越瑞士阿尔卑斯山脉的平轨铁路线南段的施工经验

ResearchTunnel Engineering—ArticleThe Ceneri Base Tunnel:Construction Experience with the Southern Portion of the Flat Railway Line Crossing the SwissAlps Davide Merlini⇑,Daniele Stocker,Matteo Falanesca,Roberto Schuerch Pini Swiss Engineers,Lugano6900,Switzerlanda r t i c l e i n f oArticle history:Received5April2017 Revised11August2017 Accepted22September2017 Available online7April2018Keywords:Deep and long tunnelDifficult ground conditions Support designRisk managementTunnel monitoringLarge cavernsLow overburdenOverpass tunnelNumerical analysis Excavation in urban area a b s t r a c tThis paper summarizes the experience that was gained during the construction of the15.4km long Ceneri Base Tunnel(CBT),which is the southern part of theflat railway line crossing the Swiss Alps from north to south.The project consisted of a twin tube with a diameter of9m interconnected by cross-passages,each325m long.In the middle of the alignment and at its southern end,large caverns were excavated for logistical and operational requirements.The total excavation length amounted to approx-imately40km.The tunnel crossed Alpine rock formations comprising a variety of rock typologies and several fault zones.The maximum overburden amounted to850m.The excavation of the main tunnels and of the cross-passages was executed by means of drill-and-blast(D&B)excavation.The support con-sisted of bolts,meshes,fiber-reinforced shotcrete and,when required,steel ribs.A gripper tunnel boring machine(TBM)was used in order to excavate the access tunnel.The high overburden caused squeezing rock conditions,which are characterized by large anisotropic convergences when crossing weaker rock formations.The latter required the installation of a deformable support.At the north portal,the tunnel (with an enlarged cross-section)passed underneath the A2Swiss highway(the major road axis connect-ing the north and south of Switzerland)at a small overburden and through soft ground.Vertical and sub-horizontal jet grouting in combination with partial-face excavation was successfully implemented in order to limit the surface settlements.The south portal was located in a dense urban area.The excavation from the south portal included an approximately220m long cut-and-cover tunnel,followed by about 300m of D&B excavation in a bad rock formation.The very low overburden,poor rock quality,and demanding crossing with an existing road tunnel(at a vertical distance of only4m)required special excavation methods through reduced sectors and special blasting techniques in order to limit the blast-induced vibrations.The application of a comprehensive risk management procedure,the execution of an intensive surface survey,and the adaptability of the tunnel design to the encountered geological conditions allowed the successful completion of the excavation works.Ó2018THE AUTHORS.Published by Elsevier LTD on behalf of Chinese Academy of Engineering and Higher Education Press Limited Company.This is an open access article under the CC BY-NC-ND license1.IntroductionThe Ceneri Base Tunnel(CBT)is the southernmost portion of the New Railway Link through the Alps(NRLA)crossing the Swiss Alps from north to south[1].The client is the AlpTransit Gotthard Ltd., on behalf of the Swiss government.The NRLA is designed to create a continuousflat-rail connection from Basel to Milan,which will reduce travel times,increase the efficiency and sustainability of freight traffic,and connect Switzerland to the European high-speed railway network(Fig.1)[1].Along with the CBT,the main elements of the NRLA consist of the Lötschberg Base Tunnel (34.6km total length),which was constructed from1999to2006 and opened in2007,and the Gotthard Base Tunnel(57km total length),which was begun in1999and opened in December2016.The CBT is characterized by a twin tube system with a single-track railway tunnel with a length of15.4km linked through cross-passages that are spaced approximately325m apart (Fig.2)[1].The total excavation length amounts to approximately 40km and had an excavation volume of approximately4Â106m3. The cost of the works was about2.1billion Euro.The construction works started in1997with the drill-and-blast (D&B)excavation of the geological exploratory tunnel(3.1km in⇑Corresponding author.E-mail address:davide.merlini@(D.Merlini).Engineering4(2018)235–248 Contents lists available at ScienceDirectEngineeringlength),which was located approximately in the middle of the alignment (Sigirino).In 2008,an intermediate access adit (2.3km)—running almost parallel to the exploratory tunnel—was excavated using a gripper tunnel boring machine (TBM)(Fig.3(a)).Two large caverns (with a cross-section of about 270m 2)were excavated for logistical and operational reasons.The caverns were connected to the end of the intermediate access adit and of the exploratory tunnel.The excavation of the main tunnels started in 2010from the intermediate heading of Sigirino.The north (approximately 8.3km)and south (approximately 6.3km)tunnels were excavated simultaneously using D&B method (Lot 852,about 90%of thetotalFig.2.