Anisotropic stiffness of soil is very important for predicting ground movements and the serviceability limit state of geotechnical structures. Although stiffness anisotropy of various sedimentary clays is recognized, anisotropic stiffness of unsaturated loess (an Aeolian deposit) is rarely studied. As soil structure and suction play critical roles in the behaviour of unsaturated loess, the effects of hydro-mechanical loading on anisotropic small-strain stiffness of compacted and intact loess should be investigated.

The principal objective of this research is to examine evolving anisotropic stiffness of unsaturated compacted and intact loess. A suction-controlled triaxial apparatus, equipped with bender elements and hall-effects sensors, was used. To investigate the anisotropic s...[ Read more ]

Anisotropic stiffness of soil is very important for predicting ground movements and the serviceability limit state of geotechnical structures. Although stiffness anisotropy of various sedimentary clays is recognized, anisotropic stiffness of unsaturated loess (an Aeolian deposit) is rarely studied. As soil structure and suction play critical roles in the behaviour of unsaturated loess, the effects of hydro-mechanical loading on anisotropic small-strain stiffness of compacted and intact loess should be investigated.

The principal objective of this research is to examine evolving anisotropic stiffness of unsaturated compacted and intact loess. A suction-controlled triaxial apparatus, equipped with bender elements and hall-effects sensors, was used. To investigate the anisotropic stiffness at very small strains (below 0.001%), wetting-drying tests and isotropic compression tests were carried out. Furthermore, constant suction and constant-? shear tests were conducted, to study the small-strain (between 0.001% and 1%) stiffness.

It is found that, different from other saturated sedimentary clays, ?_ℎℎ (corresponding to the horizontal shear plane) of unsaturated intact loess is less than ?_ℎ? (corresponding to the vertical shear plane) at very small strains. This is mainly because horizontal layering is absence in the intact loess whereas it is often found in other sedimentary clays. This stiffness anisotropy is found evolving significantly with a change in suction. Similar trend was observed in the compacted loess, implying that suction can induce stiffness anisotropy even at isotropic stress state.

During shearing at small strains, intact loess exhibits stiffer response than the compacted loess at deviatoric strain below 0·025%, mainly because more clay aggregates accumulate at the inter-grain contacts in the intact loess. Contrary to the typical behaviour of compacted soils, small-strain stiffness of the compacted loess increased with a decreasing suction. The increase may be caused by significant wetting-induced collapse during the wetting process. In contrast, the shear stiffness of the intact specimens remains unchanged, owing to slight swelling observed during wetting. Comparing the stiffness degradation of loess to that obtained from widely-used design chart proposed by Vucetic & Dobry (1991), the design chart is found unconservative for the stiffness degradation of both compacted and intact loess at small strains.

Furthermore, a bubble model for unsaturated soil is developed within the framework of kinematic hardening and bounding surface plasticity. To model effects of recent suction history, an elliptical elastic bubble is defined inside a modified Cam-clay bounding surface. The elastic bubble is considered as a function of suction, degree of saturation and plastic volumetric strain. Translation of the elastic bubble is governed by suction, degree of saturation and stress increments. Moreover, hardening modulus depends on not only stress and void ratio but also suction, degree of saturation and distance between the elastic bubble and the bounding surface. The proposed model is evaluated using suction-controlled constant−? shear tests on completely decomposed tuff. It is evident that the new model is capable of well capturing effects of recent suction history on non-linear stress-strain relation and shear modulus degradation at small strains.