Results of previous work in presplitting blasting show that the roles of shock wave and gas action are conjectural and that theories of fracture initiation and fracture propegation are often in disagreement.

In order to investigate these discrepancies experimentally, the importance of fracture surface condition is stressed, and the information which can be derived from fracture surface morphology is reviewed. The general features of crack extension behavior, especially the observed relation between different spacing of holes, are investigated in tri-dimensional models of plexiglas.

Detailed visual examination by the naked eye and through optical and scanning electron microscopes reveals that the fracture surface produced by the simultaneous detonation of small decoupled explosive charges in a row of boreholes is well delineated, and that the individual events involved in presplitting are sculptured in the fracture surface. This allows for the direct observation of the characteristic morphological features, an assessment of crack initiation, a determination of the sequence of fracturing, and of crack propagation. Proper interpretation of these characteristic morphological features can reveal the influence that changing the experimental conditions of hole spacing and decouplinf ratio has on the fracture surface morphology, the point of crack initiation, and the crack propagation.

The maximun critical distance observed between boreholes for the formation of a presplit fracture by the action of shock waves alone, is approximately six times the diameter of the borehole. Under these conditions, the fracture originates at the middle point between boreholes and is characterized by individual fracture elements symmetrically oriented toward the point of crack initiation, thus forming a "honeycomb" fracture surface. Theoretically, from this maximun critical distance, it is found that the rate of attenuation of the shock wave is proportional to the inverse of the square root of the radius of the borehole. Over six hole diameters, smooth conchoidal fracture surface and concentric rib marks result from the extension of initial shock-wave-generated cracks by the action of gas expansion. These are the common mode of fracture surface in presplitting. Small shear fractures extended toward the boreholes from the initial shock-wave-generated cracks characterize the boundary of the boreholes.

Specially designed experiments and optical and scanning electron microscopy reveal that fracturing begins at "flaws" which are activated by the shock wave. This wave spreads in all directions around the borehole and forms circulars-like elements which coalesce, extending radially until the action of the shock wave is attenuated. The interaction of these enlarged "flaws" forms well-delineated geometric figures which are somewhat modified depending on the relative violence of the process and on the time of activation. Under the subsequent action of gas pressure, this process continues and controls the entire mechanism of presplitting. It is suggested that if reliable control of the decoupling ratio is used to induce radial cracks at the boundary of the borehole wall, losses of gas expansion energy, in well confined charges, are at a minimum. Finally, several areas for future research are outlined.

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¹ Pennsylvania State University, The Graduate School, Department of Mineral Engineering, Marzo 1975.