Effects of Drainage Conditions on the Shear Strength of Unsaturated Soil

Aiming at the examination whether two factors of the net normal stress and the suction can express the shear strength of unsaturated soil under different conditions of drainage, triaxial tests were conducted on unsaturated specimens of a clayey silt under drained and undrained conditions. In drained tests with the pore-air pressure u_a and the suction s held constant (D-tests), the volume contraction occurred as well as the draining of the pore water. Two kinds of undrained tests were conducted with u_a regulated. In undrained tests with u_a constant (Ua-tests), s decreased during shear and the shear strengths became lower those in D-tests. Another kind of undrained tests in which u_a was regulated in order to keep s constant (Us-tests) resulted in simultaneous increases in u_a and u_w, and the state of stresses reached one corresponding to the unconfined compression. Shear strengths from Us-tests were much lower than those from D- and Ua-tests. Measured shear strengths were analyzed using an expression for the shear strength as a linear function of s and the net mean normal stress; results showed that, although the expression would be satisfactorily accepted, the effects of the difference of the testing conditions were detected.