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Focusing mainly on the compressive-shear (C-S) stress, the existing soil strength theories fail to well describe the soil strength under tenso–shear (T-S) stress and are not suitable for analyzing the tensile strength and T-S coupling strength. Taking the T-S strength into account, a three-dimensional nonlinear strength model of soil is established. The failure function on the π plane is a Lade Lade-curved triangle in principal stress space, which reasonably reflects the effect of medium principal stress. The failure function on the triaxial compression meridian plane is divided into the T-S and C-S zones, in which the two failure functions are smoothly connected at the closed stress point. To easily combine it with the specific elastoplastic constitutive theory, the proposed strength model is transformed into the D-P criterion in transformed stress space using the transformation stress method, which provides a theoretical foundation for numerical calculations. Compared with the true triaxial test results and numerical slope examples, the proposed model gives a good description of the nonlinear soil strength under complex stresses and the instability failure mechanism of the slope. [ABSTRACT FROM AUTHOR]

An unsaturated soil constitutive model considering the influence of microscopic pore structure can more accurately describe the hydraulic–mechanical behavior of unsaturated soil, but its numerical implementation is more complicated. Based on the fully implicit Euler backward integration algorithm, the ABAQUS software is used to develop the established hydro-mechanical coupling constitutive model for unsaturated soil, considering the influence of micro-pore structure, and a new User-defined Material Mechanical Behavior (UMAT) subroutine is established to realize the numerical application of the proposed model. The developed numerical program is used to simulate the drying/wetting cycle process of the standard triaxial specimen. The simulation results are basically consistent with those calculated by the Fortran program, which verifies the rationality of the developed numerical program. [ABSTRACT FROM AUTHOR]

Most soil mechanics theories are limited to strain hardening and shrinkage under high compressive stresses, and there are some shortcomings in the selection of suction or degree of saturation as the water content state varies in the constitutive models of unsaturated soil. Based on the triaxial shear tests of unsaturated compacted soil (a silt of high plasticity) with different water content and confining pressure (low-confining), a shear dilatancy model of unsaturated soil based on the mass water content is proposed in this paper. The influence of the water content on the shear deformation characteristics of the unsaturated soil is analysed. The stress–dilatancy relationship and the prediction equation of the minimum dilatancy rate of the unsaturated soil under different water content and different confining pressure are provided. Selecting the mass water content as the state variable, a constitutive model suitable for the dilatancy of unsaturated soil is established. The method of determining model parameters based on the mass water content is analysed. The applicability of the model is verified by comparisons between the predicted and experimental results. [ABSTRACT FROM AUTHOR]

The interparticle bonding effect due to water menisci plays an important role in the hydromechanical coupling properties of unsaturated soils. This paper presents an unsaturated hydromechanical coupling model that considers the influence of matric suction, degree of saturation, and microscopic pore structure on the interparticle bonding effect. The enhanced effective stress and bonding variable are selected as constitutive variables. The bonding variable is correlated with the ratio between unsaturated void ratio and saturated void ratio. The deformation characteristics of unsaturated soils are described based on the bounding surface plasticity theory. A soil–water characteristic model that considers deformation and hydraulic hysteresis is integrated into the constitutive model to achieve hydromechanical coupling. The proposed model can effectively describe the hydromechanical coupling characteristics and the meniscus bonding force of unsaturated bimodal structure soils; the model parameters can be easily obtained through routine experiments. The experimental results of unsaturated isotropic compression, the wetting/drying cycle, and unsaturated triaxial shear tests are used to validate the capability of the proposed model. [ABSTRACT FROM AUTHOR]

This paper presents a novel elastic-viscoplastic constitutive model for reproducing the time-dependent behaviour of coarse-grained soil considering particle breakage, by integrating the Unified Hardening (UH) model, the elastic-viscoplastic (EVP) model and the overstress theory. The relationship between the degree of particle breakage and the loading rate is established, and the state variables associated with the critical state of coarse-grained soil are derived to jointly consider both time and particle breakage. A new three-dimensional elastic-viscoplastic constitutive model is then constructed through combining the one-dimensional viscoplastic hardening parameter with a secondary consolidation coefficient considering particle breakage. The proposed model requires 19 parameters and it can well describe the influence of time-dependency and particle breakage to the shear, dilatancy, and compression behaviours of coarse-grained soil with various confining pressures or initial void ratios. Comparisons between the model predictions and experimental data are employed to validate the capability of the proposed model to replicate the time-dependent behaviour of coarse-grained soil. [ABSTRACT FROM AUTHOR]

This paper presents a study of the tensile strength of unsaturated soil by a DEM model in a novel uniaxial tensile test device. For validation and comparison, traditional triaxial shear test of unsaturated soil are also conducted. In the DEM model, the capillary effects and some other cementation effects are modelled by a bond, whose strength is a function of the moisture content and void ratio in uniaxial tensile tests and also the confining pressure in triaxial tests. To compare the DEM simulations with experiments, the bond strength function is calibrated through a quantity measurable in both laboratory and DEM simulations such as the shear strength in triaxial tests or the uniaxial tensile strength in uniaxial tensile tests. The comparison shows that the proposed model is able to capture the phenomena observed in experiments. Most importantly, through investigation of the grain-scale data such as the motion, force chains and development of fractures, it is possible to explain some macroscopic observations such as the form of shear bands in the sample, the influence of the moisture content on the shear and tensile strength, etc. [ABSTRACT FROM AUTHOR]

Modelling of unsaturated soil stress–strain relationships under cyclic loading with constant matric suction is a topic that is poorly addressed by the current literature. Using the theoretical framework of the bounding surface plasticity and Barcelona basic model (BBM) combined with the concept of fractional accumulated strain rate, a bounding surface model for accurately describing the unsaturated soil stress–strain relationship with constant matric suction is established here. The fractional accumulated strain rate is introduced to consider the influence of cyclic loading history. The movable mapping centre rule is used to describe the hysteresis characteristics of the dynamic stress–strain curve during the unloading process. The shape of the dynamic stress–strain curve can be described reasonably by considering the cyclic loading process as the three processes of initial loading, unloading, and reloading. The cyclic loading test data of unsaturated silt, sand, and lean clay with constant matric suction is used for parameter calibration and model validations. Results show that the proposed model can accurately capture the dynamic stress–strain relationship of unsaturated soil with constant matric suction. [ABSTRACT FROM AUTHOR]

This work presents a fully coupled constitutive model for thermo-hydro-mechanical behaviour of unsaturated soils. This model, with 18 parameters, can consider the primary features of unsaturated soils, such as mechanical responses at various temperatures and matric suctions, drying/wetting behaviour (water retention curves and volumetric strain) at various temperatures and net stresses, and thermal volumetric behaviour at various matric suctions. A numerical example on a drained heating-drying-isotropic loading-wetting path is used to show the basic properties and performance of the proposed model. Finally, the proposed model is validated against experimental data for temperature/suction-controlled isotropic tests, temperature-controlled wetting-drying tests, suction-controlled heating-cooling tests and temperature/suction-controlled triaxial tests. [ABSTRACT FROM AUTHOR]