Controlling backbreak and enhancing fragmentation in dragline bench blasting—a geo-engineering approach

Abstract

Dragline bench blasting contributes to about 200 million m3 of overburden excavation in India. Large-sized bench blasts with multiple rows (8–10 nos.) and holes per row (18–20 nos.) are carried out to obtain a dragline cut measuring, typically, 80 m in width and 200 m in length. Backbreak is generally observed in such blasts, which deteriorates blasting efficiency, fragmentation, and dragline utilization. Rockmass disposition, blast design, explosive selection, and blast-induced ground vibration were found to be the key contributing factors affecting backbreak and fragmentation. In this research work, in situ P wave velocity profiles were generated using 24-channel seismic refraction tomography for competency mapping of the rockmass. Seismic tomography identified three sonically distinct layers up to a depth of 22–30 m. Near-field vibration monitoring was conducted to record ground vibration signatures using two triaxial borehole geophones installed within a distance of 40 m from the last row of the blast and the vibration levels up to 839.71 mm/s and backbreaks extending up to 9.9 m were observed. Fragmentation analysis was carried out using scaled photography and WipFrag software. Behavior of backbreak and mean fragment size with peak vector sum of ground vibration, in situ P wave velocity, and bench stiffness was examined. This was followed by development of mathematical models for predicting backbreak and mean fragment size. Suitable blast design, charging scheme, and delay configuration were suggested based on rockmass competency, threshold peak particle velocity, and desired fragmentation. The suggested pattern was implemented and found to control backbreak within 5.5 m and mean fragment size at 43.38 cm.