Gallerie e Grandi Opere Sotterranee n.71 - Dicembre 2003
EVALUATING THE REQUIRED FACE-SUPPORT PRESSURE IN EPBS ADVANCE MODE Dr. Giordano Russo GEODATA SpA Torino (Italy)
1.INTRODUCTION The stability of the face is one of the most important factors in selecting the adequate method of excavation of a tunnel. This is particularly true for mechanised tunnelling and specific boring machines (TBM), as, for examples, the Earth Pressure Balanced (EPBS) shield and the Slurry Shield, have been developed in the recent decades for managing the instability of the excavation profile in unfavourable geotechnical and hydrogeological conditions, with challenge external constraints. As a logical consequence, the evaluation of the stabilizing face pressure is a critical node for both the design and the construction phase. In spite of the importance of the subject, specific recommendations or technical norms are not known as common guidance for the design, and, in the current practice, different approaches are often employed both to evaluate the stability condition of the face and to assess the required stabilizing pressure. The present note deals with these aspects and gives a contribution on the basis of theoretical and experimental considerations, with particular reference to the realization by EPBS of the Porto Light Metro in Portugal (Guglielmetti et al., 2002; Grasso et al., 2002). In the first part of the paper (2), some referenced methods for evaluating the stability of face are presented and are compared using practical examples. Then (3), the consequent problem of defining the adequate design face pressure is treated through analysing international practice and experimental research in laboratory (AFTES, 2001). Finally, the principal results of this study are evaluated on the basis of mentioned experience of the Porto Metro, deriving practical indications for establishing a correct design approach.
2.ANALYSIS OF THE STABILITY OF THE FACE 2.1 Limit equilibrium methods The study of the stability of the tunnel face is a complex problem and a very detailed solution can be developed only on the basis of three-dimensional numerical analysis. However, in many cases also the reference to the so-called methods of limit equilibrium (LEM) gives satisfactory solutions, representing still an important practical tool for design, especially when based on tri-dimensional failure models. A comprehensive treatment of these subject is furnished by W. Broere (2001). In table 2.1, some referenced LEM, applicable to the general condition of cohesive-frictional ground, are shown. The static stability of the face could not be a sufficient design criterion for avoiding settlement on the surface: particularly for shallow tunnels in urban environment, it is also necessary the stability of the excavation both around the shield and the lining (Aristaghes & Auturi, 2003), as well as to preserve the hydrogeological conditions. For this reason, in the practical applications of limit equilibrium methods in section 2.2, the equalizing water pressure in the working chamber of EPBS is always highlighted.
Gallerie e Grandi Opere Sotterranee n.71 - Dicembre 2003 2
Tab.2.1: Selected methods for analysis of face stability in cohesive-frictional ground Method and Basic formulations Scheme 1. Method of Jancsecz and Steiner (J&S, 1994) According to the model of Horn (1961), the three dimensional failure scheme consists of a soil wedge (lower part) and a soil silo (upper part). The vertical pressure resulting from silo and acting on soil wedge is calculated according the Terzaghi's solution. A three dimensional earth coefficient ka3is defined as: ka3=(sincos-cos2tan-Kcostan/1.5)/ (sincos+ sin2tan) where: K[1-sin+tan2(45+/2)]/2 =(1+3t/D)/(1+2t/D)
2. Method of Leca & Dormieux (L&D, 1990) This method is based on the upper and lower limit theorems with a 3D-modelling. The upper(+) and lower (-) limit solutions are derived by means of cinematic and static method, respectively, so giving an optimistic (by defect) and pessimistic (by excess) estimation of the stabilizing pressure. In the case of dry condition, the stabilizing face pressure T is equal to (Ribacchi, 1994): T =-c*ctg+Q**D/2+Qs*(s+ c*ctg) Q,Qs = adimensional factors (from normograms), function of H/a and , where:a = radius of the tunnel; H= thickness of the ground above the tunnel axis.
Note: a third failure mechanism refers to blow-out failure in very shallow tunnel (T is so great that soil is heaved in front of the shield).
