ABSTRACT

ABSTRACT

Recent applications of explosives and blasting agents to rubble rock have led to requirements for more elaborate design and analysis methods. In most blasting uses, it is necessary not only to fracture the rock, but also to move the broken rubble in a predictable manner. Many in situ extraction techniques require rubblization to take place in a confined region where rock motion is a predominate factor in creating a permeable broken bed. In this paper, two analytical methods are presented which describe the large rubble motion during blasting. These methods provide the blast designer with a tool for evaluation and further refinement of blasting patterns and timing sequences. In both these methods, the rock medium is represented by a series of discrete, discontinuous regions. The use of discontinuous techniques rather than the classical continuum methods, results in better approximations to the rubble motion. These regions are set in motion by pressure loads from the explosive. The motion of these regions is then calculated numerically using interaction laws between regions in contact. The basis for these models or methods is presented along with the background for selecting explosive pressure loads and rock mass material behavior. Typical examples, including both cratering and bench blasting geometries, are discussed which illustrate the use of these models to predict rubble motion. Such engineering representations appear to provide a practical method for use in predicting rubble motion and a tool for design evaluation of blasting in confined geometries.

INTRODUCTION

Rubblization of rock begins with fracturing due to high amplitude stress waves followed by translation and rotation of the broken fragments due to explosive gas pressures. These events are interrelated and together form the basis for behavior of rubble blasting. In many blasting situations the final location and the porosity of the rubble or "muck" pile are critical. Blasting in confined regions, such as underground formation of rubble filled rooms for in situ processing, is very strongly influenced by the rubble motion. During blasting operation, the rock motion is dictated by the response of rock to pressures generated by detonation of the blasting agents. The blast pressure is related in an interactive manner to the degree of confinement. The rock surrounding a blasting agent must provide enough confinement such that the energy from the explosive is imparted into the adjacent rock. If enough confinement is not provided, a great deal of energy is converted into undesirable high rubble "fly" velocities or into shock wave energy in the air. If the explosive is buried too deeply, most energy is used in crushing a very small amount of rock and in generating high rock vibration levels. In this case, the resulting muck pile remains in place and the desired rock rubblization will not occur. Thus rubble motion is interrelated with the explosive energy dissipation. In most blasting designs (Langefors and Kihlstrom, 1978), these confinement effects are accounted for by using empirical and semi- empirical formulas which provide a means to select appropriate spacing, depth, size and timing of explosives. These methods have evolved through years of experience, mainly with quarry and tunnel blasting.

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