ABSTRACT
We review the research conducted at the University of Wisconsin in the last two decades in the area of true triaxial testing of rocks. We designed and fabricated equipment (the UW true triaxial testing system) capable of applying three different compressive loads on mutually perpendicular faces of cuboidal rock specimens. The equipment was used to carry out extensive series of true triaxial experiments in an array of rocks, from strong crystalline to weak porous clastic. The results revealed the substantial effect of the intermediate principal stress on failure characteristics, in terms of the failure stress, failure-plane angle, and failure mode. True triaxial failure criteria incorporate the intermediate principal stress, and expose the inadequacies of the commonly employed criteria, such as the Mohr and Mohr-Coulomb theories, which consider only the two extreme principal stresses.
