Shear banding in true triaxial tests and its effect on failure in sand

The occurrence of failure, mechanisms that create failure, and soil behavio r in the vicinity of failure have been investigated. One mechanism is smoot h peak failure, in which the soil continues to behave as a continuum with u niform strains, and smooth peak failure is followed by strain softening. An other mechanism is shear banding, whose occurrence in the plastic hardening regime limits the strength of the soil. True triaxial tests have been perf ormed on tall prismatic specimens of Santa Monica Beach sand at three relat ive densities in a modified version of a cubical triaxial apparatus to stud y the effect of shear banding on failure in the full range of the intermedi ate principal stress. The experiments show that the strength increases as b [=(sigma (2) - sigma (3))/(sigma (1) - sigma (3))] increases from 0 to abo ut 0.18, remains almost constant until b reaches 0.85, and then decreases s lightly at b = 1.0. Shear banding initiates in the hardening regime for b-v alues of 0.18-0.85. Thus, peak failure is caused by shear banding in this m iddle range of b-values, and a smooth, continuous 3D failure surface is the refore not generally obtained for soils.