Geotechnical factors affecting the application of pre-split blasting to rock slopes

Abstract

Previous approaches to pre-split blasting have tended to concentrate on the mathematical theory of dynamic stress wave interaction, whilst recognising some interaction with quasi-static stresses induced by expanding gases in the borehole. However the decoupling introduced during pre-splittng is specifically designed to reduce dynamic effects and to emphasize quasi-static effects, and it can be argued that the process has more in common with hydrofracture than with conventional use of explosives. Investigation of the mechanics involved during the fracturing process around both single and multiple line - charges in model testing in polyester resin proved the quast-static gas component of energy release to be the predominant mechanism controlling fracture growth around blast holes and in the formation of pre-split fractures. Both field and test observations indicate that the predominant geotechnical factor affecting the relative success of pre-splitting is the orientation of major discontinuities and or sets in relation to the pre-split line. Decreasing discontinuity intersection angle is shown to progressively increase overbreak from ninety degrees to twenty degrees, below which a dramatic increase in overbreak is observed with a failure of the pre-split in the final face. Discontinuity frequency is shown to have no major discernable affect on the success of pre-splitting. The effects of further varying geotechnical factors on the success of pre-split blasting are discussed, including anisotropy, grainsize, texture, weathering, ground water, stability and geostatic stress fields.