Anisotropic soils exhibit complex mechanical behaviours under various loadsing conditions, e.g., reversible dilatancy, three-dimensional failure strength, fabric anisotropy, small-strain stiffness, cyclic mobility, making it difficult to comprehensively capture these characteristics within a single constitutive model. Failure to capture anisotropic soil behavious may result in poor predictions in geotechnical engineering. Hence, to provide a unified prediction for the mechanical responses of anisotropic sand and clay under both monotonic and cyclic loading conditions, a fabric-based anisotropic constitutive model, i.e., the CASM-CF, is developed within the framework of the standard Clay and Sand Model (CASM) in this paper. Effects of small-strain stiffness and anisotropic elasticity are incorporated into the stiffness matrix to capture the stiffness variation over a wide strain range and reversible dilation. The fabric tensor defined by particle orientation and its evolution law are integrated into the CASM-CF model through the Anisotropic Transformed Stress (ATS) method. The plastic modulus is modified by considering cyclic loading history and stress reverse to better predict the mechanical responses of soils when subjected to cyclic loadings. The newly proposed model is employed to predict the mechanical behaviours of clay and sand under various strain scales and stress paths, including monotonic, cyclic, proportional, and non-proportional loading conditions, in the literature. Conclusions can be drawn that the model performs satisfactorily under various stress paths, and it has the potential to be used in the analysis of practical geotechnical applications of wide range.

The discrete element method (DEM) is adopted to investigate the influence of the particle shape on the smallstrain stiffness and stiffness degradation of granular materials during triaxial compression tests. Clumped particles are used to simulate irregular granular particles. The simulation results show that a more irregular particle shape causes an increase in the initial stiffness at very small strains and more delayed stiffness degradation. The micromechanism is explored on the basis of the analytical stress-force-fabric relationship, which reveals that increased particle irregularity leads to higher relative contribution of the tangential force anisotropy to the deviatoric stress. The achievable slip ratio and the mechanical coordination number also increase with increasing particle irregularity, resulting in larger resistance to deformation. An equivalent spherical particle analysis method is proposed, which reveals that the irregularity of particle shapes significantly increases both the sliding resistance and the rotational resistance between two particles, resulting in greater stability in the contact network and thus contributing to higher macroscopic stiffness and slower stiffness degradation.

Small strain properties of subgrade fill material are essentially required for the accurate estimation of deformation behavior of railway subgrade. Many attentions have received on small strain properties of soils under the isotropic stress state or low shear stress level. The high level of shear stress and stress ratio induce reduction in small strain stiffness and thus present the potential challenge to the deformation stability of the subgrade. However, there is not much attempt to investigate the small strain properties under high stress ratio. This paper explores the effects of stress path and stress state on small strain stiffness Gmax and Poisson's ratio v of heavily compacted fully weathered red mudstone (FWRM) under a broad range of stress ratio, via a series of stress-controlled triaxial and bender element tests. Three stress paths, named as constant stress ratio (SSP), constant confined pressure (VSP), constant axial stress (HSP) with stress ratio up to 33.0 were considered. Low level of shear stress slightly promotes Gmax, while a significant reduction of Gmax is triggered as the stress ratio exceeds a critical value. A unified correlation between the critical stress ratio and confined pressure is developed. The evolution of Poisson's ratio is also described by a unified three-dimensional feature surface, which influence of stress path is identified by the location and shape of the surface.

Stiffness of soil at very small strains G0 is mainly affected by void ratio, effective stress and suction. Empirical equations considering those factors have been proposed to estimate G0. However, for collapsible soil like loess, variations in suction might induce changes in void ratio of soil. The combined effect of these two factors poses challenges in accurately estimating of G0. This paper first presents an experimental study on the G0 of collapsible loess under various conditions, including as-compacted states, wetting/drying and K0 loading. G0 is estimated from shear wave velocity obtained with bender element technique. The changes of G0 with respect to void ratio, suction, effective stress, and wetting under K0 stress conditions are evaluated. Test results reveal that power relationships can be defined between G0 and void ratio, suction and effective stress, respectively. The changes in G0 along wetting/drying shows an S shape due to the different dominant effects on soil structure, as well as the induced non-uniform volume changes when suction change at different zones. Under K0 loading, G0 decreases upon wetting at stresses below the compaction stress, while it increases upon wetting at stresses above the compaction stress, due to the combined effects of densification caused by volume collapse during wetting and softening induced by suction decrease. Finally, a G0 model considering net stress and suction as independent stress variable is proposed. This model could effectively capture the change of G0 during wetting, drying and loading, as well as upon wetting under K0 loading for collapsible loess.

Assessing the stability of embankments during railway operation is paramount for ensuring safety of railway. However, directly measuring the strength of fill materials can be challenging when the railway is in use. A strong correlation has been observed between soil shear strength and small strain stiffness. By establishing a robust correlation between shear strength and small -strain stiffness in the laboratory, considering various of factors, and combining it with field measurement of in -situ soil small stiffness might be an effective way to this problem. This study focuses on a type of filling materials commonly used in southwestern parts of China for railway construction: fully weathered red mudstone (FWRM) and its lime -treated counterpart (LFWRM), as the objects. A series of triaxial and unconfined compression tests were conducted to examine the effects of water content, confined pressure, and lime treatment on the shear strength and small strain stiffness of FWRM and LFWRM. The results show that the strength and stiffness of FWRM significantly decrease with increasing water content, while LFWRM specimens demonstrate good resistance. All LFWRM specimens displayed a brittle shear behavior. Empirical correlation was established for FWRM and LFWRM. The relationship for LFWRM is water content independent, meanwhile for FWRM is strongly dependent upon whether soil is saturated or not. The ratio of small strain stiffness to strength (E max /q max ) for FWRM decreases substantially after saturation, whereas it remains almost constant for LFWRM. The reduction in strength and stiffness can be attributed to the degradation of the soil fabric due to increasing water content, where the pore size distribution (PSD) of FWRM changes significantly with increasing water content due to aggregate swelling. However, for LFWRM, the PSD remains bimodal, which is due to the cementation bonding observed between lime -treated aggregates that explains the stable structure and improved performance of LFWRM.

In this paper, the tunnelling-induced deformation in anisotropic stiff soils is analysed using FE modelling. The influence of material description is investigated rather than an advanced simulation of the tunnelling method. A new hyperelastic-plastic model is proposed to describe the anisotropic mechanical behaviour of stiff highly overconsolidated soil. This model can reproduce the superposition of variable stress-induced anisotropy and constant inherent cross-anisotropy of the small strain stiffness. Additionally, a Brick-type framework accounts for the strain degradation of stiffness. Formulation of the novel model is presented. The tunnelling-induced deformation is first investigated in plane strain conditions for a simple boundary value problem of homogeneous ground. The influence of initial stress anisotropy and inherent cross-anisotropy is inspected. Later, the results of 2D simulations are compared with the analogous results of 3D simulations considering different excavated length of the tunnel sections. The tunnelling process is reproduced by introduction of a supported excavation and a lining contraction stage in undrained conditions. Finally, the tunnelling case study at St James Park is back analysed using the proposed material model in plane strain conditions. The obtained calculation results are compared with the field measurements and discussed.