ABSTRACT: A new true-triaxial loading apparatus was designed, fabricated and calibrated. Its purpose is to determine the compressive strength of hard rocks under the most general stress conditions. The apparatus has the capability of applying three independent principal stresses on a prismatic specimen, and enables rock strength testing under stress conditions encountered at depths reaching 10 km. An extensive series of true tri- axial tests in Westerly granite and KTB amphibolite reveals that the intermediate principal stress has a significant effect on rock strength, contrary to the implicit assumption made by Mohr-type failure criteria. Test results were used to define a new generalized strength criterion for each rock in terms of stress-tensor invariants.

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