The failure of rock

Virtually no information concerning the non-elastic behaviour of hard rock is available, as the systems usually used to determine the strength of rock specimens become unstable as soon as the peak of the strain-stress characteristic is passed and its slope becomes negative. An analysis of the energy changes associated with the extension of Griffith cracks in tension and compression is made, which shows that the non-elastic behaviour can be described by a curved Griffith locus in the strain-stress plane. Only when the slope of this locus dϵ/dσ > 0, is a material intrinsically brittle and liable to spontaneous fracture. When dϵ/dσ < 0, crack extension can proceed by unstable fracture or stable failure, depending upon the amount of energy supplied to the material by the loading system. The effect of triaxial confinement is to reduce the negative slope of the Griffith locus and make the material more ductile than it is in uniaxial compression. Micro-seismic activity during the testing of rock specimens can be accounted for by assuming that rock is a heterogeneous material containing elastic inclusions, and analysing the fracture of these inclusions in terms of the above. Experimental data obtained from uniaxial compression tests on Tennessee marble and St. Cloud granite is presented and is shown to be in accord with the analytical predictions.