Destress blasting in hard rock mines - a state-of-the-art review

CIM Bulletin, Vol. 1, No. 1091, 2006

H.S. Mitri and M.R. Saharan

The file is a zipped PDF document.Destress blasting or preconditioning, a rockburst control measure, has been tried to control strainbursts since the beginning of the 20th century. The concept of destress blasting evolved from the observation that the zone of highly fractured rock immediately surrounding some deep underground openings seems to offer some shielding to both the occurrence of and damage from rockbursts. It was argued that extending and maintaining this zone of the fractured rock ahead of a face can reduce both the occurrence and effects of rockbursts. Destress blasting was again re-evaluated for South African gold mines in the late 1980s.The current practice in South Africa has notable departures from the original concepts of destress blasting with the main objective of destress blasting being to activate already existing fractures rather than initiate new ones, and locating the blastholes in zones that are already fractured. Also, the position and depth of destress blastholes should be confined to the already fractured zone for an effective application. The aim of destress blasting is to shift stress concentrations and associated seismic activities deeper into the rock. The extent of the already fractured zone is generally 3 m to 5 m from the active face. Therefore, destress blasting should be part of a regular production cycle to be effective in continuously transferring seismic activities away from the face in a systematic manner. A low shock and high gas energy explosive (ANFO types) has a better effect in the opening and extending pre-existing fractures, hence should be employed. Destress boreholes should be equally far from both the hangingwall and the footwall to keep them free of blast-induced damage and to minimize associated ground control problems. The borehole spacing of 3.0 m is established for the 38 mm diameter boreholes based on experience, borehole endoscopy, and ground penetrating radar.In North America, destress blasting is more widely practised, particularly on sill and crown pillars in deep, narrow-vein mines. The following summarize the prevalent notions for destress blasting particularly applied to pillars and stopes of the steeply dipping veins of metal mines in North America: The main objective of destress blasting is to fracture highly stressed stiff rocks. The resultant effect of such exercises is to transfer mining-induced stresses away from the working areas of the mine. Destressing involves a change in rock mass properties as well as a reduction in stresses after destress blasting. The blasthole depth is a function of the desired destressing area, the magnitude of the mining-induced stresses, the mining method, and the available mechanization in the mine. Boreholes of 9 m to 10 m depth for crown-and-sill pillars have been reported. A higher magnitude of the induced stresses may necessitate destressing for the whole stope in advance with 20 m to 25 m deep boreholes. Although powder factors as low as 0.2 kg/m3 were used in vein mines, explosive energy levels close to those used in production blasting have been used. Emulsion-type explosives as well as ANFO-type explosives are equally used. Three case studies are presented in order to illustrate the complexities associated with destress blasting application in the field: sill pillar destressing at the Strathcona mine, Falconbridge; ore vein destressing at the Star Mine, Coeur d’Alene mining district; and the Brunswick Mine 29-9 Pillar destressing. It is demonstrated from the case studies that destress blasting is still poorly understood in spite of numerous research efforts and generous support from the mines and government agencies over a long period of time. There is a need to undertake the research at a micro-mechanical level to understand the development and growth of blast-generated fractures in confined rock mass and associated changes in the stress regime due to this dynamic fracture growth. An investigation of this nature is currently underway at McGill University.