Yielding supports

In tunnelling with gripper TBMs, the same yielding supports can be applied as for conventional tun- nelling. The Northern Section of the Vereina Tunnel (Switzerland, D = 7.64 m) and the Sections Amsteg (D = 9.58 m) and Faido (D = 9.43 m) of the Gotthard Base Tunnel (Switzerland) may be mentioned as practical examples.

The applicability range of the "yielding principle" is, nevertheless, strongly limited by the fixed ge- ometry of the tunnelling equipment and the required clearance profile. The design of the yielding support must be considered when selecting the boring diameter and the dimensions of the back-up equipment in order to avoid costly (and sometimes dangerous) re-profiling works or jamming of the back-up equipment {3-13, 5-13, 3-14, 5-14, 6-14}. The continuous adjustment of the boring diame- ter using overboring techniques is still not sufficiently reliable today (cf. Section 4.3.2) and, if at all feasible, the increase that can be achieved in the boring diameter is also limited (Table 3). On the other hand, the choice of a fixed, but larger boring diameter for the entire tunnel is often not eco- nomical.

It should be also noted, that the deformability of the lining leads to a reduction of its loading, but weakens the longitudinal arch action and thus leads to an increase in the ground pressure acting upon the shield {5-10}. Furthermore, allowing larger deformations may lead to major loosening phenomena or to a softening of the ground. This has to be taking into account in the design of the deformable tunnel support. In addition to an appropriate structural detailing, a sufficiently high yield pressure is very important for safety (Anagnostou and Cantieni, 2007; Cantieni and Anagnostou, 2009b), but may be difficult to be achieved in combination with shotcrete because (in contrast to conventional tunnelling) the advance rates are high relative to the time needed for shotcrete hard- ening. Steel sets with sliding connections are advantageous in this respect. Furthermore, long bolts (> 5–6 m for common cross-sections of traffic tunnels) are indispensable, particularly for coping

with non-uniform rock convergences and in order to ensure the stability of deformable tunnel sup- ports during the yielding phase. It should be noted that systematic bolting (in contrast to shotcrete or steel sets) does not consume bored space, thus leaving more space free for the ground defor- mations to occur. The support pressure achievable by bolts is, nevertheless, rather low (0.1–0.3 MPa).

In tunnelling with shielded TBMs, coping with squeezing pressure may necessitate very thick seg- ments, which, besides increasing the required boring diameter, are difficult to handle. Deformable segmental lining systems specifically for shielded TBMs have therefore been the subject of inten- sive past and current research and development (e.g., Billig et al., 2007a; Schneider et al., 2005;

Vigl, 2003). Such deformable linings could be applied either with a "conventional" TBM or, as pro- posed by Baumann and Zischinsky (1993), Robbins (1997) and Wittke-Schmitt et al. (2005), in combination with the alternative TBM concepts illustrated in Figure 8a–e. A deformable segmental lining can be realized basically in two ways: either, (i), by arranging a compressible layer between the ground and the lining (the ground experiences convergences, while the deformations of the lin- ing remain small, Figure 12a); or, (ii), by arranging special deformable elements in the longitudinal joints of the lining that allow for a reduction of its circumference (Figure 12b).

Yielding layers between segmental lining and ground

The basic idea has been proposed and was patented in England in 1979 – J. Mowlem, UK Patent application GB 2013 757 A, cf. Schneider et al. (2005). For a TBM drive through swelling ground, Lombardi (1981) proposed the application of a compressible layer consisting of polyurethane foam.

Wittke-Schmitt et al. (2005) investigated the possibility of using expanded clay as a backfilling ma- terial in combination with the alternative TBM concept of Figure 8c. (Note that in order to allow ra- dial ground deformations of 1.20 m and considering the deformability of the expanded clay, the concept requires a radial annular gap of 2.40 m, thus leading to a boring diameter of 15.10 m.) Vigl (2003) presented a "convergence-compatible" segmental lining. The segments in this so-called

"CO-CO-system" incorporate at their extrados supporting ribs which are in contact with the rock.

Figure 12. Concepts for deformable segmental linings: (a) compressible layer between rock and lining;

(b) deformable elements in the longitudinal joints between the segments.

