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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]

The strength of granular materials during triaxial compression is investigated via a grain scale analysis in this paper. A 3D Discrete Element Method (DEM) program provides the triaxial strength data and helps to validate the micromechanical analysis. Some standard methods in statistics are employed first to quantitatively examine the assumptions made when deriving the stress-force-fabric (SFF) equation. After careful validation, a more concise format for the SFF equation is proposed for triaxial compressions. With this SFF equation, the strength is found to be jointly contributed by the magnitudes of the contact force anisotropy and fabric anisotropy. The influence of the initial void ratio, confining pressure and loading direction on the development of contact force anisotropy and fabric anisotropy is examined and presented. With similar techniques, the “force” term in the SFF equation is further decoupled, and an equation is obtained such that it explicitly links the contact force term with the friction coefficient between grains, a tensor defined as a statistic of the normal contact forces and a tensor defined as a statistic of the mobilisation status of contacts. Based on this equation, another equation regarding the stress ratio of granular assembly is obtained, and it clearly indicates two sources that contribute to the phenomenological friction nature of granular assembly. These two sources are caused by the contact force at the grain scale. The first is the anisotropy of the average normal contact forces, and the second is the mobilisation of contacts. [ABSTRACT FROM AUTHOR]

Soil is a multiphase material with irregularly shaped particles, the behaviour of which is highly nonlinear, anisotropic, and inelastic. Anisotropy and its evolution characteristics are crucial in soil's constitutive modelling, particularly for clayey soil. A new anisotropic constitutive model for remoulded clay is proposed based on improved yield surfaces and rotational hardening rules for initial and induced anisotropy. The logarithmic interpolation function is adopted to describe the volumetric deformation characteristics of clay in an arbitrary stress state, and a flexible yield surface shape is obtained by changing the newly introduced parameter n. This new parameter can be determined by fitting the undrained shear strength or stiffness of stress–strain curves. A key feature of the proposed evolutional rule for anisotropy is that the principal direction of the yield surfaces in the stress plane can be consistent with the isotropic stress axis when the critical state is reached. The proposed elastoplastic model shows good capacity for describing laboratory results from oedometer tests with constant stress ratios, reconsolidation tests with various stress ratios, triaxial compression tests, and triaxial extension tests under both undrained and drained conditions. [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]

The extraction process of methane hydrate involves interactions of multiple fields, phases, and loading stages. By extrapolating deductions from the thermodynamic equation in granular thermodynamic framework and amalgamating with coupled flow theory and linear non-equilibrium theory, a comprehensive Thermo-Hydro-Mechanic-Chemical (THMC) coupling model for hydrate-bearing sediment is established in this study. The mechanical field is established by energy dissipation conservation, the hydraulic field considers the increase in gas and liquid permeability due to temperature gradients, while the thermal field incorporates the rates of seepage, hydrate dissociation, and volumetric strain. Meanwhile, Darcy's law is explained as general deductions from the thermodynamic equation without the need for assumption. The proposed model is applied from the perspective of unit scale and coupling field, and verified with the test data. The modelling results show that the deduced model can capture the strain-softening, dilatancy, and stress paths behavior under various drainage conditions, and the volumetric strain, methane gas production and variations in heat exchange caused by the hydrate dissociation can also be predicted. Furthermore, the mechanical response and dissociation properties of sediment samples with various parameter values are discussed. [ABSTRACT FROM AUTHOR]

This paper established a coupled thermo-hydro-mechanical mechanism in view of the soil particle rearrangement for saturated/unsaturated soils under the framework of granular thermodynamics. This model avoids concepts such as the flow rule, yield function, dissipation potential function, and hardening criterion. The effects of the loading path and the soil structure are reflected by constructing a density function of elastic potential energy considering the structural strength of a solid skeleton with a degradation factor. That is, the changes in the stress states are closely related to soil particle rearrangement and the transitions between different matter phases. A generalized effective stress principle is derived and is suitable for the coupled thermo-hydro- mechanical process. The deduced generalized phase stresses differ from the classical effective principle based on linear elastic porous media and can automatically consider the impact of the stress path, temperature path and soil structure. The established model spans the complete process from unsaturated to saturated soils and is verified by the typical test results. [ABSTRACT FROM AUTHOR]

The stability of slurry trench is of importance in diaphragm wall construction. The local stability of slurry trench under seepage application is investigated in this research. In the current research, the local instability of slurry trench is identified as the soil collapse in an instable sublayer and a three-dimensional translational failure mechanism is developed. In this mechanism, the local stability is initialed by the movement of a single rigid block and the seepage application is accounted by introducing the analytical model of semi-confined seepage. The stability is analyzed through kinematic approach and the upper bound solution of safety factor is deduced as a result. The stability is evaluated by optimizing the mechanism for the local failure. Using the current solution, the seepage effect on the stability is analyzed. Regardless of seepage, the stability is mainly dependent on the shear strength of the sublayer. Under seepage application, the failure zone is enlarged and the safety factor decreases as a result. The effect of seepage on the stability is significantly influenced by piezometric head difference Δ p and leakage ratio λ of the sublayer. The validity of the current solution is examined by investigating the case of diaphragm wall installation in Suzhou, China. [ABSTRACT FROM AUTHOR]

This paper investigates the passive instability of the working face of large-diameter shield tunneling in frictional soils by developing a new mechanism. The proposed composite mechanism comprises rotational and translational blocks, and failure is involved within the movement of the blocks. The upper bound solution of support pressure for passive instability is derived using a kinematic approach. The stability analysis explores the critical support pressure and associated failure mode by optimizing the mechanism. The proposed mechanism indicates that the failure in the cover layer is fully involved in the translational movement, while in the crossed layer, the failure is partially involved in rotational movement. The mechanism is influenced more significantly by the frictional angle than by cohesion and surcharge on the ground surface. The proportion of rotational blocks to translational blocks decreases with increasing φ ′. In the presence of a rotational block, the proposed composite mechanism outperforms the previous translational mechanisms in predicting the critical support pressure and associated failure mode. Finally, the current solution is validated by comparing with previous centrifuge tests, where agreement on the critical support pressure and the associated failure mode is found. [ABSTRACT FROM AUTHOR]