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is paper presents a simple fractal model to quantify the effects of initial porosity on the soil-water retention curve and hydraulic conductivity of unsaturated soils. In the proposed conceptual model, the change of maximum pore radius, which largely determines the change of the air-entry value, is directly related to the fractal dimension of pore volume (D) and porosity change. e hydraulic properties of unsaturated soils are then governed by the maximum pore radius, the fractal dimension of pore volume (D), and the fractal dimension of drainable pore volume (D d ≤ D). e new fractal model removes the empirical fitting parameters that have no physical meaning from existing models for porosity-dependent water retention and hydraulic behaviour and employs parameters of fractal dimensions that are intrinsic to the nature of the fractal porous materials. e proposed model is then validated against experimental data from the literature on soil-water retention behaviour and unsaturated conductivity.

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This paper presents a number of experimental results of suction-controlled triaxial tests on a compacted weakly expansive soil with different suctions and suction histories. In terms of suction control methods, the high suction level (from 3.29 MPa to 38 MPa) was realized by the vapor equilibrium technique and the low suction level (from 0 kPa to 800 kPa) was controlled by the axis translation technique. The results of triaxial tests indicate that the specimen with higher suction shows higher strength and smaller contractive and higher dilative volumetric strains and the average skeleton stress ratio (q/p') at failure deceases with increasing the suction in the high suction range (3.29 MPa ~38 MPa). Given that the suction during shearing is constant (e.g. 200 kPa), the specimen dried to higher suction in history shows higher strength and smaller contractive volumetric strain. Experimental results also show that high pre-applied suction (i.e., the maximum suction in the history) can lead to peak strength, post-peak softening and shear dilation. Three different methods (pressure plate, filter paper and vapor equilibrium methods) were employed to study the soil-water retention behaviour of the unsaturated expansive soil. The test results indicate that combining above three different methods is able to determine the soil-water characteristic curve in the entire suction range (0~367 MPa). Test results of the expansive soil also show that the void ratio keeps decreasing along with the increase of the suction in the entire suction range.

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In order to investigate the influences of drying and wetting cycles, initial degree of compaction, and water content on shear strength of the Yudong subgrade silt, a series of direct shear tests were performed at saturated and unsaturated states. The test results show that effects of the drying and wetting cycles, water content, and compaction degree on cohesion are more evident than those on the internal friction angle. According to the test data, a formula for the cohesion was proposed, which accounts for the drying and wetting cycles, water content, and degree of compaction. Because Bishop’s strength formula for unsaturated soils could not be applied to Yudong silt, a formula is given based on Fredlund’s formula for predicting the shear strength of unsaturated Yudong silt from the soil-water characteristic curve.