Gallerie e Grandi Opere Sotterranee n.71 - Dicembre 2003 3
3. Method of Anognostou and Kovari (A&K,1996) This method is based upon the silo theory (Janssen, 1895) according to the tri-dimensional model of sliding mechanism proposed by Horn. The analysis is performed in drained condition, and a distinction between the stabilizing water pressure and effective pressure in chamber of EPBS is presented. If a gradient between water pressure in the chamber and in the ground exists, destabilizing seepage forces occur and a higher effective pressure is required at the face. However, accepting this flow, the total stabilizing pressure is lower than in the case of imposed hydrogeological balance. The effective stabilizing pressure () is:= F0D-F1c+F2 h-F3ch/D If the material in the chamber is in a fluid state, =0 and solving the equation for h the necessary water pressure for equilibrium is derived. F0,F1,F2,F3= adimensional factors from normograms, function of H/D and .
Note: The original analysis consider ko=0.8-0.4 for the prism and for the wedge (tunnel level), respectively.
2.2 Practical application of the methods A practical application of these methods refers to the design and construction follow-up of the new light metro of the city of Oporto (Portugal), currently under construction by EPBS. As an example, a section with the features reported in table 2.2 is analysed (reference to the above scheme of A&K method) and the results are summarised in Table 2.3. Tab.2.2: Main features of the examined section Geology Complex conditions: prevalent completely weathered granite
(W5) and/or residual soil (W6), with local presence of boulders of relatively less weathered granite (W3/W4).
=10-12kN/m3; c=0-20kPa; =30-34; ko=0.5(assumed); K=10-5-10-7m/s Contour conditions
H=18.2m; ho=14.8m; D=8.7m
Note: K=coefficient of permeability Before comparing the results of Table 2.3, the following should be noted: taking into account the residual uncertainty, the analyses have been
performed considering both a mean (mean shear parameters in tab.2.2) and a worst scenario (lowest values): note that the latter coincides with the design approach;
A&K analyses are performed supposing an hydraulic equilibrium, so that water flow and seepage forces do not occur. In other words the groundwater pressure is completely compensated by the fluid pressure in
Gallerie e Grandi Opere Sotterranee n.71 - Dicembre 2003 4
the working chamber. Hence h=0 and the water table is not modified, avoiding the risk of settlement on the surface;
the method of L&D refers to dry condition: an approximated solution is proposed arranging the original formula in terms of effective parameters and adding the groundwater pressure. According to this approach a weighted value of the unit weight of the ground (f(,)) is considered in the equation of equilibrium;
the results are directly expressed in terms of the required stabilizing pressure: in particular, if the required T is higher than groundwater pressure it means that the tunnel face is unstable even in the condition of hydrogeological balance.
Tab.2.3: Results of the analysis of stability (meanworst scenario; at the tunnel crown/at tunnel invert) Method Notes T(dry)(kPa) T (kPa) T (kPa) 1.(J&S) (115/221)(130/238) 54/7369/90
Upper limit (+) 220 (61/148)(74/161) (013) 2.(L&D) Lower limit (-)* 1030 (62/149)(81/168) (120)
3.(A&K) (61/148)(82/169) 021 Note: *Ribacchi (1994); the values in parenthesis are extrapolated for comparison, taking into account that the existing groundwater pressure is 61/148kPa. According to the results in the table, it is possible to observe that: for the worst scenario, all the analyses confirm the instability of the
face and the consequent necessity of a confinement; if the mean values are used, only for J&S method the hydrogeological balance is not sufficient for stabilizing the face;
there is a practical coincidence between the results of L&D (lower limit) and A&K methods; as derivable from the paper of the latter Authors, a more significant difference among the two methods should be expected for both higher values of cohesion and larger diameter of excavation. These results are also in good agreement with experimental findings from centrifugal models (Chambon e Cort, 1994);
according to the results from the L&D methods, and in particular to the reduced difference between the upper and the lower limit analysis, the solution appears well defined;
the confining pressures obtained with J&S are higher than the other methods and, as in some way expectable, intermediate between them and the active effective pressure Ka (87/117107/139kPa).
3.DEFINITION OF THE DESIGN FACE PRESSURE In the previous section, the practical application of different approaches verified the instability of the tunnel face at the examined section. This implies the necessity of an adequate confinement and, more in general, of a suitable tunneling method: in the cited example a fully mechanized excavation by means of an EPBS. The EPBS principle involves a cutting wheel operating in front of a chamber entirely f