(a)Layout of the CBT [1]with photographs of (b)the north portal of Vigana (Camorino),(c)the intermediate heading of Sigirino,and (d)the south portal ofVezia.Fig.1.(a)The layout of NRLA project;(b)the profile of the Gotthard axis [1].236 D.Merlini et al./Engineering 4(2018)235–248excavation).The remaining portion of the tunnel was completed in backward driving lots at the north(Lot853)and south(Lot854) portals.In order to minimize the time and cost of construction,a total of eight excavation faces were operated simultaneously.The Sarèjunction caverns(Fig.3(b))are located in south of CBT and extend for a length of approximately400m.They are charac-terized by a variable geometry of the section,which consists of a conical pattern with a maximum cross-section of about260m2. At the end of the Sarèjunction caverns,there are two stub tunnels of approximately150m each,which will allow the construction of the future extension of the NRLA project through Italy.The excavation of the CBTfinished in March2015and January 2016with a breakthrough at the south and north portals,respec-tively.The civil works are currently under completion,and the installation of the railway infrastructure systems began in the summer of2017.The CBT is scheduled to be opened in2020.The cross-section of the CBT(Fig.4)has an excavation surface area of48–87m2and an optimized horseshoe shape[2].The shape is designed to meet the multiple requirements that result from the operational needs of the railway line,along with other require-ments related to the project execution and its geomechanical envi-ronment.A double lining is constructed along the entire CBT;the external lining provides temporary stability during construction,while the inner lining provides long-term stability for the tunnel for the entire project life(100years).The inner lining is mostly constructed using non-reinforced concrete,and is only reinforced over the tunnel portions that are characterized by poor geological conditions or geometrically com-plex zones(e.g.,connections with cross-passages).A drainage sys-tem is placed between the two linings and waterproofing is placed at the arch[3].Groundwater is collected from the lateral drains to a central drain,which evacuates the water toward the north portal.The inner lining was cast far from the excavation face.This allowed the rock mass to experience larger deformations;as a result,based on the ground response curve,this procedure reduced thefinal load on the inner lining(thus allowing a reduction in the thickness of the inner lining and in its reinforcement).Static vali-dations of the inner lining were systematically performed during construction,using the crosscheck information gained during excavation.2.Geological investigation campaign and geological conditions expected before excavationThe entire CBT is situated in the crystalline bedrock of the Southern Alps[4],as shown in Fig.5.The overburden of the tunnel varies from about10m to a maximum of850m.A detailed survey campaign with a total bored length almost equal to that of the main tunnel,and with single boreholes of lengths up to700m, was executed during the period1991–2008.Based on geological and geotechnical conditions,it was possible to distinguish and characterize47homogeneous sections in order to perform an assessment of probable risk scenarios[5],as shown in Fig.6.The lithostratigraphic units crossed by the tunnel can be divided into two areas:in the north,the Ceneri Zone(10.4km); and in the south,the Val Colla Zone(5km).The Ceneri Zone is primarily formed of gneiss and,to a lesser extent,of basic and ultra-basic rocks(e.g.,amphibolites and serpentinites)thathaveFig.5.Forecast geological longitudinal profile of theCBT.Fig.4.(a)CBT cross-section;(b)3D rendering of the completedCBT.Fig.3.(a)The gripper TBM used for the excavation of the Sigirino access adit;(b)the D&B excavation of the Sarèjunction caverns.D.Merlini et al./Engineering4(2018)235–248237all undergone considerable metamorphism (amphibolite facies).The whole area is also affected by complex tectonics,of which the main consequence is a high dip angle for the schistosity,litho-logical contacts,and axial plane folds.The Val Colla Zone also includes a series of paragneiss and orthogneiss,combined with intermediary basic rocks (hornblende schists).Above all,it is their structural geometry that sets these two areas apart.In the Val Colla Zone,the planar structures follow sub-horizontal trends,while they are more vertical in the Ceneri Zone.The interface between these two areas,which is 600m in thickness according