Evolution of effective charge weight per delay for prediction of ground vibrations generated from blasting in a limestone mine

Abstract

A major concern when blasting at surface mines is the generation of ground vibrations and air blasts and their effects on nearby structures. The current accepted practice is to control the weight of explosive in a delay, say 8 ms. Uncertainties in vibration analysis from blasts can be attributed to the problem of determining the actual amount of explosive detonating within a specific time interval, and the number of holes effectively contributing to the resulting particle velocity. This paper describes the results of a systematic study carried out to investigate these effects. It involved a total of over 29 blasts, employing one-, two- and three-hole calibration studies, and regular production blasts. In the latter, the bench height varied from 5.5 – 9.5 m, and boreholes loaded with ANFO explosives were 100 mm and 160 mm in diameter. Six tri-axial vibration sensors were located on ground surface along a linear array from 50 – 750 m from the blast, to investigate the changes in amplitude, frequency and duration with distance. The study compares single-hole blast signatures with multi-hole ones, and provides guidelines on determining the ‘effective’ charge weight per delay in vibration analyses of such blasts.