Innovative tunneling construction method to cope with squeezing at the Saint Martin La Porte access adit (Lyon-Turin Base Tunnel)

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1 Innovative tunneling onstrution method to ope with squeezing at the Saint Martin La Porte aess adit (Lyon-Turin Base Tunnel) G. Barla Politenio di Torino, Department of Strutural and Geotehnial Engineering, Torino, Italy ABSTRACT: Reent innovations in yield-ontrol support systems applied to onventional tunneling in diffiult ground onditions assoiated with squeezing are disussed in this leture. The Saint Martin aess adit along the Base Tunnel of the Lyon-Turin railway link is presented as a ase study to illustrate some of the developments whih have been implemented to deal with squeezing onditions enountered during exavation in the Carboniferous Formation. Also disussed are in situ performane monitoring, laboratory investigations, and advaned modeling studies performed with the aim to improve the understanding of squeezing behavior, in view of the design of the Base Tunnel. 1 INTRODUCTION Tunnel onstrution in squeezing onditions is very demanding due to the diffiulty in making reliable preditions at the design stage. Even during exavation suh onditions are not easily antiipated. Squeezing ours in low strength and high deformability ground in ombination with great depth and, eventually, high water pressure, assoiated with large time-dependent deformations. Squeezing may vary over short distanes due to flutuations in the mehanial and hydrauli properties of the rok mass. If onsideration is given to the onstrution of deep tunnels (suh as the Alpine Tunnels), alignment onstraints, and unertainties of geologial exploration, it is not always possible to avoid the diffiult ground whih may result into squeezing onditions. Therefore, the seletion of the most appropriate exavation-onstrution method to be adopted is highly problemati and unertain. In mehanized tunneling, due to the fixed geometry and the limited flexibility of the TBM (Tunnel Boring Mahine), allowable spae to aommodate ground deformations is restrited. On the ontrary, in onventional tunneling, although a onsiderably larger profile an be exavated in order to allow for large deformations, inevitably exavation will take plae with a low rate of advane. It is however true that, if the work at the fae is well planned and appropriate stabilization measures are implemented, exavation may proeed at an aeptable rate of advane even in severely squeezing onditions. On the ontrary, if anything goes wrong in mehanized tunneling, exavation is signifiantly hindered and in ases might ome to a omplete standstill. The ase study presented in this leture deals with the Saint Martin La Porte aess adit (Lyon-Turin Base Tunnel) whih is being exavated through the Carboniferous Formation. This tunnel has experiened squeezing onditions. Reent innovations in yield-ontrol support systems adopted in onventional tunneling are disussed together with in situ performane monitoring, laboratory investigations, and advaned modeling studies whih have been arried out with the aim to gain in the understanding of squeezing behavior. 2 PROJECT BACKGROUND The Saint Martin La Porte aess adit is a vital part of the early works for the Lyon-Turin Base Tunnel, whih is at the entre of the axes linking the North and South, and East and West Europe (Figure 1) and is to be exavated between the portals in Italy and Frane (Figure 2). At present two aess adits (La Praz and Modane) are omplete whereas the Saint Martin La Porte adit is being exavated. These adits are essential in understanding the geologial, geomehanial, and hydro-geologial onditions along the alignment and for the seletion of the exavation method to be used. They will also provide multiple faes for onstrution, and be used for ventilation aess for maintenane and resue teams if neessary.

