Initiation of residual stress zone during blasting in jointed granite rock mass in operation of Priargunsky Industrial Mining and Chemical Union

Abstract

Russian and foreign technical literature says that blasting in jointed rock mass induces zones with altered physical properties and geomechanical behavior beyond the perimeter holes. These zones are identified as the crushed zone, radially fractured zone, spalling zone and shattered zone. The shattered zone lacks scientific attention although this zone can reach the width of (30–170)db (db—blasthole diameter) in open pit mines and (25–75)db in underground mines. The earlier implemented research answers the question on quantitative change in the stresses and strains of a jointed rock mass during blasting. The initiation mechanism of the shattered zone remains unclear. This study aims to find the initiation mechanism of the shattered zone in jointed rock mass and to determine its stress state after blasting. Three series of full-scale experimental studies have been performed in jointed granite rock mass using acoustic emission methods, ultrasonic techniques and deformation measurements. It is found that in the shattered zone, blocks in the jointed rock mass displace radially from the blast holes with deformation of the joint surfaces and with elastic strains preserved after blasting. For this reason, this zone is qualified as the zone of blast-induced residual stresses. The article gives the formula for the residual radial compressive stresses under short-delay multi-row blasting. The numerical calculation using this formula and the actual mine data prove the formula validity. The method of destressing blasting is proposed to unload rockburst-hazardous rock mass from stresses in the areas of the confining pressure phenomena such as spalling and sloughing. This method has been trialed in Priargunsky’s mines in granite rock mass (1400 m long area was unloaded from stresses in mines). The authors highly appreciate participation of I. I. Shishkin, B. M. Belyaev, V. M. Pankov and V. A. Pazdnikova in the experimental research.