Experimental study on wetting water permeability of unsaturated intact loess under isotropic stress

The unsaturated water permeability is one of the key parameters in performing consolidation analysis for unsaturated soil systems. A series of unsaturated infiltration tests are performed on the intact loess with various void ratios at null stress and with the same void ratio at different isotropic stresses using the independently developed triaxial equipment for shearing and permeability measurement of unsaturated soils, whereby the suction is reduced incrementally by the direct addition of small amount of water to the sample at a constant isotropic stress. The influences of void ratio, stress, degree of saturation and suction on the water permeability are analyzed. The water permeability at null stress is compared with that at applied stress. The permeability functions to describe the relationships of water permeability versus degree of saturation and suction are respectively proposed for the intact loess at applied stress. The results show that the void ratio and stress have the effect on the relationships of the water permeability versus degree of saturation and suction, and little influences are found on the latter as the suction value is higher than certain threshold suction. The water permeability as functions of both degree of saturation and suction is the same as long as the void ratio is the same at both null stress and different stresses. The relative water permeability versus suction can not be normalized, and the relative water permeability increases with the decreasing void ratio or increasing stress for the same suction. Interestingly, there are all unique relationships of relative water permeability as functions of both degree of saturation and suction ratio (defined as suction to air-occlusion suction ratio). The wetting water permeability for the intact loess is not described by the vG-M model at a given void ratio. The proposed permeability functions can predict the water permeability for the intact loess with both the increase of degree of saturation and the decrease of suction during wetting at a constant isotropic stress. The predicted results are in good agreement with the test ones.