This thesis is primarily an account of experimental work which has had as its objective an improvement in the efficiency of practical rock excavation. It is, secondarily, an attempt to explain the performance of rock cutting machinery in terms of three different varieties of retarding force which combine to oppose the "cutting force" which has to be provided by the rock cutting machine if it to excavate rock.
Chapter 1 sets the context of the work within practical mining, Chapter 2 describes machinery which has used varieties of cut-and-break for the practical excavation of rock, Chapter 3 describes laboratory work leading to the construction of a field-trial size machine, Chapter 4 describes laboratory trials with that machine, Chapter 5 recounts field trial experience.
Chapter 6 is devoted to a discussion of retarding forces as they apply to rock cutting tools sliding across a rock surface. Conventional sliding friction, it is suggested, may be but one of several different kinds of retarding force experienced by a rock cutting tool. For convenience, in Chapter 6, all the retarding forces are called "friction", since each is a retarding force experienced by the tool as a result of the dissipation of energy by the tool. Retarding forces, it is suggested, may arise as a result of the cutting action of the tool, "Evans Friction", Fe, as a result of sliding friction, Fs, and as a result of surface distortion of the rock by the tool, F d· The various retarding forces are, it is suggested, combined to form the cutting force, Fc, in the form Fc= Fe+ Fs + Fd, and this relationship is invoked in an interpretation of the observed relationship between the specific energy of excavation and the advance-per-tum of the cutting wheel when cutting conventionally, (ie, with no breaking action in addition to cutting).
Chapter 7 opens with a theoretical discussion in terms of self-similarity and the known proportionality between force and depth of cut which suggests that the normal force applied to a tool would be expected to increase as the square-root of the area of the wear-flat of a tool. Wear according to this rule would conserve the 'settlement depth' of the tool, since this quantity, which may be defined as the distance to which a tool may be expected to sink into rock as a consequence of the elastic deformation of the rock, is a constant if the force increases as the square root of the area to which it is applied. The hypothesis that tool wear conserved the settlement depth, and the implications of the possible truth of this hypothesis, tested as far as is possible with data sets from the literature, form the substance of the remainder of this chapter.
Chapter 8 comprises suggestions for future research. The remainder of the thesis comprises appendices: Appendix 1 deals with the design of the experimental machine, Appendix 2 lists the rock strengths used in the laboratory work, Appendix 3 comprises the cut proformas mentioned in the text, Appendix 4 comprises size distributions for excavated material, Appendix 5 is the numerical elastic model, Appendix 6 summarises most of the existing rock cutting theories. The list of references cited is to be found at the end of the thesis.