to forecasts,is formed by the ‘‘Val Colla Line (VCL),”which is mainly composed of mylonites (fine-grained schists,often with a high mica content)resulting from intense pre-Alpine ductile deformation,probably dating back between 290million and 240million years.Diaclases run through the entire rock mass and the project crosses some fault areas (fragile Alpine tectonics,especially Insubric phase).Although the rock mass is covered by alluvial,detrital,fluvio-glacial,and glacial Quaternary sediments,the digging for the base tunnel only goes through loose materials in the areas around the north and south portals.During the construction works,only minor water inflows were expected and the permeability of the rock mass was found to be generally low.3.Excavation methodsThe excavation methods were based on the geological prognosis results and on other requirements such as the environmental impact,logistics,and the timing/investment ratio.Besides the two shorter conventional tunneling works at the portals,the main excavations were carried out from the underground cavern located at the intermediate heading of Sigirino.Based on extensive geolog-ical investigations,the final design phase was closed with a high confidence in the selected excavation methods of the CBT.To sum-marize these methods,TBMs were planned for the tubes (a single-shield TBM for the south direction and a double-shield TBM for the north direction),while D&B method was limited to a short connec-tion from the Sigirino access adit to the Val Colla Line,and the cross-passages.In the review phase of the final design,the excavation of the northern portion of the CBT by means of TBM was abandoned due to the high risk of shield jamming (estimated tunnel radial conver-gence up to 30cm)in correspondence to the fault zones (22fault zones,having a possible extent ranging from 10to 110m).For theexcavation of the southern portion,both TBM and D&B methods were tendered as possible excavation methods.For economic rea-sons,the contractor who was awarded the construction of the main tunnels (Lot 852)selected the D&B method for the excavation of the entire tunnel (this option was about 10%cheaper than the TBM option).The owner and the contractor mutually decided to use a highly mechanized D&B method using efficient and modern heading installations [6],as shown in Fig.7.Each installation was planned to lead financial and technical improvements and consists of a ventila-tion platform that can be moved independently,a heading platform,an invert platform,a crusher,conveyors,and a single-track suspen-sion rail [7].4.Rock support classesA total of 10standard rock support classes (see the cross-sections in Fig.8)were designed in order to deal with the geotech-nical conditions that were expected along the tunnel drive of the CBT.The rock support classes range from SPV 1to SPV 10;the SPV 1to 4classes are characterized by flat invert slab,frictional bolts,and shotcrete reinforced with fibers;the SPV 5and 6classes are characterized by curved invert slab,fully grouted bolts,shot-crete reinforced with mesh or fibers,and reinforcing ribs;and the SPV 7to 10classes are characterized by a pseudo-circular shape,radial and face injected bolts,shotcrete reinforced with mesh,and reinforcing ribs with yielding elements [8,9].It should be noted that the distance between the location of the rock support installation and the tunnel face depends on the rock support class;in general,the higher the rock support class,the smaller the distance between the rock support installation and the tunnel face.The rock support classes SPV 3-accelerated (SPV 3acc)and SPV 6A were conceived during construction;the stan-dard rock support class SPV 3acc is an intermediate classbetweenFig.6.Risk scenarios assumed in the design phase of theCBT.Fig.7.The highly mechanized D&B method used for the CBT excavation.238 D.Merlini et al./Engineering 4(2018)235–248SPV2and SPV3that allowed a faster advance rate(in particular toward the north),while SPV6A is characterized by the installation of Toussaint–Heintzmann(TH)ribs and a larger over-excavation. The larger over-excavation allowed larger rock mass convergences (15cm at the tunnel crown and23cm at the wall);this reduced the load on the lining,and thus decreased the need to install the heavier—and more time consuming—rock support classes SPV7 to10.Fig.9shows the daily advance rate and costs for each standard rock support class,as well as a comparison between the cumulated tunnel lengths of installed rock support classes as foreseen in the design phase and those installed during construction.Due to the encountered geotechnical conditions,the lighter rock support classes SPV1and2were not adopted during construction;the ‘‘medium-to-heavy”rock support classes(SPV4to10)were installed over the majority of the tunnel length.A selection of the most appropriate rock support is essential to ensure the safety of the construction works while complying with the project time schedule and costs(i.e.,minimization of the project risks).Theflow chart in Fig.10shows the phases of the decision-making process that were adopted for the CBT;this pro-cess was aimed at mitigating and managing risk scenarios while ensuring the optimal choice of rock supports,according to Ref.