Blast induced rock mass damage around tunnels

Drilling and blasting is a preferred method of rock excavation world-wide due to low initial investment, cheap explosive energy, easy acceptability among the blasting engineers and, possibility to deal with different shapes and sizes of openings. Although, drill and blast method has witnessed significant technological advancements, it has inherent disadvantage of deteriorating surrounding rock mass due to development of network of fine cracks in it leading to safety and stability problems. The damage in the peripheral rock mass culminates in the form of overbreak and damaged zone beyond overbreak. In some cases the projects cost has increased more than 15% because of overbreak. Although significant efforts have been made to assess damage to the surrounding rock mass using different methods, the solution based on easily available site parameters is still lacking. Authors have carried out field investigations at five different tunnels located in Himalaya, India to study blast induced damage for wide range of rock mass quality Q values (0.03-17.8). In addition to Q, specific charge, perimeter charge factor, maximum charge per delay, advancement and confinement factors have also been used. Data sets of 113 experimental blasts are collected from the five tunnel sites. All the parameters, easily available to the site engineers, have been used for developing an empirical correlation to estimate the rock mass damage around the tunnel, which is discussed in the paper. The proposed empirical correlation has been validated using ultrasonic tests on rock core samples obtained from one of the experimental location.