WT ON SITE Mastering of Squeezing Rock in the Gotthard Base

he term “squeezing rock” originates from the pioneering days of tunnelling Tin the . The various descriptions of rock pressure were already classified into three groups, namely loosening rock pressure, swelling pressure and squeezing pressure. Thus, the observed rock behaviour was often described using terms like spalling, swelling and squeezing. As long ago as the last century, it was understood that these three types of rock pressure were caused by fundamentally different physical mechanisms (Kovári 1975). They may also act in a superimposed way; and thus it is conceivable that, in a rock of low strength containing clay minerals, the failure processes are accompanied by the swelling phenomenon. Squeezing rock is characterised by the tendency to reduce the cross-section of the opening (Figure 1). The reduction in size of the opening in course of time is called “convergence”. The actual creep potential of the rock under the given stresses is a basic requirement for the occurrence of squeezing Fig. 1: Squeezing rock reduces the cross-section. rock. Since the lining resists the convergence, the pressure acts as a reaction, so that rock tion and operation methods and type of Simplon “a rock was encountered pressure and rock deformation are directly temporary lining, it was essential, besides appearing as a dough, mainly consisting of soft related to one another. With respect to the information of the actual geological condi- micaceous limestone”. Overcoming this short lining, the rock pressure is regarded as a tions, to describe the construction process as section took seven months (Pressel 1906). loading, and with respect to the rock, it acts precisely as possible. Of the seventeen case The rock pressure decreases with increasing as a lining resistance; thereby, two distinct studies from , Japan, Austria, Turkey and rock deformation. In earlier days, in extreme aspects (action and reaction) of the same , the conclusion is that the trend squeezing rock conditions, a big contraction phenomenon are expressed. If the rock pres- in modern traffic tunnel construction is to of the cross-section was accepted and the sure exceeds the bearing capacity of the excavate large areas in the tunnel profile, subsequent re-profiling, as well as changing lining, it will be damaged or even destroyed, even in squeezing rock conditions, in order to the type of lining, were the only possibilities and the rock deformations continue until a allow a high degree of mechanisation. Usually of controlling the rock pressure. new state of equilibrium is reached. By not this necessitates a systematic support of the The existence of ground water or high pore fulfilling the planned clearance of the face which, thanks to the technological pressures aids the development of rock pres- minimum excavation line with the temporary developments in recent decades, can be sure and rock deformation. This observation lining, re-profiling the rock is unavoidable rationally executed. is confirmed repeatedly by the favourable (Figure 2). Such repair work is time- effect of rock drainage using an advanced consuming and involves high costs. Constructional Experience pilot or parament tunnel. Recently, within the framework of a From worldwide experience in tunnelling in As a rule, the rock deformation is not research project (Staus & Kovári 1996) squeezing rock, the following empirical facts uniformly distributed over the excavation commissioned by the project management of emerge (Kovári 1998): cross-section. Often, bottom heave is practi- AlpTransit of the and Large long-term deformations or large long- cally irrelevant, although in the side walls and the BLS AlpTransit Co., the experiences term rock pressures only occur in rocks of the roof large deformations occur. In many gained in the last 25 years in traffic in low strength and high deformability. A cases, moreover, the deformation of the face, squeezing rock zones were studied and pronounced creep capacity is an important and its stability, respectively, do not present presented according to unified points of view. prerequisite for the occurrence of this type of any practical problems. For full face excava- The report is intended to heighten our aware- rock pressure. Phyllite, schist, serpentine, tion of large cross-sections the stabilisation ness of the various forms of squeezing rock, claystone, tuff, certain types of Flysch, and of the face is necessary, involving time- and thus consolidate our understanding of the weathered clayey and micaceous metamor- consuming measures (Lunardi 1995). underlying relationships. Since the rock phic rocks are typical examples of such rock The intensity of the rock deformations and behaviour is inseparable from the construc- types. In excavating a 42 m stretch in the of the rock pressure, respectively, in a stretch 234 SQUEEZING ROCK IN THE GOTTHARD

