Modeling Unsaturated Soil Response Under Suction-controlled Multi-axial Stress States
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In this research work, a novel suction-controlled true triaxial testing apparatus has been developed to test 3-in (7.5-cm) side, cubical specimens of unsaturated soil under controlled-suction states for a wide range of stress paths that are not easily achievable in a conventional cylindrical apparatus. The equipment is a mixed-boundary type of device, with the specimen seated on top of a high-air-entry ceramic disk and between five flexible (latex) membranes on the remaining sides of the cube. The new cell is an upgraded, more elaborate version of the one previously reported by Hoyos (1998), featuring two independent pore-air and pore-water pressure control systems via a PCP-5000-UNSAT pressure panel. Matric suction states in the specimens are induced during testing via the axis-translation technique. The technique is implemented by utilizing the s = ua testing concept (uw = 0). The paper outlines the full development of the new cell, including details of its main components and the step-by-step assembling process. Results from a series of constant-suction hydrostatic compression (HC), conventional triaxial compression (CTC), triaxial compression (TC), and triaxial extension (TE) tests on lightly compacted silty sand are presented. An attempt was made to calibrate and validate the Barcelona model (Alonso et al. 1990) using results from CTC and TC tests. The operational true triaxial apparatus will play a fundamental role in the complete characterization of unsaturated soil behavior under multiaxial stress paths that are likely to be experienced in the field.