(1. Institute of Geotechnical Engineering，Xi?an University of Technology，Xi'an，Shaanxi 710048，China；2. Shaanxi Provincial
Key Laboratory of Loess Mechanics and Engineering，Xi?an University of Technology，Xi?an，Shaanxi 710048，China)
Abstract：Stepped wetting tests were performed on collapsible loess under null stress and net vertical stress by using refitted direct shear apparatus for unsaturated soils. The variation of deformation and suction were measured during wetting. The influences of void ratio at the beginning of wetting(null stress) and net vertical stress on deformation and water retention behaviour of collapsible loess were analyzed during wetting. The wetting water retention behaviour under null stress was compared with that under net vertical stress. The models describing the relation of void ratio versus the suction and the water retention behaviour on the degree of saturation versus the suction were proposed under hydro-mechanical coupling. Having considered hydro-mechanical coupling，the relationship of void ratio and normalized suction was described by a logarithmic function and the influence of net vertical stresses on the water retention behaviour was reflected by the variation of void ratio. The relationship of the suction and degree of saturation under hydro-mechanical coupling was the same as that under null stress for the same void ratio. When the suction was less than a certain threshold value，the variations of the void ratio either at the beginning of wetting or induced by hydro-mechanical coupling had great influence on the relationship of suction and water content. A modified V-G model considering the variations of the void ratio was put forward. The model described the wetting water retention behaviour under null stress and net vertical stresses. The predicted results were in good agreement with the test results.
VAN GENUCHTEN M T. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils[J]. Soil Science Society of America Journal，1980，44(5)：892–898.
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