related to the construction of the approxi- Wiesmann argued in a qualitative way, basing mately 20 km long , which his considerations on experience known to has a maximum depth of overburden of 2,100 m. him of tunnelling in squeezing rock, on the The Simplon Tunnel I was constructed in findings from triaxial tests and on the stress the period 1898 – 1906, and Simplon Tunnel conditions in an elastic plate containing a II between 1912 and 1921. The long hole under in-plane loading. He recognised, construction time for the second tunnel was and also gave clear reasons for, the relation- due to the European war. The Alpine geolo- ship between rock pressure and deformation: gist Heim warned in an article (1878) that “With each fraction of a millimetre with was much acclaimed by professional which the rock mass moves, the amount of colleagues at the time, that, in his opinion, pressure acting (on a lining) decreases”. insuperable difficulties would be encountered The first computational model for when tunnelling at great depth. He main- describing the stress redistribution in a plate tained that “for each rock one needed to with a hole in it taking into account a failure envisage a column so high that its weight criterion comes from the bridge engineer exceeded the strength of the rock and there- Maillart (1923), who in 1923 considered the fore the foot of the column would be crushed. idea of a “protective zone” to be outdated. In Depending on the strength of the rock this fact, this represents a considerable scientific column will be higher or lower, but the envis- advance, to speak of separate plastic and aged conditions would always occur.” Under elastic regions, whereby the rock mass is “strength” Heim understood the uniaxial stressed to the limit of its triaxial strength or strength of the rock (Kastner 1962). He where this is no longer the case. From Fig. 2: Reprofiling of the rock is unavoidable. believed that reaching this strength, “hydro- Maillart we also get the pregnant formulation static conditions” would dominate and he “As long as we require a tunnel lining, which of squeezing rock usually varies considerably. coined the term “latent plasticity”. Further, can withstand an external rock pressure, the For the same excavation support, the same he assumed that “the internal friction would strength of the rock will be increased and thus depth of overburden and the same lithological be so reduced under the all round pressure enabled to develop a self-carrying capacity”. type, often sudden changes of convergence that a stress redistribution would occur The subsequent internationally well estab- of several magnitudes difference may be without cleavage and the rock begins to flow, lished theoretical developments led to the observed over a short distance. This is one of just like ice flows in a glacier (Heim 1878). “characteristic line method”, which permits the main reasons for setbacks, which in some The material would try to flow into the tunnel quantitative assessment of the rock pressure. cases may occur despite wide experience and opening. From this he concluded that, beyond Under characteristic line, one understands a well-founded knowledge of the engineers in a certain critical depth, depending on the type the functional relationship between the radial charge. of rock, the tunnel construction work would displacement at the edge of a hole and the The influence of the depth of overburden become impossible to control technically. It resisting force acting there. Thus, the charac- on the types of rock pressure could not, up till was Wiesmann (1912), one of the chief super- teristic line is limited purely theoretically to now, be empirically observed in an unam- vising engineers on the construction of the the axisymmetric conditions: this applies both biguous way. The reason for this is that the Simplon Tunnel, who discovered the error in to the cross-sectional shape (circle) and to changing deformation properties and strength the reasoning of Heim. Firstly, for the behav- the material properties (homogeneity, properties have a much greater influence on iour of the rock surrounding the tunnel it is isotropy), the primary state of stress (hydro- convergence and pressure than the over- not the uniaxial, but the triaxial, compressive static condition) and the lining resistance. burden effect. Thus our knowledge of the strength that applies: “The bearing capacity For a detailed analytical derivation of the unfavourable influence of overburden is based of enclosed bodies, this is the governing rock characteristic line see references (Fritz 1981, on theory. strength”. He could already consult the results Kovári 1986, Brown 1983, Panet 1995). Finally, we would like at this point to draw of the von Kármán’s (1911) triaxial