Empirical charts for estimation of minimum response time ( T min ) in free face blasting

Abstract

In drilling and blasting, it is widely accepted that the optimisation of delay timing (interhole and inter-row) can have a positive impact on fragmentation, muckpile characteristics, damage control and ground vibrations. In surface (free face) blasting in particular, minimum response time, also referred to as burden movement time T _min has shown to be a useful engineering index to help with the selection of optimal interhole and inter-row delays. Minimum response time is defined as the time between explosive detonation and rock mass movement. It is hypothesised that in order to improve breakage through the cooperation of adjacent explosive charges, delay intervals should be less than the expected minimum response time; conversely, in order to provide adequate relief between rows of holes and thus reduce confinement, delay intervals should exceed the expected minimum response time. This paper presents an empirical approach to estimate T _min values for different blasting conditions. The proposed model is explained and empirical charts developed with the view to provide a practical tool for first pass calculations.