2 Figure 1. The Lyon-Turin railway link. Present and future tansalpine railway and road tunnels. The overburden along the tunnel in the zone of interest ranges from 300 to 600 m. Exavation takes plae in dry onditions. In order to assess the rok mass quality during exavation detailed mapping of the geologial onditions at the fae is undertaken as depited in Figure 4. The perent distribution of strong (sandstones and shists) and weak (oal and lay-like shales) roks at the fae is determined. Also adopted is the Geologial Strength Index (GSI) lassifiation aording to Hoek and Marinos (2000). Based on this systemati mapping performed from hainage 1100 to 2000 m, the distribution of strong and weak rok mass omponents is illustrated in Figure 5. It is shown that the perentages of weak roks and strong roks are highly variable, although partiularly high is the perentage of weak roks (40-60 %) between hainages m and m, whih also represent the tunnel lengths where in general the largest deformations did our. FN 4 hpg Figure 2. The Base Tunnel between Saint-Jean de Maurienne (Frane) and Venaus (Italy) inluding the three aess adits. At present, onsideration is being given to extend the tunnel length from 53 to 57 km, with the South Portal loated near to Susa (Italy). q The Saint Martin La Porte aess adit (Figures 2 and 3) is being exavated through the Carboniferous Formation, Zone Houillère Briançonnaise-Unité des Enombres (hsg in Figure 3), whih is omposed of blak shists (45 to 55%), sandstones (40 to 50%), oal (5%), lay-like shales and atalasti roks. hpg marin Figure 4. Typial geologial onditions at the fae of the Saint Martin La Porte aess adit ( - sandstone, a - layshales, -oal, et.). Figure 3. Geologial profile along the Saint Martin La Porte aess adit. A harateristi feature of the ground observed at the fae during exavation (Figure 4) is the highly heterogeneous, disrupted and fratured ondition of the rok mass whih exhibits squeezing problems. This formation is often affeted by faulting that results in a degradation of the rok mass onditions. Figure 5. Perent distribution of weak roks and strong roks along the aess adit.

3 With interest in the potential for squeezing, the perentage strain in the rok mass surrounding the tunnel, defined by tunnel onvergene/tunnel diameter was omputed (see paragraph 4 below for the onvergene data). By knowing the in situ stress defined by the produt of the depth below surfae and the unit weight of the rok mass, and assuming the degree of diffiulty assoiated with tunneling in squeezing rok to depend on the level of perent strain (Hoek, 2001), the distribution of the uniaxial ompressive strength of the rok mass is as shown in Figure 6. Several support systems were used in the Carboniferous Formation. However, it soon beame apparent that a stiff support would not be feasible in the severly squeezing onditions enountered. The design onept finally hosen was based on allowing the support to yield in a ontrolled manner, while using full-fae exavation with systemati fae reinforement by fiber-glass dowels. The support system initially implemented onsisted of fae-reinforement, yielding steel ribs with sliding joints (TH, Toussaint-Heintzmann type), anhors and a thin shotrete layer in a horseshoe profile. These setions of the tunnel (P7) underwent very large deformations with onvergenes up to 2 m and later needed to be re-profiled (Figure 7). In order to improve the working onditions and to ontrol the level of deformations, a novel yielding support system was implemented with a near irular ross setion (DSM). The exavation-onstrution sequene finally adopted onsisted in the following (Figures 8 and 9): Stage 0: fae reinforement, inluding a ring of grouted fiber-glass dowels around the opening perimeter, designed to reinfore the rok mass over a 2 to 3 m thikness. Figure 7. Photograph showing the deformed horseshoe ross setion (P7) in the bakground. This is ompared with the new ross setion implemented (DSM), following re-profiling Figure 6. Convergene and distribution of rok mass strength along the aess adit. 3 SUPPORT SYSTEMS Stage 1: mehanial exavation arried out in steps of one meter length, with installation of untensioned rok anhors (length 8 m) along the perimeter, yielding steel ribs with sliding joints (TH type), and a 10 m thik shotrete layer. The tunnel is opened in the upper ross setion to allow for a maximum onvergene of 600 mm. Stage 2: the tunnel is opened to the full irular setion at a distane of 30 m from the fae, with appliation of 20 m shotrete lining, yielding steel ribs with sliding joints (TH type) with 9 longitudinal slots (one in the invert) fitted with hid- Con (highly Deformable Conrete) elements. The tunnel is allowed to deform in a ontrolled manner so as to develop a maximum onvergene whih should not exeed 400 mm. Stage 3: installation of a offered onrete ring at a distane of 80 m from the fae. Swellex Mn12 Untensioned rok anhors length 8m R m thik shotrete Yielding steel ribs with sliding joints (TH type) Untensioned rok anhors length 8m Figure 8. Design details of the yielding support system used in the Saint Martin La Porte aess adit. Stage 1.