tests on The first works on its practical application attention to the following: marble from the year 1905. Secondly, the to the determination of the rock pressure Firstly, it is not possible to give a precise behaviour of a rock in a plastic state cannot come from Mohr (1964) and Lombardi quantitative definition of the term “squeezing be compared to that of a fluid. In a viscous (1971). In 1964, Pacher proposed a character- rock”. There is agreement, however, that the (Newtonian) fluid it is only a question of time istic line of a special kind, which should time effect is one of the most pronounced and until a hydrostatic stress state develops. Due enable the tunnelling engineer to optimise unmistakable characteristics of squeezing to internal (Coulomb) friction, however, the lining resistance. Müller (1978) spoke of rock. It is governed – as explained above – by rocks behave quite differently. After the creep the “Pacher concept of the deliberate stress the combined action of the rock properties, and relaxation processes fade away there relaxation, to achieve a minimum of lining the pore water pressures and the depth of remains, due to the cohesion and internal thickness for temporary lining”. He maintains overburden. friction, a deviatoric component of stress state that “the theory of the overall concept of the Secondly, in overcoming a stretch of which allows a difference in principal stresses New Austrian Tunnelling Method (NATM) squeezing rock, it is particularly important to – for axisymmetrical conditions between the is based on Pacher’s characteristic line for have a correctly formulated contract between radial and tangential stresses – in the rock linings”. In 1994 Kovári raised objections to the client and the contractor. This is under- surrounding a tunnel. As one of the first, such a theory and clearly showed that the standable, since the costs per tunnel metre are Wiesmann recognised the significance of the optimisation of the lining resistance following high and the rate of advance is slow. stress redistribution in the vicinity of an NATM is fundamentally flawed, since its underground opening, as well as the influence requirement of having a trough-shaped rock Squeezing Rock Phenomenon of the failure state on the stress redistribution, characteristic line according to Pacher has no The first theoretical works to explain the in that he called the zone of rock affected by proper theoretical basis (Kovari 1994). Kolymbas phenomenon of squeezing rock are closely stress redistribution a “protective zone”. (1998), in a recently published text book in 235 SQUEEZING ROCK IN THE GOTTHARD

m.a.s.l. 3000

2000

1000

Aar-Massif MTM Gotthard-Massif Pennine Gneiss zone Fig. 3: Longitudinal geological profile of the .

his chapter on NATM supports the Pacher also be penetrated by the Lötschberg Tunnel rock properties must be reckoned with than curve, in that he writes, it is reasonable, in its southern section, the Gotthard Base originally predicted: about 70% of a zone of although it could not up till now be verified Tunnel will pass through further complex 1.1 km width consists of weak rocks, by measurements or numerical simulations. crystalline rocks: the Middle Tavetsch Massif, including kakiritic phyllite (a rock reduced Thereby, Kolymbas justifies those who have the Gotthard-Massif and the Pennine Gneiss to a loose condition during the formation of always doubted the reports of Rabcewicz, Zone; they consist predominantly of granite, the mountains; formerly the term mylonite Müller and others regarding the alleged use of gneiss and schist or slate. Of special impor- was employed). Our subsequent discussion the Pacher curve for model projects of NATM. tance are the long stretches in the Middle concentrates on this stretch as well as the The realisation of the large recently planned Tavetsch Massif (MTM) and in the conditions in the above mentioned Clavaniev projects in the Alps makes the retraction of Clavaniev Zone (CZ) bordering in the Zone, which consists of clayey kakirite, phyl- the claims of NATM more urgent than ever. northern part, where rocks of low strength litic schist and gneiss types of the Aar-Massif. and high deformability are expected (Figure 4). Squeezing Rock in the The old crystalline Middle Tavetsch Massif Sedrun Concept Gotthard Base consists of highly varying rock types: gneisses The construction lot Sedrun stretches over a The overall project of the Gotthard Base to a succession of steep zones of soft phyllites distance of about 6 km of the tunnel, the Tunnel has been described in detail elsewhere and schists. In the period 1995-97 two deep tunnel being driven from the foot of the shaft (Gehriger 1994, Kovári 1995, Zbinden 1997), trial boreholes were sunk in the southern and northwards and southwards. This lot also which is why we restrict ourselves here