ABSTRACT:

In Alpine fault zones with high in-situ stress, tunneling can result in substantial and enduring ground deformations. The magnitude of deformations is closely tied to the applied excavation- and support concept, though typically, they can reach tens of centimeters. The principle of yielding is based on the idea that as deformation increases, the need for support capacity decreases.

Yielding elements, as a part of the shotcrete lining, represent the state-of-the-art method for tunneling in Alpine fault zones. These elements help to prevent the shotcrete lining from being overstressed during the early curing process. This is particularly important close to the tunnel face, where the deformation rate is highest, and the shotcrete strength is at its lowest. Furthermore, long-term creep deformations of the rock mass can be absorbed with such a lining system. This lining system provides a higher support resistance level than steel sets, thus allowing for controlled deformations at a reasonable cost. This approach has proved effective and cost-efficient in numerous projects with challenging geotechnical conditions. Radial deformations caused by the rock pressure are transformed into a tangential closure of the ductile lining. Hence, the load-dependent stiffness of the yielding elements must be less than the time-dependent stiffness of the shotcrete at all times to avoid overstressing. Since displacement development depends on the distance to the face and time, the tunneling process requires high flexibility in the excavation and support concept. The Semmering Base Tunnel is a single-track, double-tube railway tunnel currently under construction in Austria. With the alignment of the 27.3 km long tunnel, major fault zones under high overburden have to be crossed. An extraordinary excavation- and support concept is applied to control the vast deformations of up to 200 cm. A temporary pilot tunnel is excavated to increase pre-relaxation of the highly stressed ground and ease excavation conditions for later tunnel widening. For both tunnels, a shotcrete lining with yielding elements is applied. A novel yielding element was invented to overcome the drawbacks of existing systems. This novel HS-EPS yielding element consists of a high-strength expanded polystyrene, which makes it lightweight and enables safe and rapid installation. Furthermore, the modular setup of the element permits on-site adaption to react to actual deformation patterns and changing ground conditions. Recent experiences from the application at the Semmering Base Tunnel are presented.

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