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Precise testing methods have been applied to an experimental study of the effect of the intermediate principal stress σ2 on failure of a granite, a limestone, and a dolomite rock at room temperature. σ2 affects failure in two ways. Strength at failure increases with σ2 by an amount that is proportional to, although smaller than, the amount of the least compression σ3; this effect of σ2 is much more pronounced for brittle than for ductile material. The angle between faults and maximum compression direction is markedly reduced as σ2 is increased from the value σ3 to σ1; this effect is pronounced for both brittle and ductile material.
Kiyoo Mogi (Sun,) studied this question.