to a northern parts of the Middle Tavetsch Massif includes the construction of one of the tech- summary of the basic elements. to a depth of almost 2,000 m (Figure 4). They nically difficult multifunctional stations. The The 57 km long Gotthard Base Tunnel supplement three older shorter boreholes and construction concept for driving the tunnel forms together with the 34.5 km long both reached to below the level of Gotthard tubes in squeezing rock is based on the care- Lötschberg Base Tunnel the heart of the Base Tunnel. In the southern part of the fully prepared geotechnical model of the rock, AlpTransit project, which was accepted by a Middle Tavetsch Massif, with the start of the whose elements were determined using the clear majority in two Swiss referendums. The construction of the two access tunnels to the results of trial boreholes. Important findings project offers the possibility of moving the top of the blind shaft in Sedrun over a length with regard to the mechanical properties of greater part of the cross-alpine freight traffic of 1.3 km, the first in situ information could the rock were also given by the laboratory from road to rail and to ensure the connec- be obtained; it was shown that the rock tests on the core samples. Of particular tion of Switzerland to the European High encountered so far is better than was interest were the drained and undrained Performance Rail Network for passenger predicted. In the northern part, on the other triaxial tests with accurate determination and traffic. The trains will pass below the Alps at hand, a more unfavourable distribution of the control of the pore water pressures. Despite a maximum gradient of 1.25 % at depths of up to 2,300 m. Due to the increased speed for Fig. 4: Trial boreholes in the region of the Middle Tavetsch Massif. passengers and freight in both base tunnels the system is like that of a flat railway system. N S For the base tunnel, a system comprising m.a.s.l. two single- tunnels with cross connec- tions and the possibility of a change of track was chosen. Two multi-functional stations allow, amongst other things, an emergency stop and can be reached from outside by means of access tunnels and a vertical shaft; the latter serves during the construction phase as a point of intermediate attack. The vertical shaft in the vicinity of the village of Sedrun is situated in a zone of rock with favourable rock properties, has a depth of 800 m and an inner diameter of 7.5 m. The sinking of this blind shaft was begun in 1998 and completed in February, 2000. Geology In Figure 3 the longitudinal geological profile Aar-Massif Middle Tavetsch Massif (MTM) Gotthard-Massif is shown. Besides the Aar-Massif, which will 236 SQUEEZING ROCK IN THE GOTTHARD

the complex structure of the kakiritic phyllite zones a high degree of mechanisation for the Table 1. Squeezing rock: data on remarkably uniform and useful results were excavation work is possible. By “poorest” excavation and support measures obtained from the total of 39 tests carried out zones is meant the rock regions in which, as a Tunnel excavation radius 5.09 – 6.54 m (Vogelhuber 1998). consequence of the excavation work, espe- The geotechnical model corresponds cially large deformations (inward movement Over-excavation 0.30 – 0.70 m overall to a several hundred metre-long of the face, reduction of the cross-section) or Area of full section 81 – 134 m2 homogeneous, isotropic rock mass with a by their retardation or prevention the devel- Length of round 1.0 m depth of overburden of 900 m. The rock opment of extremely high rock pressures can Steel fibre reinforced concrete consists of a series of qualitatively different be expected. The project engineers (Ehrbar & (protective layer) 0.05 m rock zones, described by an appropriate geot- Pfenninger 1999) were responsible for the Steel arch TH 44/70 echnical model. For the rock type considered task, based on the available information, of spacing 1.00 – 0.33 m to be the least favourable the following values defining a length of the “poorest” zone, to weight per TM 2.5 – 9.4 to of elastic (i.e.Young’s) modulus E, angle of represent it in a model and to design the Shotcrete behind face 0.35 – 0.50 m internal friction ( and cohesion c were corresponding normal cross-section including assumed: E = 2 GPa, ( = 23°, c = 250 kPa the construction concept. In better rock Rock bolting (drained conditions). conditions the concept of the chosen proce- length 8 – 12 m In the following, we leave out a discussion dure should retain its validity, in that the ultimate load 320 kN of the computational investigations and individual measures should only need to be anchor per TM 96 – 288 m mention only that the characteristic line adjusted in their intensity (reduction). Anchoring of face method provides useful information on the The design for the Middle