Analysis of shear banding in true triaxial tests on sand

A series of true triaxial tests have been performed on rectangular prismati c specimens of Santa Monica Beach sand at three different relative densitie s to study the occurrence of failure, mechanisms that create failure, and s oil behavior in the vicinity of failure. One mechanism is smooth peak failu re, in which the soil continues to behave as a continuum with uniform strai ns, and smooth peak failure is followed by strain softening. Another mechan ism is shear banding, whose occurrence in the plastic hardening regime limi ts the strength of the soil. Presented here are analyses based on theoretic al conditions for localization and subsequent shear banding and on the resu lts of the true triaxial tests. Thus, the strength increases as b [=(sigma (2) - sigma (3))/(sigma (1) - sigma (3))] increases from 0 to about 0.18, r emains 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 rang e of b-values, and a smooth, continuous 3D failure surface is therefore not generally obtained for soils. The experimental results agree with the theo retical conditions for the occurrence of shear banding and its consequent e ffect on the 3D failure stress states for soil.