Design and construction aspects of deep tunnels (with particular emphasis on strain softening rocks)

Abstract

A series of railway tunnels will be dug through the European Alps at depths exceeding 2000 m over long stretches. The prevailing high stresses are expected to cause rock burst, large deformations and creep, depending on the nature of the rock. In the first part of the paper the theoretical relations between support pressure and tunnel wall displacements are briefly discussed. Emphasis is given to the post-failure behaviour of the rock mass and its influences on the tunnelling conditions. Frequently encountered strain softening rock types are shown to exhibit a strong dependence of the tunnel stability on the softening rate, which itself varies with the applied confinement pressure. Also discussed are results of laboratory tests and field observations which yield a critical deformation value for a given rock type. Beyond this value, the required support pressures are shown to increase sharply. Systematic monitoring of the rock deformations due to tunnelling can help to define the most adequate support measures and to improve the input values for static calculations by back analyses. Deep tunnels require support types able to control the tunnel wall displacements efficiently. They should oppose significant support pressures from incipient deformations up to large displacements. Various constructive solutions are discussed for-drill & blast as well as for TBM excavation. The last section briefly addresses time-dependent tunnel deformation and their influence on the time of placing the final lining.