ABSTRACT
The shock-wave theory of rock failure in blasting has been earlier proposed and is now used to further define the geometry of crater formation. The fundamental principle of the theory holds that major rock failure is caused by reflection in tension of the primary shock wave. Mathematical expressions for the intensity of the shockwave at any distance from a charge, the relation of rock strength to the pressure intensity of the shockwave, and the relation between weight of charge to depth of overburden have been derived previously. Experimental work described has now permitted the extension of the theory to describe more fully the shapes and intensities of the shockwave in different rocks using different explosives. Blastability coefficient, the ratio of the compressive and tensile strengths of rock, is proposed as controlling the maximum number of slabs formed in a blast. The distance of throw of fragments in blasting is shown to be a function of the shape of the shock wave.
INTRODUCTION
The present paper is a survey of the shock-wave theory of blasting and its applications to practical problems especially that of fragmentation. Mathematics has been avoided to make printing easier and for further mathematical aspects of the theory the interested reader is referred to a series of original papers on the subject. (Hino, 1954, 1955, 1956, and in preparation, as listed in the Bibliography at the end of this paper.)
FRAGMENTATION OF ROCK THROUGH BLASTING *
INTRODUCTION
The fundamental idea of the shock-wave theory of blasting is this: The Detonation of an explosive charge produces a crushed zone of rock near the charge. As the compressive strength of rock is rather high this range of crushed zone is limited to the vicinity of a charge, while a shockwave with high peak pressure but of short duration propagates outwards as a compressive wave without producing any breakage of rock. At a free facet his compressive wave reflects as a tension wave because at a free face there should be no force. As the tensile strength of rock is much smaller than the compressive strength of rock a fracture occurs at a point where the effective tension, that is, the difference between the intensity of tension wave and remaining compression, exceeds the tensile strength of rock. The distance between this point of the first main fracture and the first free face is defined as "thickness of the first slab." This dimension determines the sizes of fragments because dimensions of a fragment other than thickness are proportional to the thickness. If there remains a compressive wave after the detachment of this first slab which moves outwards with a momentum entrapped within its thickness from the original shockwave, then similar processes are repeated at the newly produced free faces until the intensity of the remaining compressive wave is reduced to below the tensile strength of rock.
In this section, the behavior of the shock wave has been described on the basis of simple experiments where a shockwave propagates in one dimension.
