Design and construction aspects of deep tunnels (with particular emphasis on strain softening rocks) (Reprinted from Tunnels under Pressure)

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 exp ected to cause rock burst, large deformations and creep, depending on the n ature of the rock. In the first part of the paper the theoretical relations between support pressure and tunnel wall displacements are briefly discuss ed. Emphasis is given to the post-failure behaviour of the rock mass and it s influences on the tunnelling conditions. Frequently encountered strain so ftening rock types are shown to exhibit a strong dependence of the tunnel s tability on. the softening rate, which itself varies with the applied confi nement pressure. Also discussed are results of laboratory tests and field o bservations which yield a critical deformation value for a given rock type. Beyond this value,the required support pressures are shown to increase sha rply. 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 suppo rt types able to control the tunnel wall displacements efficiently. They sh ould oppose significant support pressures from incipient deformations up to large displacements. Various constructive solutions are discussed for dril l & blast as well as for TBM excavation. The last section briefly addresses time-dependent tunnel deformations and their influence on the time of plac ing the final lining. (C) 2001 South African Institute of Mining and Metall urgy Published by Elsevier Science Ltd. All rights reserved.