Tunnelling operations, apart from fixing the alignment and lining comprise some intermediate operations. The adoption of any one of a number of possible methods for tunnelling depends on the nature of the soil profile. A Tunnel Boring Machine is used, which can work on full face of the tunnel at one time, by varying the type of tools to suit the type of soil, slush to rock. Clayey soils are generally dealt with by the shield method, a method gradually refined over the years with the advent of tunnelling machines. Tunnelling in rock is essentially a process comprising drilling and blasting of rock and removal of muck. Crownbars are provided between the ribs and the exposed surface of the tunnel roof as it is difficult to make the ribs fit correctly against the exposed surface. In tunnels with small cross-section, the crown joints may be ‘kicked’ up or the ribs set high to accommodate the concrete delivery pipe.