[10].The decision-making process involves the designer,the construction site manager,and the contractor.The six phases of the decision-making process shown in Fig.10 are described as follows:(1)Planning.This entails an interpretation of the geological survey(along the profile and from the surface),laboratory investi-gation results,previous monitoring data,and results of numerical calculations in order to evaluate the most probable risk scenarios.(2)Standard cross-section choice.This is based on the consid-erations of the design phase,and the standard cross-section is chosen.The choice is made in agreement with the designer,con-struction site manager,and contractor,and all the information relating to the excavation(i.e.,type and number of installed safety measures,risk scenarios encountered,tunnel advance length,etc.) is collected in the module for safety measures.(3)Excavation.This is performed according to the standard cross-section chosen.(4)Check of the correct standard cross-section chosen.This is done through interpretation of geotechnical survey and monitoring data.In the case of successful testing,it is possible to proceed with the selected standard cross-section.In the case of too-conservative or too-light supports,the standard cross-section must be updated.(5)Verification of structural safety and serviceability.If the standard cross-section is too light,leading to an increase in defor-mations and possible cracks in the concrete,etc.,the structural safety must be checked.If the structural safety is not compromised, advance work can continue with a new,stronger cross-section.If structural safety is not achieved,replacement actions and the installation of additional supports must be carried out before exca-vation can continue.The choice of rock supports is made in agree-ment with the designer,construction site manager,andcontractor. Fig.9.(a)Daily advance rate and costs forecasted;(b)comparison between the forecasted standard cross-section and the executedone.Fig.8.Standard cross-sections used for the CBT excavation and related risk scenarios.D.Merlini et al./Engineering4(2018)235–248239(6)Design and execution of replacement measures.The works must be performed in the shortest possible time while ensuring optimal safety conditions.Tunnel excavation can only continue once the replacement measures and installation of addi-tional supports are completed.During tunnel construction,the geological conditions ahead of the tunnel face were evaluated by means of systematic probe dril-ling.Core drilling was carried out only before entering known major fault zones.A detailed face mapping was carried out after each round length.5.The tunneling experienceIn general,the rock mass conditions that were encountered during the excavation agreed well with the ones expected in the design phase(based on the extensive survey campaign). However,due to the intrinsic complexity of a long and deep tunnel and the complex behavior of some rock formations, unexpected events occurred during excavation.The following sections describe the major challenges that were encountered during excavation.5.1.Squeezing conditions and strong asymmetric convergences in the intermediate heading of Sigirino and CBT northThe excavation of the logistics caverns and the neighboring CBT toward the north was performed within the Giumello gneiss (GiuG)formation,under a high overburden with maximum values of approximately800m.Using the two-dimensional(2D)distinct element code UDEC TM [11],the stress-strain behavior of the different types of rock mass encountered was evaluated.The main geological and geotechnical structures(joints,fault system,and disturbed zones)were defined on the basis of the forecast for each macro-structural area and on the basis of the geometrical characteristics(i.e.,tunnel direction and overburden).For the CBT excavation,according to the numerical results, maximum displacement would occur close to the fault zones and should amount to about10cm.Higher deformations than those predicted were observed during the excavation in the geological formation of GiuG[12]. Compared with the prognosis,the geological response of the rock mass showed behavior that was strongly influenced by the fault zones(i.e.,faults and shear zones formed by cataclasites of up to 1m thickness,or cataclasites with kakirite of up to20cm thickness)subparallel to the direction of excavation and to