The Effect of Triaxial Stress Systems on the Strength of Rocks at Atmospheric Temperatures

Abstract

The experimental work described in this paper was carried out in order to discover more about the laws governing the effect of the stress system on brittle fracture, with particular reference to rocks. Consideration of the implications of Griffith's theory of brittle fracture when an arbitrary, uniform plane stress system is applied (Murrell 1964a, b) enabled a number of fracture criteria to be postulated for triaxial stress conditions which required experimental investigation. An apparatus has been built which enables cylindrical rock specimens to be subjected to various combinations of principal stress (tension with compression, triaxial compression with the intermediate principal stress equal either to the major or the minor principal stress) with or without a controlled pore pressure, and at the same time enables the axial and diametral strains to be measured. Confining pressures of up to 4 kb can be applied, with pore pressures up to 2 kb. A new method of measuring the tensile strength of very brittle materials is described. Experiments were carried out on an isotropic sandstone.