ABSTRACT
In few engineering projects are the feasibility, the planning, the costing, the design, the techniques used and the risk of serious accidents during construction so dependent on the geology of the site as in tunnelling. The principal geological consideration is ease of excavation of the trench. The principal factor controlling rate of progress and costs in constructing a tunnel in strong rocks is usually relative ease of excavation. The traditional method is to blast out successive sections of the tunnel by drilling a pattern of holes in the rock, charging them with explosives and firing. The properties in bulk, particularly the shear strength, of the material forming the cutting: the stability of a cutting in rock is usually dependent on the occurrence of joints and other planes of weakness, and on the amount of cohesion and the friction across these planes.
