Macro-micro tests of cohesive soil under varied normal and shear stresses subjected to drying-wetting cycles

The mechanical behavior of cohesive soil is sensitized to drying-wetting cycles under confinements. However, the hydromechanical coupling effect has not been considered in current constitutive models. A macro-micro analysis scheme is proposed in this paper to investigate the soil deformation behavior under the coupling of stress and drying-wetting cycles. A new device is developed based on CT (computerized tomography) workstation to apply certain normal and shear stresses on a soil specimen during drying-wetting cycles. A series of tests are conducted on a type of loess with various coupling of stress paths and drying-wetting cycles. At macroscopic level, stress sensor and laser sensor are used to acquire stress and strain, respectively. The shear and volumetric strain increase during the first few drying-wetting cycles and then become stable. The increase of the shear stress level or confining pressure would cause higher increase rate and the value of shear strain in the process of drying-wetting cycles. At microscopic level, the grayscale value (GSV) of CT scanning image is characterized as the proportion of soil particles to voids. A fabric state parameter is proposed to characterize soil microstructures under the influence of stress and drying-wetting cycle. Test results indicate that the macro- and micro-responses show high consistence and relevance. The stress and drying-wetting cycles would both induce collapse of the soil microstructure, which dominants degradation of the soil mechanical properties. The evolution of the macro-mechanical property of soil exhibits a positive linear relationship with the micro-evolution of the fabric state parameter.