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Rock strengths and rock failure criteria 115

Figure 3.24 Critical state line (defined by s 1 ¼ 3s 3 ) suggested by numerous high-
pressure triaxial strength tests in dry rocks. Note the closeness of the unconfined
strength (UCS) circle to the confining pressure s crti (Barton, 1976). ‘J’ represents jointed
rock. The magnitude of 4 c is 26.6 degrees when s 1 ¼ 3s 3 . For porous rocks, effective
stresses should be used to replace the total stresses.
itself, as illustrated in Fig. 3.24. This critical state can be considered as the
separation between the brittle and ductile deformation of the rock as
illustrated in Fig. 3.24. In ductile and brittle deformation stages, the rock
behaviors are very different (Mogi, 1974). The rock loaded at a low
confining pressure (small s 1 s 3 ) is brittle as shown in Fig. 3.25, meaning

Figure 3.25 Schematic triaxial stressestrain curves for rock loaded at different
confining pressures and the brittleeductile transition. (Plotted based on Mogi, K., 1974.)