The principal objectives of this research were to investigate and to model coupled hydraulic and mechanical characteristics of an unsaturated expansive soil....[ Read more ]

The principal objectives of this research were to investigate and to model coupled hydraulic and mechanical characteristics of an unsaturated expansive soil.

17 triaxial tests were carried out to investigate hydraulic and mechanical behaviour of the unsaturated expansive soil along various stress paths, including wetting-drying, compression and shearing. In each test, sufficiently long duration was permitted to ensure suction equalization. The longest duration of one test was up to seven months. Irreversible swelling upon wetting, irreversible contraction upon drying and accumulated swelling after a wetting-drying cycle were observed. These volume changes decreased with an increase in net mean stress. Based on double-structure concepts proposed by Gens & Alonso (1992), an SD (suction-decrease) yield curve was identified from the irreversible swelling upon wetting and was found to be inclined at an angle of 30° to the net mean stress axis, smaller than the assumption made in Barcelona Expansive Model. Irreversible contraction upon drying indicated yielding on SI (suction-increase) yield curve proposed by Alonso et al. (1990). Identified yield suction upon drying was significantly lower than the maximum experienced suction, indicating coupled movements of SD and SI yield curves. Reagarding hydraulic behaviour, at suctions less than 100 kPa, both absorption and desorption rates decreased as net mean stress increased. At suctions larger than 100 kPa, soil-water characteristics were not influenced by net mean stress. Suction history was found to produce a significant influence on isotropic compression and shear behaviour. This was caused by changes of soil state (both void ratio and degree of saturation) resulting from cyclic wetting-drying. Smaller degree of saturation made the soil become more resistant to yield, exhibiting a hardening effect on mechanical behaviour.

Two new constitutive models (Model-A and Model-B) were developed. Model-A was intended to improve existing elasto-plastic models for simulating the coupling of hydraulic and mechanical behaviour of unsaturated soils. This model was modified from a state-dependent elasto-plastic model proposed by Chiu & Ng (2003). Two main improvements were considered, i.e. to simulate hydraulic hysteresis and stress-dependent soil water characteristics and to capture observed key features of deformations in cyclic wetting-drying. Reasonable agreement between model predictions and test results was achieved.

Model-B was developed to explore the possibility to capture the influence of suction history on mechanical behaviour. This model was modified and extended from an elasto-plastic model presented by Wheeler et al. (2003). Two main improvements were considered, i.e. to simulate wetting-induced plastic swelling and to consider soil behaviour at deviator stress state. This model is able to simulate the observed influence of suction history on compression and shear behaviour. This is because that plastic change of degree of saturation was regarded as another hardening parameter for mechanical behaviour in addition to plastic volumetric strain.