Shear modulus and damping ratio of unsaturated kaolin measured by new suction-controlled cyclic triaxial device

The cyclic triaxial test is the most widely used way to measure the cyclic stress strain properties of saturated soils because of its availability in most geotechnical laboratories. Further, it can be viewed as a suitable tool with which to analyze the cyclic behavior of unsaturated soils in a wide strain range. In this study, a new stress path suction-controlled cyclic triaxial device is presented. It is fitted for suction-controlled testing by the axis-translation technique. Pore-air and pore-water pressures are applied at the top and bottom of the specimen by using two electro-pneumatic regulators. The capability of the new apparatus is demonstrated in this paper with the results of some suction-controlled cyclic triaxial tests. The tests are carried out on compacted specimens of lean clay that are 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. Volumetric behavior and evolution of the yield curves along suction equalization, constant suction ramped isotropic compression, and post-cyclic compression stages are discussed on the basis of the Barcelona Basic Model elasto-plastic framework. The stiffness and damping ratio of the specimens are subsequently presented, highlighting the dependency of the Young's modulus (E), shear modulus (G), and damping ratio (D) on the shear strain amplitude, number of cycles, and suction. The results show that E and G increase, whereas D decreases significantly by increasing suction at similar strain levels and number of cycles.