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University of Newcastle (2011)

Constitutive modelling of hydromechanical behaviour of unsaturated soils

Zhou, Annan

Titre : Constitutive modelling of hydromechanical behaviour of unsaturated soils

Auteur : Zhou, Annan

Université de soutenance : University of Newcastle

Grade : Doctor of Philosophy (PhD) 2011

Geotechnical problems associated with unsaturated soils are common in semi-arid climates such as Australia’s, where the upper soil layers are naturally unsaturated. Once a soil becomes unsaturated, its mechanical and hydraulic properties become different from its saturated counterpart. The behaviour of unsaturated soils is so complex that most constitutive models developed for saturated soils are no longer valid when soils become unsaturated. Development of constitutive relationships that can explain and reproduce key hydro-mechanical features of unsaturated soils has become a crucial issue in recent years. This thesis intends to develop robust and reliable constitutive models for the hydro-mechanical behaviour of unsaturated soils, which is still one of the most challenging problems in geomechanics. The thesis is an assembly of five peer-reviewed journal papers. The first two papers provide a systematic validation of the volume change equation in the Sheng-Fredlund-Gens (SFG) model and the derived shear strength criterion against experimental data. The SFG model recently presented by Sheng et al. (2008) provides a consistent explanation of yield stress, shear strength and volume change behaviour of unsaturated soils as functions of suction. All these functions are based on one single equation that defines the volume change with suction and stress changes. Experimental data for air-dried specimens prepared from a slurry condition as well as data from initially compacted soil specimens are analysed. The advantages and limitations associated with various proposed shear strength criteria in the literature are discussed. The third and fourth papers present an alternative method to couple the hydraulic component with the mechanical component in a constitutive model for unsaturated soils, as well as its application to quantify the effect of initial soil density on the soil-water characteristic curve. A new coupling mechanism is proposed based on the fact that soil-water characteristic equations are usually obtained for constant stress, not constant volume. The proposed coupling mechanism also satisfies the intrinsic relationship between the degree of saturation and the volumetric strain for both drained and undrained conditions. Numerical examples are presented to show the performance of the proposed method in predicting soil behaviour along drying and loading paths. Finally the approach is validated against experimental data for different soils. The last paper presents a novel approach for interpreting unsaturated soil behaviour in the stress – saturation space. In the new approach, all constitutive laws are presented by effective stress and degree of saturation. A new volume change equation is proposed in terms of stress and degree of saturation, to give better explanation of the non-linear change of soil compressibility under constant suctions. The soil compression index is assumed to be a function of the effective degree of saturation and is interpolated from the known compressibility at the fully saturated state and that at a dry state. The yield surface is established in the plane of the effective degree of saturation and the Bishop stress. An alternative approach (proposed and validated in the third paper) to simulate hydraulic hysteresis and hydro-mechanical interaction is introduced, which enables calculation of effective degree of saturation under complex stress and suction paths. The proposed volume change equation, and the corresponding yield surface, are generalised to the triaxial stress state by incorporating it with Modified Cam-clay model and the consistent framework of the SFG model (validated in the first paper). The basic properties and performance of the proposed constitutive model are illustrated through some numerical examples. Finally, the proposed model is validated against a variety of experimental data (drained/undrained, isotropic/triaxial tests on reconstituted/compacted unsaturated soils).

Mots clés : constitutive modelling ; hydromechanical behaviour ; unsaturated soils


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