ABSTRACT

ABSTRACT

We have instrumented well-controlled free-face blasts with six- component borehole strain gages, in order to determine the complete strain tensor as a function of time due to explosive loading. The strain tensors have been diagonalized to determine the principal strain time-histories, and further processed to determine the octahedral shear strains as a function of time. Principal strain time-histories, when compared with a computer model, indicate the arrival of various direct and reflected phases. Strong shear strains coincide with the expected arrival time of shear waves. These shear waves are probably instrumental in the fragmentation of confined rock due to explosive loading, and should be incorporated in models of the fragmentation process.

INTRODUCTION

Many mining operations rely on blasting to initially fragment rock masses for subsequent processing. In certain applications, (i.e., oil shale retort formation, blasting operation success is critical to resource recovery. In more conventional mining operations, blasting is still the most energy-efficient means of fragmenting rock. Optimal use of explosive energy in mining operations requires a thorough understanding of the underlying physical principles. Both pressurization due to rapidly expanding explosive gases, and strain waves generated by the interaction of the gases with the borehole walls undoubtedly affect fragmentation. Although the effect of expanding gases was once deemed most important, in the past few years the importance of strain waves and their interaction with flaws has received increased attention. Because the controversy over which effect is predominant is thoroughly reviewed by Winzer et al. (1983), we will only briefly discuss previous strain measurement studies here. Analytical predictions of the strain wave from an explosive column have become progressively more sophisticated, from the initial work of Sharpe (1942), through models of Heelan (1953), Jordan (1962), and Favreau (1969). Plewman and Starfield (1965) proposed a novel numerical approach in which a cylindrical explosive column is modeled as a linear series of spherical charges detonated with delays corresponding to the detonation velocity of the explosive. The resultant waveform is then a repeated summation of the strain waves generated by a spherical charge, appropriately lagged, and attenuated according to path- length differences. The U.S. Bureau of Mines did an extensive series of field studies in the 1950's and 60's to determine the response of rock to explosive [e.g., Duvall and Atchison (1957); Atchison and Tournay (1959)]. Comparing some of the strain-gage records obtained with the numerical model of Plewman and Starfield, Starfield and Pugliese (1968) found generally good agreement. The model simulated neither an s-wave from the explosive column nor any reflections. The tests were done in quarry floor, so that reflections from a vertical free face were not present, rock movement was somewhat confined, and the rock was only damaged from preceding blasts. It is noteworthy that no significant tensile strains were observed in the original records or the models. We have worked almost entirely with bench blast configurations, because that is the usual operating condition in quarries. In a typical bench blast, complicated strain histories in the affected rock may arise from several causes. First, strain waves generated by the explosive column will have a broad pulse-width due to the finite detonation velocity of the explosive. This is evident from the work discussed above. Second, the rock through which the waves will propagate will have discontinuities due to jointing, layering, and damage from previous blasts, which may attenuate strain wave

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