Ductile–brittle quantitative evaluation of rock based on post-peak properties under true triaxial stress

Abstract

Abstract To quantitatively evaluate the influence of high true three-dimensional stresses on the ductile–brittle behaviours of rock in deep underground engineering, a series of true triaxial compression tests with different stress levels were carried out on marble and four kinds of granite. The influences of true triaxial stress states ( σ 2 , σ 3 ) on the post-peak characteristics were analysed, and a new normalized ductile–brittle evaluation index was proposed based on post-peak energy conversion characteristics of rock under true triaxial stresses. The ductile–brittle behaviours of rock were divided into four qualitative levels, namely, ductile-brittleness, transitional, brittleness and super-brittleness, and the influences of true triaxial stress states on the ductile–brittle behaviours of rock were quantitatively investigated. The research shows that as σ 2 increases or σ 3 decreases (that is, the differential stress between σ 2 and σ 3 increases), the brittleness of rock increases, and its increase rate gradually decreases and tends to be stable, transforming from ductile-brittleness to transitional, brittleness and super-brittleness and resulting in super-brittleness being easily induced by low- σ 3 and high- σ 2 conditions. When the differential stress between σ 2 and σ 3 is small, the intrinsic characteristics of rock itself have an obvious influence on ductile–brittle behaviours. When the differential stress between σ 2 and σ 3 is large, all kinds of rocks can exhibit super-brittle behaviour. The change of stress controls the evolution of rock ductile–brittle behaviours, and high-stress controls rock brittleness. The rock brittleness under true triaxial stress is significantly higher than that under conventional triaxial stress at the same σ 3 . σ 2 induces an increase in rock brittleness and causes the decay rate of brittleness to decrease with increasing σ 3 , and σ 2 increases the upper limit of σ 3 for brittle failure of rock. The enhancement effect of σ 2 on rock brittleness must be considered when evaluating the brittle failure of deep surrounding rock under high-stress conditions; otherwise, the risk of brittle failure may be underestimated.

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