PRE-FORMED LININGS IN TUNNELLING PRACTICE

Based on a detailed description of the applications for preformed metallic and concrete segmental lining in Europe and North America paper concludes that the use of pre-formed segmental linings instead of in situ placed linings in tunnels will predominate where the physical characteristics of the ground require the placing of a support system quickly after excavation and where economic considerations show that a permanent lining can be achieved in a single process. These linings are more usefully employed in soft ground, possibly water bearing, varying from sands and gravel through cohesive clays, mudstones and chalks rather than hard rock conditions although even in regard to the latter the choice must depend on the degree of competence of the rock. A reduction in lining thickness in some grounds may be achieved by allowing the newly excavated cavity at a tunnel face to redistribute ground stresses before placing the permanent lining. Because of the time element, and the need to protect the tunnel, either a temporary flexible support is required or pre-formed linings must be devised which will partially collapse through frangible packings to take up the initial deformations of the ground. In the former case a permanent lining would eventually be formed to encompass the temporary work, while the latter in a single construction perhaps provides the more economic solution.