ABSTRACT
Blasting is resorted to while dealing with hard rock and soft rock of compact nature. Special care has to be taken while blasting. The blasting operation has to cover just the profile and the tunnel has to advance progressively. It has to be done in such a way as to minimise shock, vibration and any other disturbance to the rock structure around the profile of the tunnel. It should be such as to ensure minimum ‘overbreak’, i.e., the extra section over and above the minimum section required to be removed (see Figure 7.1). This is particularly important when the tunnel is to be lined. Otherwise, extra lining material, an expensive item, will have to be used. The basic principle is to loosen the volume of virgin rock in such a way that when it is removed, the length of the tunnel has advanced in the correct direction, and conforming as nearly as possible to the correct cross-section.
