Abstract
Many quarries and shallow open pit mines have to deal with blocky rock conditions. The geometry and geomechanical nature of structural networks in place dictate to a large extent the outcome of drilling and blasting operations. Blasting engineers and field practitioners have to adapt to the overall mechanical behavior of the rock mass and the specific orientation and potential incidence of the structures in place with the blasting layout. Understanding the influence of geological features in blasting outcomes, and incorporating these notions into practical designs often rely on experience from practitioners and site-specific adjustments. The present study investigates the influence of very blocky conditions on blasting requirements from a series of large-scale experimental trials.
Joint and discontinuities affect in different ways the fragmentation by blasting behavior of rocks. Joints can act as reflective surfaces enhancing local fracture initiation, or dampen the main shockwave responsible for rock fracturing mechanisms. Joints also represent preferential failure planes which may dictate the size of larger blocks. This work investigates blastability (i.e. effort required to blast) for very blocky geological conditions. Blastability indexes provide a frame of reference for comparison purposes between different operational or geological conditions. This study investigated blastability of a very blocky geology through single hole blast (SHB) test indexing.
SHB indexing consists in carrying a series of single hole blast tests parallel to a free face at different burden intervals. The resulting crater is mapped using topographical mapping tools (LiDAR in this case) to quantify the burden-dependent relationship for cratering dimensions. SHB craters are described through a power formulation yielding two index parameters: a burden dependent coefficient and a site specific exponent. These index parameters can be used to contrast different settings or operational parameters, or derive site-specific burden-dependent specific charge guidelines.
In this study, 12 SHB tests were carried at a limestone quarry in the greater metropolitan area of Montreal. Tests carried followed closely the normal operational conditions (e.g. diameter, charge, stemming, etc.). The geological setting presented a blocky to very blocky structural framework. The SHB tests were drilled along pre-existing bench faces with varying incidence angle from dominant vertical structures in place. The results show constant cratering mechanisms up to a certain burden value. This burden threshold appears to be dictated in part by joint spacing. Bench face orientation appears to influence the burden dependent coefficient for the crater model considered. Additional considerations are given to taking into account SHB indexing considering blocky conditions.