In weak rock or under high overburden, considerable displacements occur during excavation of tunnels and galleries. The strains developing in many cases exceed the deformability of standard linings, frequently leading to severe damages and the necessity of costly repairs. To allow for a safe and economical tunnel construction, strategies have to be used, which guarantee support characteristics compatible with the strains, and at the same time utilize the supports as much as possible. After a review of traditional methods, mainly used in mining in the past recent developments to deal with high displacements in combination with modern standard supports, such as shotcrete and rock bolts are shown. The different systems currently available are critically reviewed. For the design of such supports the development of the expected displacements must be predicted and the time dependent properties of shotcrete considered. Special tools have been developed to predict displacements. A relatively simple analysis method to design shotcrete linings with integrated steel elements, based on predicted displacements and the transient lining properties is used to demonstrate the effectiveness and practical applicability of the various systems available.
Large displacements during excavation of tunnels due to poor rock and high stresses are a challenge for designers and contractors. Displacements can reach several tens of centimeters, in some cases displacements of one meter and more have been reported [1,2]. Associated with those large displacements are difficulties in predicting their magnitude and development, as well as problems of the limited deformability of standard supports. Tunnel supports on the one hand should provide as much resistance against deformation as possible, on the other hand should be able to sustain the large imposed strains. Various methods have been developed over the decades to cope with the difficulties. This paper addresses some aspects of consequences of large displacements in relation to the lining design. This includes a review of support techniques used in the past and recent developments of yielding supports, and the experience made with their application on site.
2. DEVELOPMENT OF SUPPORTS FOR LARGE DISPLACEMENTS
A traditional method in mining when experiencing large displacements was to use U-shaped steel sets with sliding couplings in combination with wire mesh or lagging.
Figure 1. Destroyed steel set support with sliding couplings. Photo: DMT(available in full paper)
Figure 1 illustrates the deficiencies of such supports. In particular in cases of anisotropic deformation the steel sets buckle, and costly and dangerous repairs are required. Timber supports had to be replaced many times until stabilization was reached.
2.1. Timber elements
With the introduction of concrete and shotcrete linings in the late nineteen fifties, the previous problem of excessive loosening diminished, but the comparatively low deformability of concrete lead to destruction of the lining in case of larger displacements. Rabcewicz proposed timber elements integrated into a concrete lining as early as 1950 to provide sufficient deformation capacity of the system . Depending on the required ductility and resistance different types of timber can be used.
Figure 2. Concrete support with integrated timber element, as proposed by Rabcewicz (available in full paper)