Extent of Rock mass Damage Induced by Blasting in Tunneling

Drilling and blasting is preferred method of rock excavation world-wide due to low initial investment cheap explosive energy, easy acceptability amongst the blasting engineers, possibility to deal with different shapes and sizes of openinings. Although drill andblast method has witnessed significant technological advancements, it has inherent disadvantage of deteriorating surrounding rock mass due to developemt of network of fine caracks in it leading to safety and stability problem. The damage in the peripheral rock mass culminates in the form of overbrak and damanged zone. Overbreak increases project cost by more than 15%. The damaged zone extends beyond overbreak. Although significant efforts have been made to assess damage to the surrounding rock mass using different methods, easier solution based on easily available site parameters is still lacking. Authors have carried out field investigations at five different tunnel construction project sites located in Himalaya, India to formulate an empirical correlation for prediction of blast induced damamge for wide range of Q values (0.04 – 17.8). The proposed correlation is based on specific charge, perimeter charge factor, maximum charge per delay, advancement and confinement factor and rock mass quality rating Q. All the parameters used in empirical correlations are readily available to the site engineers and does not require laboratory testing. Data sets of 113 experimental blasts are collected from te five tunnel sites. The proposed empirical correlation has been validated using ultrasonic tests on rock core samples obtained from one of the experimental location

Investigation of excavation damage zone in Himalayan tunnel

Expreiments were carried out to measure blast induced damage in 99 MW L7T-Singoli-Bhatwari hydroelectric power project, Rudraprayag. Repetitive NX size rock core up to 5.0 m depth were taken from side wall and face with triple trube core barrel drill. The rock samples were tested using ultrasonic techniques. Ultrasonic tests on NX size core reveal that the 2.0 m of the zone surrounding the opening are adversely affect by the tunnel excavation blasting process. The ultrasonic velocities reduce to 70% of the average values in the initial 0.5 m from the excavation line. It is also observed that characterization of blast induced damage zone beyond 2.0 m with ultrasonic techniques is inconclusive due to natural variation in ultrasonic velocity on account of the anisotropy of the roc