Some Basic Problems in Rock Breakage by Blasting and by Indentation

Abstract: This thesis is devoted to discussing some basic problems in rock breakage by blasting and by mechanical excavation, and contains a detailed summary and six appended papers numbered as Papers A to F. The central problem treated in this thesis is the interaction between either explosion products or mechanical tools and rock. In order to understand the breakage process, the mechanical behaviour of rock-like materials under the action of environmental conditions is first discussed. Based on experimental and theoretical studies, Paper A examines the effects of conning stress and pore fluid on the deformation and failure of structured (sintered) coal. A theoretical model is established for describing the post-failure region. According to the model the deformation and failure of coal are divided into three categories, depending on the value of a dimensionless parameter, related to the effective confining stress and the matrix strength. This explains the experimental phenomena concerned. The results from experimentsalso show that the absorbed gas in the coal specimens has no significant influence on the stress-strain curves. The pore pressure in the effective stress is therefore the pressure offered only by the free gas. In addition, different pore fluids with the same effective confining pressure may correspond to different effective stress-strain relations. The treatment of deformation and failure of structured coal in this paper emphasises the mechanisms of pre-existing crack growth and new crack nucleation under various environmental conditions, with the deformation and failure behaviour of the "model" material (structured coal) providing an illustrative example. Therefore, the qualitative behaviour obtained from the structured coal can be naturally extended for brittle rocks. In Paper B, a complete relationship between burden and either blasthole diameter or specific charge for a single hole blast is first established and then extended for the cases of multiple holes in Paper C. This relationship is based on energy conservation law and consists of three teems which are related to the surface energy to form the long side cracks, the volumetric energy to fragment the rock, and the energy needed to move the rock, respectively. For the sake of convenience in practical use, several approximate formulas describing burden as a power function of the blasthole diameter with a parameter of spacing-burden ratio are derived from the complete relationship and dimensional analysis. The exponents of the diameter in the formulas are 2, 1, 5/6 and 2/3, and these decrease with the increasing scale in blasting. The coefficients of the power formulas are expressed as functions of the physical properties of the rock and explosive. The valid ranges of the formulas established depend on the rock properties. The linear relation, derived as a special case with intermediate sizes of blasthole diameters, is in good agreement with Langefors' well-known formula It therefore gives a physical explanation of Langefors' formula and explains why it can only be used in that range. The non-linear approximate formulas were tested against the regression curves from real values in surface and underground mines, and a better agreement was achieved with surface blasting. Our newly developed formulas express the constants in empirical formulas with governing parameters, such as rock properties, explosive properties, and blasting scale. Therefore, they have more adaptability and are more precise compared with empirical formulas. Papers D, E and F deal with rock indentation. Rock indentation by a bit is the fundamental process in most mechanical excavation methods. The bit is forced into the rock, and the rock near the bit is crushed into small or large fragments, leaving subsurface cracks in the remaining rock wall. A conceptual model of crack structure in rock caused by mechanical excavation is presented based on a large number of experiments and a theoretical overview, and the main factors governing the indentation events are summarized. Functional relationships relating indentation force and either indentation depth or length of radial/median crack to the various quantities characterising the physical event (namely the shape and the size of the indenter and the properties of the rock, etc.) are established through similarity analysis. These forces are induced by hemispherical, truncated, or cylindrical indenters. Typical parameters characterising rock behaviour under compression and readily obtainable in the laboratory are involved in these formulas. The formulas obtained can serve for general use in various kinds of rock with different diameters of the three indenter types. This provides a new and more profound understanding of the physical mechanisms of rock indentation phenomena, and it also provides a great deal of information for practical use, such as full face boring and percussive drilling. Modification of the above-mentioned formulas due to the influence of confining stress, pore-fluid pressure, variation of the micro cracks involved in the rocks is suggested. The possible influence of loading rate on the results predicted by using the established formulas is also discussed in the thesis.