Mastering of Squeezing Rock in the Gotthard Base

WT ON SITE Mastering of Squeezing Rock in the Gotthard Base he term “squeezing rock” originates from the pioneering days of tunnelling Tin the Alps. 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 tunnel “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 Italy, Japan, Austria, Turkey and rock deformation. In earlier days, in extreme aspects (action and reaction) of the same Switzerland, 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 Swiss Federal Railways 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 tunnels 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 Simplon Tunnel, 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.

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