schis-tosity that significantly increased the structural anisotropy (Fig.11).The displacement monitoring performed by three-dimensional(3D)stations and multipoint extensometers showed high values of deformation of30–40cm for the CBT’s northwest advance.A strongly asymmetric pattern was found,with velocity values that were not assumed to decrease even at an outstanding distance from the face(Fig.12)[13].As a consequence,important cracks in thefirst lining occurred(Fig.12).To achieve an optimal understanding of the deformation behavior of the rock mass,sev-eral back-analyses[14]were performed using numerical calcula-tions(Fig.13).The latter allowed a correct validation of the mitigation measures as well as the redesign of thefirst lining and of the second lining.Due to the structural anisotropy of the rock masses,asymmetrical convergences frequently occurred dur-ing the excavations of the CBT north(leading to the local formation of cracks in the primary lining).The convergences were higher in the east side of the tunnel crown and in the west side of the tunnel wall.The asymmetric convergences required the installation of longer bolts(compared with the standard rock support classes) and/or the increase in the bolting density over15%–20%of the overall north CBT excavation.5.2.Excavation through a major fault zone under high overburden in CBT south:The Val Colla LineThe major fault zone that was expected and encountered during the CBT excavation is the so-called VCL[15,16],anintensely Fig.11.Geological plan of the study area.(a)Forecast;(b)crosscheck.NE:northeast;NW:northwest.Fig.10.Aflow chart for the choice of rock support class:①planning;②standardcross-section choice;③excavation;④check of the correct standard cross-sectionchosen;⑤verification of structural safety and serviceability;and⑥design andexecution of replacement measures.240 D.Merlini et al./Engineering4(2018)235–248tectonized zone with a length of 658and 529m in the east and west tubes,respectively.During excavation,the VCL was encoun-tered about 200m earlier than expected (Fig.14).The major issue related to the deviation between the geological forecast and the encountered geology was related to the higher overburden (approximately 450m instead of 350m),and thus to the higher stress state.The encountered rock mass formations were intensely tectonized,and were as follows:Formation 1a:an alternation of phyllites,mylonite,and mica schist;Formation 1b:gneiss;Formation 2:a heterogeneous mixture of lithotypes 1a and 1b with kakirite-type fault breccia and fault-gauge;and Formation 3:a kakirite-type fault-gauge.The most unfavorable geotechnical section—which consisted of alternating fault breccia and fault-gouge-type kakirite,mylonite,and cataclasite,which was several meters thick—was found over a length of about 250m.For the excavation of rock mass formations 2and 3,the SPV 7to 10cross-sections were adopted;these are characterized by a pseudo-circular shape,radial and face injection bolts,shotcrete reinforced with mesh and reinforcing ribs,and yielding support with the presence of slots.The combination of extremely poor rock mass conditions and high overburden led to strongly anisotropic deformations (peak values up to about 80cm,Fig.15),loosening and,finally,to a major face instability (about 150m 3of material).To overcome the sec-tion where the collapse occurred,piles,shotcrete reinforced with mesh,reinforcing ribs (type TH 29),and radial and grouted face bolts were installed (Fig.16).Analytical calculations were also performed in order to check the tunnel face stability (based on Ref.[17]).Over the remaining portion of the VCL,the encountered geotechnical conditions were more favorable than expected,as shown in Table 1.Fig.14.Geological plan of the study area:forecast (green)and crosscheck (red)VCL limits.Tm:tunnel meter used in CBT;ATKm:chainage in meter of the NRLAAlpTransit.Fig.13.(a)Back-analysis conducted using the 2D UDEC TM code;(b)replacement actions underconstruction.Fig.12.(a)Monitored displacements (unit:mm)in section NW9;(b)cracks in the first lining for the CBT’s northwest advance in the GiuG formation.D.Merlini et al./Engineering 4(2018)235–248241To achieve an optimal understanding of the deformation behav-ior of the rock mass,several back-analyses were performed using finite-element method (FEM)analyses.The latter allowed a correct validation of the mitigation measures as well as the redesign of the first and second linings [18].5.3.Excavation through a major fault zone under high overburden in CBT north:The Val Mara ZoneFor the north advance,the most critical geotechnical section was represented by the Val Mara Zone (VMZ).The VMZ was char-acterized by a length of about 150m,a maximum overburden ofabout 800m,and the widespread presence of kakiritic and cata-clastic levels embedded in mixed gneiss