A Refined True Triaxial Cell for Modeling Unsaturated Soil Response under Suction Controlled Stress Paths

This paper describes a servo-controlled true triaxial (cubical) device that 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 not achievable in a conventional cylindrical apparatus. The cell is a mixed-boundary type of device, with the specimen seated on top of a high-air-entry 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 et al. (2005), featuring two independent pore-air pressure and pore-water pressure control systems via a PVC-100-UNSAT pressure panel, as well as a fully computer-driven stress application and control system via a PCP-5000-UNSAT pressure panel. Suction states, s = (u_a - u_w), are induced in the specimens via the axis-translation technique. The technique is implemented by utilizing the s = u_a testing concept (u_w = 0). The paper outlines the full development of the new cell, including details of its main components, the step-by-step assembling process and the selection of the appropriate sample compaction process and density for testing compacted clayey sand. The operational cell will play a fundamental role in the complete characterization of unsaturated soil behavior under multi-axial stress paths that are likely to be experienced in the field.