Behaviour of the crushed zone in rock indentation

Conclusions

• 1. Micro hardness tests of compressed crushed rock indicate that the plastic behaviour of powder of ductile rocks, in this case a marble, resembles that of intact rock, irrespective of the deformation history of the crushed material. The inelastic deformation of such rocks under a bit can then be treated by plasticity theory.

• 2. The deformation properties of crushed material of granite and sandstone are of a brittle nature. Preserved grain interlocking, shape of grains (original or fractured) and degree of confinement are important factors in this respect.

• 3. Crushed rock powder can reconsolidate at pressures considerably lower than the contact pressure under a button of a drill or a disc cutter. Compacted crescents are then easily explained by the confined situation and the available pressure under a drill bit.

• 4. The inelastic zone of brittle rocks is partly kept in place by friction between tool-inelastic zone, inelastic zone-elastic matrix and by internal friction. If this friction can be reduced by some chemical agent, the inelastic zone could perhaps be squeezed out. Penetration will also increase after removal of the inelastic zone between cuts, e. g. by flushing or by some mechanical method.