and intensively tectonized.In this section,as forecast in the design phase,the SPV 7to 10cross-sections were adopted;these are characterized by a pseudo-circular shape,radial and face injection bolts,shotcrete reinforced with mesh and reinforcing ribs,and yielding support with the presence of slots.During excavation,the VMZ was encountered approximately 140m (Fig.17)earlier than expected.The poor ground conditions led to strongly anisotropic deformations (with peak values up to 40cm)and finally to a face instability of 250m 3(Fig.18).Umbrella pipes,shotcrete reinforced with mesh,reinforcing ribs (type TH 29),and radial and face injection bolts were installed before restarting the excavation (Fig.19).Several back-analyses were per-formed using FEM calculations in order to check the external lining and the consequences on the inner lining;an analytical calculation was also performed in order to check the tunnel face stability (based on Ref.[17]).5.4.Excavation of large caverns under difficult rock mass conditions in CBT south:The SarèjunctionThe Sarèjunction,which is located in CBT south (Fig.2),is char-acterized by a twin tunnel system of 400m in length,a coneshape,Fig.15.(a)Radial deformations monitored in targets located in different cross-sections and different positions;(b)geomechanical model for the CBT east tube section between chainage 945and :central tunnel crown;CD:tunnel crown west side;CS:tunnel crown east side;PD:sidewall west side;PS:sidewall east side;MD:footing west side;MS:footing eastside.Fig.16.Cross-section SPV 10with yielding support and a pseudo-circular shape,as adopted for the VCL excavation.Table 1Comparison between rock mass formations and rock support classes as planned in the design phase and as built (VCL excavation southeast tube).Rock mass formationsDesign phase (total length =620m)As built (total length =658m)1a1b 231a 1b 23Section length 17%27%41%15%44%27%27%2%Cross-section typeSPV 7/8SPV 9/10SPV 4/6/7SPV7–10Fig.17.(a)Geological longitudinal section forecast;(b)crosscheck.HB:homogeneous geomechanical section.242 D.Merlini et al./Engineering 4(2018)235–248a 260m 2maximum cross-section area,and an overburden of 150m.At the end of Sarècaverns,there are two stub tunnels of approximately 150m each;these allow the excavation of the future extension through Italy,while minimizing the disturbance to the CBT.Based on core boreholes from surface,laboratory,and in situ investigation,gneiss and orthogneiss formations and two disturbed zones were forecast (Fig.20).Numerical calculation with a distinct element method code was implemented and a maximum displacement of 80mm was forecast;no problem was therefore expected on the external ring.In the execution,shortly after the first 80m of excavation,a strong anisotropic rock mass with a low dip angle sequence of layers consisting of cataclasticfault-rock alternating with schistose gneiss with better rock matrix characteristics was found [19].Compared with the analysis in the design phase,higher deformations and cracks in the first lining occurred during the full-face tunnel crown excavation (Fig.21).Through the back-analysis of massive deformation monitoring and geomechanical data gained from horizontal core boreholes,an accurate geomechanical model was built (Fig.22).With this new model,a comparative numerical analysis was performed with full-face tunnel crown excavation and with the tunnel crown exca-vated in sectors.From these results,it was decided to carry out the remaining excavation with the tunnel crown excavated in sectors (Fig.23).The supports installed comprised a shotcrete lining of 35cm reinforced with two mesh layers;unlike the design phase,injection bolts of 10m and 8m and lattice girders of four /32mm spaced 1.2m apart were adopted.Further analysis was made to excavate peculiar cross-sections of the twin tunnels that were separated by a rock pillar only 3m wide,corresponding with the terminal part of the Sarèjunction.The external lining for the stubs and the CBT tunnel consists of 25cm of shotcrete reinforced with fibers;unlike the design phase,injection bolts of 8m together with TH 25reinforcing ribs (spacing of 1m)were installed.The caverns were excavated with a mechanical method (i.e.,hammer)in order to minimize the effect of nearby excavations.Due to the supports installed and the excavation technique adopted,there were no significant cracks in the shotcrete lining and themaximumFig.18.(a)Photo of the tunnel face instability occurred in the VMZ;(b)interpretation of thecollapse.Fig.19.Cross-section SPV 10with yielding support and a pseudo-circular shape,as adopted for the VMZexcavation.Fig.20.(a)Forecast geological plan with study cross-section;(b)Sarè3predictive model with forecast displacements (unit:mm)calculated by the UDEC TM code.D.Merlini et al./Engineering 4(2018)235–248243。