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

The occurrence of failure, mechanisms that create failure, and soil behavior in the vicinity of failure have been investigated. One mechanism is smooth peak failure, in which the soil continues to behave as a continuum with uniform strains, and smooth peak failure is followed by strain softening. Another mechanism is shear banding, whose occurrence in the plastic hardening regime limits the strength of the soil. True triaxial tests have been performed on tall prismatic specimens of Santa Monica Beach sand at three relative densities in a modified version of a cubical triaxial apparatus to study the effect of shear banding on failure in the full range of the intermediate principal stress. The experiments show that the strength increases as b [=(sigma (2) - sigma (3))/(sigma (1) - sigma (3))] increases from 0 to about 0.18, remains almost constant until b reaches 0.85, and then decreases slightly at b = 1.0. Shear banding initiates in the hardening regime for b-values of 0.18-0.85. Thus, peak failure is caused by shear banding in this middle range of b-values, and a smooth, continuous 3D failure surface is therefore not generally obtained for soils.