In weak or faulted rock and high overburden considerable displacements occur during excavation of tunnels and galleries. In most cases the behaviour of the ground is not ductile, but governed by different failure modes. In the design stage it is important to identify potential failure modes under the given boundary conditions to be able to select appropriate construction methods. A particular problem under such conditions is that the strains developing often exceed the deformability of standard linings, frequently leading to severe damages and the necessity of costly repairs. To allow for 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. Recent developments of ductile elements used in combination with shotcrete and rock bolts are shown, and their efficiency 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 to predict displacements and a relatively simple analysis method to design shotcrete linings with integrated yielding elements, based on expected displacements and the transient lining properties are presented. The methods described in this paper have been successfully used in the European Alps and on other projects around the world.
The huge number of tunnels constructed during the last decades has led to the accumulation of considerable experience. Due to improved investigation and modelling methods the construction of tunnels under adverse conditions has become safe and efficient. The inherent inaccuracies in the geological and geotechnical models, as well as the simplifications in the analysis tools also play a role in the success of a project. Experience in tunnel construction in poor and faulted rocks under high overburden still is limited. However, with the construction of long basis tunnels those conditions are met quite frequently. A serious challenge is the heterogeneity of brittle faults, as well as the large, anisotropic and in many cases long lasting deformations . In addition, with high overburden the exact location and properties of fault zones usually are unknown, requiring continuous updating of the ground model during construction, as well as an adjustment of the construction methods to the actual conditions. Advanced exploration and monitoring methods are required to recognize changes in the ground quality in time, allowing for a timely adjustment of excavation and support. Special attention has to be paid to an appropriate estimation of displacement magnitudes and its timely development. The size of the excavation has to allow for the expected displacements without impairing the clearance profile. When displacements are underestimated, extremely costly reshaping is required. On the other hand, the timely development of the displacements can play a major role in the utilization of the lining capacity in case shotcrete is used. Conventional supports often are not able to sustain the large imposed strains. In the past this problem has been addressed by leaving open gaps in shotcrete linings..