A novel, computer-driven, true triaxial (cubical) apparatus has been developed to test unsaturated soil specimens under suction-controlled multi-axial stress paths that are not achievable in a conventional cylindrical apparatus. The cubical cell implemented in this research work is a considerably refined version of those previously reported by Hoyos (1998) and Laikram (2007). The refined version is a servo-controlled, mixed boundary type of cell that allows for full control and real time measurements of matric suction, net principal stresses and soil deformations along a wide range of simple-to-complex stress paths induced on cubical specimens of unsaturated soil. The specimen seats on top of a high-air-entry ceramic disk with a 5-bar entry value.;A comprehensive series of drained (constant-suction) hydrostatic compression (HC), conventional triaxial compression (CTC), triaxial compression (TC), triaxial extension (TE), and simple shear (SS) tests were conducted on compacted, 3 in (7.62 cm) per side, cubical specimens of clayey sand (SP-SC) under constant suction states. Target suction levels are induced and kept constant during testing using the axis-translation technique. Results from suction-controlled tests under axisymmetric conditions (sigma2 = sigma3) were used for a thorough calibration of the elasto-plastic, critical state-based frameworks previously postulated by the Barcelona Basic Model (Alonso et al., 1990), the Modified Barcelona Basic Model (Josa et al., 1992), and the Oxford Model (Wheeler and Sivakumar, 1995).;Results from suction-controlled conventional triaxial compression (CTC) and triaxial compression (TC) tests were used for the validation of these models in predicting stress-strain response of compacted clayey sand under 50, 100, and 200 kPa suction states. Results were also used to validate the modifications proposed under the Refined Barcelona Basic Model framework developed in the present work.;Finally, results from the series of suction-controlled triaxial compression (TC), triaxial extension (TE), and simple shear (SS) tests were used to evaluate the nature of the major, minor and intermediate principal strain response of compacted clayey sand under multi-axial shearing, as well as the influence of suction on the shape and position of the failure envelope of compacted clayey sand in the octahedral plane.
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