4 hidcon elements R m thik shotrete C Yielding steel ribs with sliding joints (TH type) Figure 9. Design details of the yielding support system used in the Saint Martin La Porte aess adit. Stage 2. As shown in Figure 10, the most important omponent of suh a yielding support system is the hidcon element, whih represents indeed the most reent tehnologial development when tunneling in squeezing rok onditions (Thut et al., 2006). A total of 9 suh elements are installed in slots in the shotrete lining between TH type steel ribs (Figure 11). The installation of this yielding support system allows ontrolled deformations to take plae as explained in priniple in the diagram of Figure 12. In the Saint Martin aess adit the hidcon elements have height of 40 m, length of 80 m, and thikness of 20 m. They have been designed to yield at approximately %, strain with a yield Figure 11. Completed installation of the yielding support system in stage 2 (30 m from the fae). The hidcon elements are visible in the shotrete gaps. stress of 8.5 MPa. Figure 13 shows the harateristi stress-strain diagrams obtained in laboratory tests. This means that with 9 elements installed, if one takes for simpliity a irular tunnel, under the assumption that eah element may attain a 50 % strain, the maximum allowed radial displaement R is equal to 20 m approximately, resulting in a total tunnel onvergene of 40 m. Also, if one takes a yield stress σ θ of 8.5 MPa, the radial onfinement stress on the surrounding rok results to be 0.3 MPa approximately. 4 PERFORMANCE MONITORING Monitoring of tunnel onvergene has been underway along the tunnel where the support system desribed above has been installed systematially, however with adaptations in the level of stabilization/support measures and the number of hidcon elements used. Convergenes are measured by means of optial targets plaed along the tunnel perimeter. A number of speial setions have been equipped with multi-position borehole extensometers. In addition, the strain/stress level in the primary and final linings is monitored. Figure 10. Details of the HiDCon elements installed between the sliding joints of the TH steel ribs before shotrete plaement. 4.1 Convergene monitoring In order to gain in the understanding of the tunnel response, it is of interest to onsider the diagram of Figure 14, whih shows the onvergenes measured in stage 1, along array 1-5, between hainage 1100 m and 2200 m approximately, with the tunnel fae being 15 m and 30 m ahead of the monitoring setion.

5 Figure 12. Simplified diagram of the yield-ontrol support system response. Figure 13. Charateristi stress-strain diagram of a hidcon element tested in the laboratory. Also illustrated in Figure 14 is the tunnel strain that has ourred (i.e. the onvergene divided by the length of the array 1-5 measured at the time of installation of the optial targets). large deformations (up to 12 % strain measured 30 m behind the fae) are assoiated with the horseshoe ross setion (i.e. the old support installed, P7) between hainages 1230 and 1400 m with the new ross setion (DSM) installed systematially starting with hainage 1400 m, the mean tunnel strain measured 15 m behind the fae is 4 % and loally never in exess of 6-7 % the m length is haraterized in general by higher perentages of weak roks and lower rok mass strength (2 MPa); following hainage 1550 m the ground onditions improved substantially (Figures 14) the 600 mm allowed onvergene with the new ross setion (DSM) has been exeeded loally whih required re-profiling of the ross setion before installing the yielding support in stage 2. Of interest, in order to appreiate the tunnel performane and ompare the old (P7) and new (DSM) exavation/support methods, is the diagram of Figure 15, whih illustrates the onvergenes 30, 80 and 120 days following the opening of the full ross setion in stage 2. Figure 14. Convergenes measured in stage 1, 15 and 30 m behind the fae, between hainage 1200 and 2100 m approximately. The following observations an be made (Figure 14): the most signifiant onvergenes in the tunnel at a distane of 15 m behind the fae our between hainages 1230 and 1550 m approximately, with a maximum strain of 4-8 % Figure 15. Convergenes measured in stage 2, 30, 80, and 120 days from onvergene targets installation, between hainages 1200 and 2100 m approximately.