Tavetsch Massif length 12 – 18 m combined action of the most important influ- and the Clavaniev Zone has in the order of ultimate load 320 kN ence factors like material parameters, primary their importance the following elements anchor per TM 80 – 210 m state of stress, lining thickness and the rock (Figure 5): circular tunnel cross-section, full Final lining 0.30 – 1.20 m deformations. The influence of the pore water face excavation, uniform systematic pressure, creep and other time-dependent anchoring of the face, deliberate over-excava- unreinforced cast-in-place concrete follows effects as well as the construction stages were tion to accommodate the convergence, steel the tunnel excavation at a distance of about not investigated in the computations, since arch linings closed to a ring with sliding 300 m. Its dimensioning is based on the they would require a knowledge of the connections (Toussaint-Heintzmann), uniform assumption that the temporary support in the specific material behaviour (constitutive radial anchoring around the cross-section as course of the long operating life (roughly 100 model) with the corresponding material para- well as a closed ring of shotcrete lining in the years) may completely loose its statical func- meters. The latter, however, could not be region behind the face. tion due to corrosion. Besides the high rock determined with sufficient reliability and In their heaviest size the steel arch linings pressures the final lining has to withstand a accuracy for practical purposes. The limita- are chosen such that at uniform convergence water pressure corresponding to a height of tions of numerical methods in tunnelling are two rings, one lying within the other, are about 100 m. Detailed investigations showed not given by the computational methods at given, whereby both a considerable lining that a top heading excavation could not be our disposal, but by the difficulty of describing resistance and a high level of safety against carried out in the most unfavourable rock the actual stress-strain relations and the lateral buckling results. The shotcrete lining zones, which is why only full-face excavation primary state of stress in the rock mass. is to be applied after the closure of the steel was considered further. To back up this deci- To recognise the limitations of geological arch linings and the full exploitation of the sion it was possible to draw upon useful predictions in relation to the average material estimated convergence, respectively, to Italian experience (Lunardi 1998 & 2000, properties and their variability along the prevent a further reduction in cross-section of Hentschel 1998). tunnel as well as the inadequacy of the above the tunnel opening. Lances in the roof region The critical hazard scenarios, which will mentioned statical computational methods and the immediate support of the face guar- apply to the tunnel excavation in this zone, have increased the importance of the plan- antee safe working conditions. The length of are collapse (instability) of the face, local ning and design work. Selecting the shape of the anchors in the face are at least 6 m, spalling of the same, exceeding the planned the cross-section, method of construction and whereby this is achieved by overlapping the limiting values of the convergence as well as operation as well as support measures should original 12-18 m long anchors. The final fall of rock from the roof region. The insta- be made such that even in the poorest rock lining with a thickness of maximum 1.20 m of bility of the face and inadmissible conver- gence are “announced” by time-dependent Fig. 5: Sketch of the proposed support measures and over-excavation in heavily squeezing rock. and forerunning rock deformations up ahead. The measurement of the distribution of the axial displacement of the face and of the radial displacement in the rock and at the boundary of the excavation provide useful indications for the current (i.e. immediate) assessment of the rock behaviour. The spatial distribution, the amount and the time varia- tion of the rock deformation, depending on the rock conditions encountered, help to define the stepwise use of the planned safety measures. In this respect the characteristic property of squeezing rock – the creep behav- iour – turns out to be an advantage for the engineer. Table 1 shows the dimensions of the 237 SQUEEZING ROCK IN THE GOTTHARD

