Shear Modulus and Damping Ratio of Unsaturated Kaolin Measured by New Suction-Controlled Cyclic Triaxial Device

This paper presents a new stress path suction-controlled cyclic triaxial device for studying the shear modulus and damping ratio of unsaturated soils at constant suction condition. The new device is fitted for suction-controlled testing by the axis-translation technique. Two electro-pneumatic regulators are used to apply pore-water and pore-air pressures at the top and bottom of the kaolin specimen. Suction-controlled cyclic triaxial tests were used to demonstrate the capability of the new apparatus. The tests were conducted on compacted specimens of lean clay that were prepared in the same initial condition and tested following different stress paths. The stress paths include suction equalization, constant suction ramped isotropic compression, series of constant suction cyclic loading, and post-cyclic constant suction ramped isotropic compression stages. Based on the experimental measurements, the volumetric behavior and evolution of the yield curves along suction equalization, constant suction ramped isotropic compression, and post-cyclic compression stages are discussed in the Barcelona Basic Model elasto-plastic framework. The stiffness and damping ratio of the specimens are shown, highlighting the dependency of the damping ratio (D), Young’s modulus (E) and shear modulus (G) on the suction, shear strain amplitude and number of cycles. The results show that the effect of the strain amplitude on D, E and G of unsaturated kaolin is qualitatively similar to that saturated kaolin. D increases, and E and G decrease as strain amplitude increases. At similar strain levels and number of cycles, D decreases and E and G increase when suction is increased.