Effect of trenching on bast-Induced ground vibration in Sri Lankan metal quarries

Abstract

Problems due to ground vibration are a matter of serious concern for the users of explosives, as it is not possible to eliminate vibration totally or to contain them at the source. Efforts need to be made in controlling the vibration to fall within the safe level, without affecting the production schedules and economic viability of the project. If ground vibrations predicted or monitored exceed the statutory limit, control measures are taken by modifying the blast design parameters. In the research, digging a trench between the vibration source and the point of observation has been studied as a technique to reduce the transmission of the Rayleigh waves. The extent to which ground vibration could be reduced has been examined by field experiments carried out at an open cast quarry situated at Arankele, Sri Lanka. Blast vibration measurements have been carried out for active isolation, where the trench is excavated closer to the blasting point. An average of ten single shot hole blasts carried out correspondingly for four different depths of shot holes namely 1 m, 1.5 m, 2 m and 2.5 m with varying depths of trench shows ground vibration reduction percentages of 6.3%, 1.2%, 7.0% and 18.3% at a point immediately after the trench keeping the depth of the trench greater than that of the shot hole in each blast, and reduction percentages at a point closer to the observation point under the same conditions for the four shot hole depths were 72.2%, 54.4%, 55.1% and 45.1% respectively. The corresponding values after digging the trench show ground vibration reduction percentages as 57.8%, 53.9%, 40.5% and 36.7% at the point immediately after the trench location and vibration reduction percentages of 55.5%, 64.7%, 56.8% and 42.3% at the point closer to the point of observation for the corresponding four shot hole depths. Results indicate that after digging the trench, ground vibration reduction levels recorded were much higher, compared to those obtained before digging the trench. However, results obtained closer to the point of observation do not indicate a reduction for each shot hole depth. This leads to the conclusion that active isolation results in a considerable reduction in ground vibration level under given geological conditions.