INVESTIGATION OF RELATIONSHIP BETWEEN ROCK FRAGMENTATION AND BURDEN STIFFNESS RATIO IN CONFINED BENCH BLASTING

Fragmentation is the fundamental concern of rock blasting, serving as a measure of blasting effectiveness. Fragmentation is sensitive to the interrelationship among the design variables which define pattern geometry. A significant relationship exists between the burden-rock stiffness L/B ratio and the degree of fragmentation of the blasted rock. Therefore, the objective of the study was to develop a model which represented a confined bench blast and to investigate the relationship between the rock fragmentation and the L/B ratio.;The model used was unique in that the radial cracks were partially pressurized. As part of the model development, an iteration technique was used to recalculate and update the borehole pressure as borehole volume increased.;An analysis was made for all models showing changes in its L/B ratio. Stress distribution analysis, deformed geometric configurations and displacement at the middle of the explosive column near the burden face was analyzed for each model. The conclusion was that, as the stiffness ratio increased, the fragmentation improved. Also, the lower L/B ratio values seemed to be more sensitive to displacement, and displaying a transition zone between L/B ratio of 3 and 5. Deformation about the borehole was symmetrical for L/B ratio equal to or less than 1.2 and as L/B ratio increased the deformed area about the borehole become more asymmetrical moving out toward the burden face.;It is generally accepted that in confined bench blasting the initial mechanism of rock breakage is the creation of radial cracks which extended from the borehole outward the burden face. Gas fills and remains in the radial cracks for a considerable time after the cracks reached the free face before movement begins. Ash, Smith and others had developed two and three dimensional finite element models to represent bench blasting. But, to date, none of these models included the existence and pressurization of the radio cracks.