Mechanical Behavior and Testing Methods of Unsaturated Soils

Abstract

The mechanical behavior of unsaturated cohesive soils under different common stress conditions, that is, static compression, dynamic shear, and repetitive compression, is considered as a function of induced initial matrix suction by desorption from the initially saturated state. The hysteresis of moisture retention is considered outside the scope of this work. The test results suggest that the moisture regime can be expressed most suitably in terms of matrix suction. The data suggest that a rather definite change in stress-strain response takes place at a critical value of the matrix suction with modulus values, in general, dropping beyond the critical suction. The direct measurement of the total soil suction in the laboratory in connection with the triaxial compression test is sought by use of thermocouple psychrometers. The total suction of the soil decreases as the applied stresses increase; the quantitative relationship is complicated by a number of factors. The matrix suction appears to be the fundamental suction component controlling the mechanical behavior during a desorption schedule. This implies certain limitations for the use of psychrometers alone in indexing the mechanical behavior of soils, especially if they have a significant osmotic suction component.