Depending principally on the characteristics and geological environment of the ground, one of the four basic tunnelling methods can be used to construct a tunnel - hand or digger shields, drill and blast, partial face machines and full face tunnel boring machines. Each of these has characteristic features which preclude their use in some strata. Thus shield tunnelling in its conventional form may be limited by the need for longitudinally stiff linings; drill and blast tends to damage peripheral rocks and may be slowed by its cyclic nature; partial face machines will not cut strong rocks, and tunnel boring machines can only be used with confidence in uniform medium strength rocks. Research and development will continue to improve performance and reduce these limitations. In order to provide an informed basis for selection, some of the important current limitations of each tunnelling method are considered in detail, together with the relevant features which may determine its successful operation. (A)

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