Investigation of the Strain-Softening Behavior of Granular Soils with a New Multiaxial Cell

The special considerations leading to the design of a new multiaxial cell used for strain-softening studies are presented. This cell has the special features of being able to accommodate large strain without introducing boundary imperfections, allowing observation of shear band formation, and permitting strain path control. The observed strain-softening behavior of a quartz sand under multiaxial stress condition, i.e., σ'1 ≠ σ'2 ≠ σ'3, are presented. Both stress path tests and strain path tests were conducted. In a stress path test, the specimen was loaded in the major principal direction in a deformation-controlled mode but with the stress response constrained along a prescribed path using computer control. Hence strain-softening response can still be observed in a controlled way. Two types of softening, prefailure and postfailure softening, are identified. Prefailure softening will occur along certain strain paths, and the associated deformation is homogeneous. In postfailure softening under a multiaxial stress condition, only limited homogeneous deformation can be sustained, and eventually there will be shear band formation. However, this shear band is not triggered by boundary imperfections but is an inevitable response of the specimen to shearing in the postfailure domain under a multiaxial stress condition.