Nature of Principal Strain Response of Unsaturated Soils Under Multi-Axial Stress States

A servo-controlled true triaxial (cubical) testing device was modified to test 10-cm cubical specimens of unsaturated soil under varying matric suction conditions. Recompacted silty sand specimens were tested following a multi-stage testing scheme by controlling the major, intermediate, and minor principal stresses. Matric suction states in the specimens were induced and maintained constant during testing by using the axis-translation technique. True triaxial testing data from a series of drained, stress/suction-controlled triaxial compression, triaxial extension, and simple shear tests, were used to evaluate the nature of principal strain response of unsaturated soils under varying matric suction conditions, and for shear stress paths not achievable in a conventional cylindrical apparatus. Normalized test data of octahedral stress versus principal strain, on the basis of matric suction level, indicate consistent stress-strain behavioral patterns. The paper presents these test results and proposes behavioral trends for unsaturated soils as a function of matric suction.