possible modifications of the various support Kovári, K. Erroneous Concepts behind the measures to the different geological condi- New Austrian Tunnelling Method. T&T tions. The most remarkable feature is the size November 1994. 2 Kovári, K. 1998. Tunnelling in Squeezing Rock, of the full section (134 m ), which might Tunnel, Int. Fachzeitschrift für unterirdisches result in the most difficult geological condi- Bauen, No. 5. tions. The increase of the excavation radius Lombardi, G. 1971. Zur Bemessung der up to 6.5 m is necessary because of the large Tunnelauskleidung mit Berücksichtigung des over-excavation for achieving the conver- Bauvorganges, Schweiz. Bauzeitung, 89. Jhg., gence, with a thick shotcrete lining and a Heft Nr. 32. Lunardi, P. 1998. History of the Bologna to greater thickness of the inner lining. From Firenze Rail Connection, Gallerie (54). Figure 6 the extra measures for material and Lunardi, P. März 1995. Progetto e costruzione di time planned for the most unfavourable rock gallerie secondo l’approccio basato sull’analisi zones can be clearly seen. The corresponding delle deformazioni controllate nelle rocce e tunnel cross-section is shown in Figure 7. nei suoli, Quarry and Construction. Maillart, R. 1923. Über Gebirgsdruck, Schweiz. Bauzeitung, Bd. 81, Nr. 14. Final Remarks Mohr, F. 1964. Schachtbautechnik, Hermann In the construction lot Sedrun of the planned Hübner Verlag, Goslar. Gotthard Base Tunnel lengthier stretches of Müller, L. 1978. Der Felsbau, Dritter Band: squeezing rock are expected. Due to over- Tunnelbau,, Enke Verlag, Stuttgart. excavation and the time-consuming and Pacher, F. 1964. Deformationsmessungen im expensive support measures the rate of exca- Versuchsstollen als Mittel zur Erforschung vation will drop to 1 m or less per working Fig. 6: Section through the temporary and des Gebirgsverhaltens und zur Bemessung permanent linings normal to the tunnel axis (least des Ausbaues, Felsmech. und Ing. Geol., day, which is why this lot could decisively Suppl.1. influence the length of construction time for favourable rock zone) after complete exploitation of the estimated convergence. Panet, M. 1995. Le calcul des tunnels par la the whole tunnel. In the design and construc- méthode convergence – confinement, Presses tion, therefore, great importance was attached Symp. „Basistunnel durch die Alpen“ (ed. R. de l’école nationale des ponts et chaussées. to the possibilities of using high mechanisa- Fechtig, K. Kovári), ETH Zurich. Pressel, K. 1906. Die Bauarbeiten am Heim, A. 1878. Mechanismus der Gebirgs- tion. The best possibility here is given by full- Simplontunnel, Schweiz. Bauzeitung, Bd. bildung, Basel XLVII. face excavation, since more working space is Hentschel, H. 1998. Unter der Erde von Staus, J. & Kovári, K. 1996. Tunnelbau in druck- available for the use of high performance Bologna nach Firenze, Tunnel 8/98. haftem Gebirge, Falldarstellungen, Forsch- equipment. Excavation in full section, Kastner, H. 1962. Statik des Tunnel- und ungsbericht, Institut für Geotechnik, ETH however, is alone for statical reasons neces- Stollenbaues, Springer, Wien. Zürich. sary, since in very difficult rock conditions Kolymbas, D. 1998. Geotechnik – Tunnelbau Vogelhuber, M. 1998. Triaxiale Druckversuche partial excavation, as for example top heading und Tunnelmechanik, Springer. im Labor, Sondierbohrung SB3.2, AlpTransit Kovári, K. 1995. The two Base Tunnels of the Gotthard Basistunnel, Interner Bericht advance, could lead to uncontrollable AlpTransit Project: Lötschberg and Gotthard, Institut für Geotechnik. construction situations. Using the method Forschung + Praxis, Band 36 (STUVA). von Kármán, Th. 1911. Festigkeitsversuche described here the systematic support of the Kovári, K. 1986. Rock Deformation Problems unter allseitigem Druck, Zeitschrift ver. dt. face for providing safety against collapse and when using Full Face Cutting Equipment in Ing. 55. safe working conditions forms a major compo- Rock. Part 1 in Tunnel 3/86, Part 2 in Tunnel Wiesmann, E. 1912. Über Gebirgsdruck, nent of the overall construction method. 4/86 (in German and English). Schweiz. Bauzeitung, Bd. 60, Nr. 7. Kovári, K. 1975. Probleme der Tunnelstatik, Zbinden, P. 1997. AlpTransit Gotthard Tagungsberichte, Dokumentation No. 12, Basistunnel, Geologie, Vortriebsmethoden, by K. Kovári Schweiz. Ingenieur und Architektenverein Bauzeiten und Baukosten, Forschung + Praxis, ETH, Zurich (SIA). Band 37 (STUVA). F. Amberg Fig. 7: Sketch of the Amberg Ing.-Büro AG, Regensdorf tunnel cross-section for over-excavation H. Ehrbar and the lining thicknesses after Electrowatt Engineering, Zurich completely exploiting the estimated References convergence. Brown, E.T., Bray, U.W., Ladanyi, B., Hoek, E. 1983. Ground response curves for rock tunnels, Journal of Geotechnical engineering, Vol. 109, No. 1. Ehrbar, H., Pfenninger, I. 1999. Umsetzung der Geologie in technische Massnahmen im Tavetscher Zwischenmassiv Nord, Proc. Symp. Geologie AlpTransit, ETH Zurich (ed. S. Löw). Fritz, P. 1981. Numerische Erfassung rheolog- ischer Probleme in der Felsmechanik, Dissertation ETH, Mitteilg. Nr. 47 des Inst. für Strassen- Eisenbahn- und Felsbau an der ETH Zürich. Gehriger, W. 1994. Das Vorprojekt des Gotthard-Basistunnels, Tagungsberichte Int. 238