Pre-Formed Linings In Tunnelling Practice

INTRODUCTION For many years, in Europe and in North America, pre-formed metallic and concrete segmental linings in tunnels have been used extensively as temporary and permanent ground support systems. In the United Kingdom the use of cast iron as a means of ground support in mining shafts dates from the first decade of the 19th century. Although at that time wood tubbing was extensively employed, cast iron was used in shafts at Wallsend and Percy Main collieries in the North East of England. The original form of cast iron tubbing consisted of cylinders which were lowered bodily into the pit and were built from the bottom up, but subsequently they were subdivided into segments which involved the making of vertical as well as horizontal joints. Principally, the practice of providing immediate pre-formed support developed because of the incompetent state of the surrounding ground and, although this still remains the over-riding consideration nowadays, several other factors are examined before finally deciding what type of lining to use. Other branches of sub-terranean technology such as tunnel thrusting or immersed tubes (both of which involve pre-formed linings) can provide alternative solutions to problems hitherto thought better solved by bored tunnelling methods. However, this paper will confine itself to those linings developed for the more traditional tunnelling methods. Usually the shape of segmentally lined tunnels is circular because this profile is the most suitable for bad ground and many tunnelling machines. This does not always lead to the most economic use of space within the tunnel and horseshoe or elliptical linings are sometimes used in suitable ground. The number of segments