6 The following remarks an be made (Figure 15): the onvergenes developed in the tunnel after 80 days of additional monitoring with the old (P7) support installed, attain a maximum of 370 mm (i.e. 3 % tunnel strain), whih takes the total tunnel losure near to 2000 mm (2 m) around hainage 1370 m omparatively, with the new (DSM) support installed in stage 2, the maximum onvergene is 600 mm (i.e. 5 % tunnel strain) around hainage 1480 m, in exess with respet to the target value of 400 mm (i.e. 3.5 % tunnel strain) Figure 16 shows the typial displaement distribution versus time on the right side, whereas Figure 17 illustrates the zone around the tunnel where the radial strain is greater than 1%, this strain limit being taken as the onset of squeezing behavior. 4.2 Deformations in the rok surround A number of speial monitoring setions were installed along the adit. In these setions, in addition to onvergene measurements also multi-position borehole extensometers were used in order to observe the rok mass response in the ground around the tunnel. For the purpose of further illustration, the monitoring results for the setion at hainage 1444 m are plotted in Figures 16 and 17. This setion has exhibited during exavation a signifiant non symmetri losure, with severe over-stressing of the primary lining and of some of the deformable elements, mainly on the right sidewall. A total of 6 multi-position borehole extensometers, eah 24 m-long, were installed in stage 1, one at the invert and rown, and two on eah sidewall, left and right. These were replaed by 15 m long borehole extensometers at the beginning of stage 2, 33 days after installation, approximately 30 m behind the tunnel fae. Figure 17. Speial monitoring setion at hainage 1443 m. Displaement versus time in the rok around the tunnel. It is lear that, where a more signifiant extent of the mobilised zone around the tunnel ours in stage 1 (Figures 16 and 17), the support system in stage 2 undergoes greater deformations. The non symmetri response of the tunnel is due to the essentially anisotropi features of the rok mass, the presene of strong (sandstones and shists) and weak (oal and lay-like shales) layers whih dip from the left to the right of the tunnel ross setion. 4.3 Tangential stress in the lining Also monitored in a number of speial setions was the tangential stress in the final lining for both the old (P7) and the new (DSM) ross setions. The results available are plotted in Figure 18 where the measured tangential stress is given versus time both at the rown and invert. It is learly shown that indeed the DSM ross setion is undergoing a level of stress whih is muh smaller than that of the P7 ross setion. Figure 16. Speial monitoring setion at hainage 1444 m. Displaement versus time around the tunnel. Figure 18. Tangential stresses at rown and invert versus time in final lining of ross setions P7 and DSM.

7 5 LABORATORY TESTS Table 1 summarises the results of laboratory tests derived from unonfined and triaxial ompression tests performed on the strong roks omponents (sandstones and shists). Table 1. Parameters derived from laboratory tests on sandstones and shists. Parameter Sandstones Shists Unit weight (γ, kn/m 3 ) Unonfined ompressive strength (σi, MPa) Hoek-Brown onstant (m i ) Tangent elasti modulus (E t, GPa) As far as the weak rok omponents (oal, laylike shales and atalasti rok) most of the attention in testing has foused on the mehanial properties of oal and its time dependent behaviour in triaxial onditions. A detailed desription of the laboratory tests performed and of the results obtained is given by Debernardi (2008) in his PhD thesis. In partiular, multi-stage triaxial tests were arried out on oal with the intent to determine its deformability and strength properties. Four ompression stages were performed with different onfining pressures of 5, 7.5, 10 and 12.5 MPa, by applying a onstant axial strain rate of 0.01 %/min, until a peak was reahed in eah stage on the stress-strain urve. At peak, the onfining pressure was inreased in order to allow for the determination of a new peak strength. This proess was repeated four times to determine four points on the failure line as shown in Figure 19. The deformability and strength parameters are given in Table 2. Table 2. Deformability and strength parameters derived from laboratory tests on oal. Parameter Tangent elasti modulus (E t, GPa) 6.9 Mohr-Coulomb parameters (, MPa) (φ, ) Hoek-Brown parameters Unonfined ompressive strength (σi, MPa) 15.3 Hoek-Brown onstant (m i ) 8.97 Also performed were a series of reep and relaxation tests on oal under different laboratory onditions (Barla et al. 2007; Debernardi, 2008; Barla et al., 2009). In partiular, aording to the stress paths shown in Figure 19, reep phases were performed in isotropi onditions (points A, B and C) and under a deviatori stress (points D and E). The reep urves for the stress state D and E are shown in Figure 20. It is observed that a seondary reep deformation is soon attained with a moderate strain rate whih is stritly related to the limited level of the mobilised strength and of the mean stress being applied. Figure 20. Results of multistage triaxial reep tests on oal. Calibration with SHELVIP model. 6 NUMERICAL MODELING Figure 19. Stress path of the multi-stage triaxial test and strength envelopes of oal. Stress path of the multi-stage triaxial reep tests. In squeezing onditions time dependent deformations are observed whenever fae advane is stopped and are likely to take plae during exavation, when it is diffiult to distinguish the fae effet from the time effet. Therefore, in suh onditions, an appropriate representation of the tunnel response is obtained only by using onstitutive models whih aount for time dependent behavior (Barla et al., 2007).

