Controlled blasting in underground construction: A case study of a tunnel plug demolition in the Neelum Jhelum hydroelectric project

Blasting is a popular method of excavation in underground construction. Blast-induced vibrations, however, if not properly controlled, can negatively affect the rock mass surrounding the tunnel. During the drill-and-blast (D&B) excavation of twin headrace tunnels through a major fault zone below the Jhelum River, a 180-m(3)/h groundwater inflow occurred from the tunnel invert. A 610-m(3) concrete plug and high-pressure grouting around the tunnel periphery were formed to stabilize the tunnel and control inflow. To complete the tunnel, the temporary tunnel plug was ultimately demolished using controlled blasting. Ground vibration monitoring data from the regular D&B operations were used to develop a site-specific vibration attenuation model. Using geotechnical data from the feasibility stage and tunnel mappings during construction, a critical particle velocity above which tensile cracking could be initiated was determined for the rock mass surrounding the plug. The critical particle velocity was incorporated into the vibration attenuation model to obtain permissible charge weights to limit blast damage to within a short distance from the charge. Using this approach, an innovative blasting pattern was adopted in which the tunnel plug was sequentially demolished by detonating a burn cut and separate contour blast rows. During blasting, rigorous supervision and data collection helped to record, evaluate, and when needed, adjust the blasting program. Overall, the tunnel plug demolition was completed in a safe and cost-effective manner.