Effective implementation of controlled blasting methodology during excavation of hard rock in the close proximity of earthen dam and tunnel

The present technical paper outlines the details of the controlled blasting techniques used to optimize blasting pattern for excavation of hard rock near the Bhira Earthen Dam in Maharashtra, India. In this connection, a series of experimental blasts were conducted by adjusting various blast design parameters at project site. The safe charge weight per delay was kept between 0.125 and 0.375 kg. The outcomes of these experimental blasts were analyzed to recommend optimized blasting patterns and methods for the overall excavation process during actual blasting operations. Blast design parameters, including the maximum quantity of explosive per delay, hole depth, burden and spacing between holes were optimized by using a site-specific attenuation equation, taking into account the proximity of the dam and tunnel from the blasting area. Peak particle velocity (PPV) level of 10 mm/s and 50 mm/s respectively were adopted as the safe vibration level for ensuring safety of the Bhira Earthen Dam and the nearby tunnel from the adverse effects of blast vibrations by analyzing the dominant frequency of ground vibrations observed and also by reviewing various international standards. Frequency of the ground vibrations observed on the dam and tunnel from majority of the blasts was found to be more than 10 Hz and 50 Hz respectively. During the entire period of blasting, the blast vibrations were recorded to be far lower than the safe vibration level set for these structures. Maximum Vibration level of about 0.8 mm/s and 35 mm/s were observed on dam and tunnel respectively which are far lower than the safe vibration level adopted for these structures. Hence, the entire excavation work was completed successfully and safely, without endangering the safety of dam or tunnel.