The ground is allowed to squeeze into the space between the ribs, which can be either empty or filled by a compressible material. Another possibility is given by the addition of a compressible layer fixed at the extrados of the segments in combination with a traditional annular grouting or a com- pressible grout (Billig et al., 2007a; Schneider et al., 2005).

A compressible annulus grouting material must have, with the exception of a high deformability of course, all of the other usual properties of gap grouting materials: easy processing, pumpability and high stability of the material. For these and for economic reasons light weight concrete is usually proposed (Strohhäusl, 1996). Schneider et al. (2005) developed the so-called "Compex", a com- pressible mortar with expanded polystyrene that can be compressed up to 50 %. Billig et al.

(2007a) reported about the development of the so-called "DeCo Grout", a cement-based pumpable mortar with expanded polystyrene pearls and foam, which is also characterized by a maximum compression of about 50 %.

Deformable longitudinal joint elements

Wood was often used in the past as a compressible element in mining (Figure 13a) and it is inter- esting to note that it was also applied in combination with prefabricated concrete elements many years ago (Lenk, 1931). Recent, mainly experimental, attempts to increase the flexibility of precast segmental linings utilize neoprene elements or hydraulic devices, which are arranged in the longi- tudinal joints (Figure 13b–c).

Brunar and Powondra (1985) reported on the development of the so-called "Meypo deformable elements", which should be placed in the longitudinal joints of a segmental lining and allow for a reduction of its circumference by 1.80 m (6 joints, each experiencing a compression by 30 cm). Ac- cording to the authors, these elements have been designed with such a high yield load (3 MN) that the lining already offers a considerable support pressure at small ground deformations and this is important in order to avoid loosening or ravelling of the ground. The Meypo deformable elements have been applied in a tunnel in the Ibbenbüren Coalmine in Germany at a depth of about 1500 m (inner diameter of the segmental lining 9.47 m). In this case, however, the tunnel was driven con- ventionally, support during excavation consisted of shotcrete and rock bolts and the segmental lin- Figure 13. Prefabricated segmental lining with compressible longitudinal joints: (a) wood (Lenk, 1931);

(b) neoprene layers and flatjacks (Croci, 1986); (c) hydraulic jacks (Baumann and Zischinsky, 1993);

(d) steel tubes (Tusch and Thompson, 1996); (e) highly deformable concrete elements (Kovári, 2005).

ing was applied as a final tunnel support. The deformable elements fulfilled expectations, but it was also realized that the costs of such a solution would be too high for it to be systematically applied in tunnelling (Maidl et al., 2001). The suggestion was therefore made of deploying reusable deform- able elements, which must be removed after the ground has deformed and before setting the sys- tem rigid by applying shotcrete into the longitudinal slots. Baumann and Zischinski (1993) proposed the use of hydraulic jacks for this purpose. These are also expensive, slowing down installation considerably and necessitating heavy reinforcement in order to overcome burst and shear forces.

As reported by Croci (1986), hydraulic jacks have also been applied earlier in the Tunnel Santomarco (Italy). In a section with squeezing crystalline and phyllitic rock, excavation was carried out mechanically under the protection of a shield (Caldarella, 1986). In order to reduce the load act- ing upon the segmental lining (45 cm thick, inner diameter 7 m) hydraulic jacks were introduced into the longitudinal joints in combination with deformable neoprene elements, which were glued onto the segments and allowed a convergence of 1 %. This application had mainly an experimental character. After 2.5 months, it became necessary to strengthen the tunnel support by applying steel sets and shotcrete.

The technical literature also includes other types of deformable elements. So, for example, Strohhäusl (1996) proposed plastic bodies, which are made from the same material as sealing gaskets and have cavities which can be filled with more or less stiff materials like polyurethane or lightweight concrete. The mechanical properties of these deformable elements can be regulated by determining the volume and the infillings of the cavities. Another possibility is the application of steel or plastic pipes, which become ovalized at a certain hoop force (Maidl et al., 2001). According to a method patented by Tusch and Thompson (1996), the steel tubes (Figure 13d) should be filled by a fluid and, in order to control the hoop force, be equipped with pressure valves. Another techni- cal option are the highly deformable concrete elements (Figure 13e) proposed by Kovári (2005), which are composed of a mixture of cement, steel fibres and hollow glass particles and collapse at a pre-defined compressive stress depending on the composition of the concrete.