In this paper, a recently developed unified critical state model (CASM-S), which is applicable to predict the mechanical behaviours of sand and overconsolidated clay, is numerically implemented into the Fast Lagrangian Analysis of Continua, i.e. the FLAC(3D), for engineering applications. The implicit integration algorithm incorporated with the line search method is employed to implement CASM-S model. The additional parameter u is calibrated by genetic algorithm, whilst the rest of the material parameters are determined following the literature upon applicable. Validation of CASM-S model and its numerical implementation has been well demonstrated by a series of drained and undrained triaxial compression tests conducted on clay and sand. In terms of stress-strain relations, volumetric versus axial strain, and negative pore pressure versus axial strain, the model predictions agree well with the experimental results. Then, a case study is performed to demonstrate the applicability of the CASM-S model to analyse geotechnical problems, e.g. the foundation pits excavated from Berlin sand within the FLAC(3D), where lateral deflections of the diaphragm wall and vertical displacements in a designated are evaluated. Conclusions can be drawn that the predictions of CASM-S model are almost identical to the field data, demonstrating a good performance in engineering applications.

This paper proposes a coupled hydro-mechanical constitutive model for unsaturated clay and sand (CASM-U) in a critical state framework. The mechanical behaviour of unsaturated soils is modelled by modifying the unified clay and sand model (CASM) with Bishop's effective stress, bounding surface concept and loading collapse (LC) yield surface. The hydraulic behaviour is described by a soil-water characteristic curve (SWCC) with nonlinear scanning law, considering the coupled effects of soil deformation and hysteresis. CASM-U is implemented into a commercial finite element software through the user-defined material subroutine (UMAT), and the implementation is benchmarked by a new semi-analytical cavity expansion solution adopting CASM-U. Finally, the performance of CASM-U in predicting hydro-mechanical behaviour of unsaturated clays and sands is examined by comparing with experimental data from tests along various loading paths, including isotropic compression, cyclic drying-wetting, triaxial shearing, and their combinations. It is shown that CASM-U can provide reasonable predictions for hydro-mechanical behaviour of unsaturated soils with a total of 15 material parameters.

In order to consider the effect of fabric anisotropy in the analysis of geotechnical boundary value problems, this study proposes a modified model based on a fabric-based modified Cam-clay model, which can account for the anisotropic response of soil. The major modification of the original model aims to simplify the equations for numerical implementation by replacing the SMP strength criterion with the Lade's strength criterion. This model comprehensively considers the inherent anisotropy, induced anisotropy, and three-dimensional strength characteristics of soil. The model is first numerically implemented using the elastic trial-plastic correction method, and then it is encapsulated into the FLAC(3D )6.0 software, and tested through conventional triaxial, embankment loading, and tunnel excavation experiments. Numerical simulation results indicate that considering anisotropy and three-dimensional strength in geotechnical engineering analysis is necessary. By accounting for the interaction between microstructure and macroscopic anisotropy, the model can more accurately represent soil behavior, providing significant advantages for geotechnical analysis.

The accuracy of a constitutive model for confined concrete largely relies on its capability to capture concrete's dilatancy behavior. In this paper, a non -orthogonal flow rule (NFR) is used to reasonably characterize the concrete's volume change law in relation to the stress state without the necessity for a plastic potential function. Then, the non -orthogonal plastic model is implemented in the finite element (FE) software ABAQUS using the implicit stress update algorithm, which employs the line search method and the numerical consistent tangent stiffness matrix to ensure the robustness and computational efficiency of FE analysis. Finally, FE simulations are performed to evaluate the constitutive model's capabilities in actively and passively confined concrete. In the latter case, steel tubes restrict the concrete's lateral deformation. An analysis and discussion are conducted regarding the impact of dilatancy behavior on concrete -filled steel tube (CFST) columns. The consistency between experimental data and simulation results demonstrates that the FE modeling with a non -orthogonal constitutive model provides an effective tool to describe the behavior of confined concrete.