8 Many onstitutive models have been proposed to desribe suh a behavior for weak rok and soil. Nevertheless, only few models an reprodue satisfatorily all the time-dependent features involved in tunnel exavation, with a reasonably simple mathematial formulation to be used in design pratie. With the phenomena observed in the Saint Martin La Porte aess adit in mind, the SHELVIP (Stress Hardening ELasto VIsous Plasti) model was formulated (Debernardi, 2008; Debernardi and Barla, 2009). 6.1 SHELVIP, a new onstitutive law SHELVIP has been derived from the lassial theory of elasto-plastiity by adding a visoplasti omponent based on the Perzyna s overstress theory. It is assumed that the time-independent plasti strains develop only when the stress point reahes the plasti yield surfae f p ( σ ) = 0 defined by the Druker-Prager riterion. Similarly, the timedependent visoplasti strains develop if the effetive stress state exeeds a visoplasti yield surfae f σ = whih is also defined by the Druker- vp ( ) 0 Prager riterion (Figure 21). NSLUCIONSG Figure 21. SHELVIP onstitutive model: visoplasti and plasti yield surfaes. The plasti yield surfae is fixed, while the visoplasti surfae an harden aording to a stressbased hardening rule. The rate of visoplasti strains, whih are both deviatori and volumetri, an be evaluated by using the visoplasti flow rule of Perzyna and depends on the deviatori state of overstress referred to the visoplasti yield surfae. The introdution in SHELVIP of a stress based hardening law is assoiated with some advantages. It allows one to evaluate the visoplasti hardening level from the stress level whih defines the threshold for development of visoplasti deformations, whih an be done by appropriate tests. In addition, as demonstrated in detail in Debernardi and Barla (2009), it permits a lear definition of eah time-dependent behavioral feature by means of a single onstitutive parameter. One may note that the overall number of parameters in the model are 11 as summarized in Table 3: 2 elasti parameters, 4 plasti parameters, and 5 visoplasti parameters. Table 3. Constitutive parameters of the SHELVIP model. Elasti Behavior E ν Young s modulus Poisson s ratio Plasti Visoplasti αp k p σt ωp γ m n l ωv Slope of the Druker-Prager s plasti yield riterion Interept of the Druker-Prager s plasti yield riterion Volumetri tension ut-off Plasti dilatany Fluidity parameter Shape fator Load dependeny fator Time strething fator Visoplasti dilatany The SHELVIP model is shown to be effetive in desribing the time dependent behavior of the weak rok omponents as demonstrated with the alibration of several triaxial reep tests performed on oal samples. As an example, Figure 20 (see above) shows a omparison between the experimental and numerial results obtained from the multistage reep tests on oal desribed above. As noted, the agreement results to be satisfatory with the material parameters of the onstitutive model shown in Table 4 below. Table 4. Parameters for the SHELVIP model. Laboratory onditions for oal (* time in days and pressure in kpa). Parameter E (GPa) ν (-) φ ( ) (MPa) α p k p (MPa) σt (MPa) ωp γ (*) m (*) n (*) l (*) ωvp (*) E E

9 6.2 Modeling of tunnel exavation The SHELVIP onstitutive model has been used to analyse the tunnel response in terms of onvergene monitored during exavation (Barla et al., 2009). For the purpose of numerial modeling the ross setions between hainage 1394 m and 1527 m have been hosen. The overburden is approximately 363 m and the initial stress state is assumed to be isotropi and equal to 9.8 MPa. The analyses have been performed with the Finite Differene Method and FLAC ode. Axisymmetri onditions have been adopted in order to reprodue the three-dimensional influene of the tunnel fae, whih is known to play a signifiant role in squeezing onditions. The tunnel ross setion is assumed to be irular, with an equivalent radius of 6 m. The total size of the model (96 m m) is very large in order to minimize the boundary effets that are very signifiant in the ase of large deformations. Partiular attention has been paid to the hronologial sequene of exavation. The ground reinforement ahead of the fae has been desribed by using an equivalent pressure of 0.1 MPa applied to the fae (Figure 22). Bak-analysis has led to the values of the