It is a well known fact that rock mass properties influence the proces
s of fragmentation considerably. Model blasts and field investigations
were carried out to find the effects of rock mass quality and joint o
rientation on tunnel blasting. Propagation of shock waves are partiall
y restricted by joint planes. It was observed that the blast results (
i.e., average fragment size and depth and cross-sectional area of the
broken zone) were considerably influenced by joint orientation. Accord
ingly, it has been concluded that loading equipment with a larger capa
city and deeper blast holes are required in formations with joint plan
es perpendicular to the tunnel axis. The number of blast holes, howeve
r, should be greater when joints are parallel to the tunnel axis. Furt
hermore, the powder factor (kg/m3) has been found to be directly relat
ed to rock mass quality (Q). Optimisation of pull, powder factor and o
verbreak is required in the case of weak formations with joints perpen
dicular to the tunnel axis. The use of contour blasting technique seem
s to be essential in poor and fair rock masses to minimise the overbre
ak, reduce the support cost and improve the stability of the opening.