onstitutive parameters listed in Table 5. Figure 23 shows the omparison of omputed and measured values in terms of radial displaement for the setion at hainage 1444 m. The agreement of the numerial results with the mean urve is rather good, notwithstanding the sattering of the monitoring data due to the high heterogeneity and anisotropy of the rok mass. Also the displaements around the tunnel monitored with the multi-position borehole extensometers are satisfatorily reprodued, as shown in Figure 24. Table 5. Parameters for the SHELVIP model. In situ onditions (* time in years and pressure in kpa). Parameter E (GPa) 0.64 ν (-) φ ( ) (MPa) α p ωp γ (*) m (*) n (*) l (*) ωvp (*) E Figure 22. Sketh of the numerial model for the sequene of exavation and reinforement (Barla et al., 2009). The radial reinforement and the first stage lining (10 m thikness) have been simulated by using an equivalent internal pressure, whih has been assumed to reah the onstant value of 0.1 MPa 5 m behind the tunnel fae, as shown in Figure 22. The influene of the seond-stage lining has been introdued by way of an additional internal pressure 30 m behind the fae, whih reahes the onstant value of MPa in a 5 m span. This value has been alulated on the basis of the yielding stress of the hidcon elements, as determined at the laboratory sale. A onstant exavation rate of 0.54 m/day has been onsidered. Figure 23. Computed versus monitored onvergenes at hainage 1444 m.

10 Antea and Politenio di Torino, Department of Strutural and Geotehnial Engineering. The author wishes to aknowledge the work of Dr Maro Barla, Mariaristina Bonini, and Daniele Debernardi who have ontributed and are ontributing very signifiantly to this researh effort. REFERENCES Figure 24. Computed versus monitored radial displaements around the tunnel at hainage 1444 m. 7 CONCLUSIONS This leture deals with the studies performed during exavation of the Saint Martin La Porte aess adit (Lyon-Turin Tunnel), whih experiened squeezing onditions during exavation in the Carboniferous Formation. An innovative tunnel onstrution method was introdued whih ouples fae reinforement by means of fiber-glass dowels with a yield-ontrol support. In situ performane monitoring, laboratory tests, and numerial modeling are desribed. It is to point out that the results obtained so far through the studies performed and briefly reported in this leture are thought to be essential in taking the deision if, where, to what extent, and by what measures an mehanized tunneling be adopted for exavation of the Base Tunnel in onditions similar to those experiened in the Saint Martin La Porte aess adit. It is thought that with the level of information available and the design tools provided a thorough analysis an be performed in order to assist in taking suh a deision. Barla, G., Bonini, M., & Debernardi, D Time dependent deformations in squeezing tunnels. Pro. of the 12 th International Conferene of IACMAG, Goa. Barla, G., Debernardi, D., & Sterpi D Numerial analysis of tunnel response during exavation in squeezing rok by using two onstitutive models. EURO:TUN nd Conferene on Computational Methods in Tunnelling, Bohum. Debernardi, D Visoplasti behaviour and design of tunnels. PhD Thesis. Dotoral Degree in Geotehnial Engineering. Politenio di Torino: 302 pp. Debernardi, D. & Barla, G New visoplasti model for design analysis of tunnels in squeezing onditions. Rok Meh Rok Engng, vol. 42, no. 2: Hoek, E., Big tunnels in bad rok. Terzaghi Leture. ASCE Journal of Geotehnial and Geoenvironmental Engineering, vol. 127, no. 9, September 2001: Hoek, E. & Marinos, P Prediting tunnel squeezing problems in heterogeneous rok masses. Tunnels and Tunnelling International, Part 1, 32 (11), 45-51; Part 2, 32 (12), Thut, A., Naterop, D., Steiner, P., & Stolz, M Tunnelling in squeezing rok-yielding elements and fae ontrol. 8th International Symposium on Tunnel Constrution and Underground Strutures, Lubljana. ACKNOWLEDGEMENTS The opportunity to prepare this key-note leture for Eurok 09 has made it possible to summarize some of the researh work on squeezing rok, with the Saint Martin La Porte aess adit as a ase study, arried out on behalf of LTF (Lyon Turin Ferroviaire SAS) through a researh ontrat (in the years from 2006 to 2009) between the Engineering Group Egis, Alpina,