After unloading, the deformation of soils cannot be stable immediately, but continues to expand over time even under constant pressure. In this paper, the expansive deformation over time when effective stress is kept constant is defined as the negative creep, while the compressive creep is described as the positive creep. The division between positive creep and negative creep is named the stable normal compression line (SNCL), on which the stress–strain behaviour of the soil is time-independent. Based on the concept of the SNCL and test results, a new formula for creep is proposed. This formula is simple in form and has less parameters, and both negative creep and positive creep can be well predicted. By incorporating this formula into the current yield function of the unified hardening model, a new time-dependent current yield function is built. Combining the yield function, a flow rule, and transformed stress method, a new three-dimensional time-dependent constitutive model considering both positive and negative creep for clays is derived and presented. The new model is then validated by test results, including multistage loading oedometer tests, triaxial undrained creep tests, and triaxial undrained compression tests at the constant strain rates.

The particle size plays an important role in the mechanical behavior of granular geomaterials. With respect to the crushable granular soils, the particle breakage under different pressures leads to the variation of the particle size distribution of soils. Therefore, it is essential to accurately model the particle fracture process to reproduce the key response of granular soils in the simulation. In this study, two simulated approaches, cluster method and replacement method, were performed to investigate the particle size effect for the crushable granular soils by three-dimensional (3D) discrete element method (DEM). DEM simulations were validated through the available results of diametric and oedometric compression tests with the help of the Weibull statistics. Based on the evaluation at particle and assemble-scale, it is found that both the cluster method and replacement method could simulate the particle strength and size dependence of sample failure with a good agreement with experimental results. The particle breakage phenomenon is also successfully modeled in DEM simulation and analyzed by the breakage index. Meanwhile, the computational efficiency was analyzed compared with two simulated methods. The findings in this study can provide a viable approach to investigate the particle breakage effect of the crushable geomaterials.

The geomaterials with original anisotropic properties formed in the natural process are usually simplified as a kind of cross‐anisotropic material. The spatial location of the depositional plane (DP) and the effective spatial mobilized plane (ESMP) in physical space is closely related to anisotropic properties; thus, the inclined angle between DP and ESMP is taken as a primary parameter governing the strength degree of geomaterial anisotropy. According to the concept of ESMP, the frictional capacity can be more effectively mobilized when the inclined angle between DP and ESMP is larger, inducing a higher stress strength. In this study, a new stress strength formula is proposed for geomaterials, which takes the cross‐anisotropic properties into account. The transformation strategy can be regarded as a strength criterion describing the convert of transversely isotropic behavior formula into an isotropic von Mises criterion formula. Based on the cross‐anisotropy strength criterion, the transformed stress (TS) equation can be derived by transforming the cross‐anisotropy stress space to the isotropic stress space. By using the proposed TS method, it is convenient to convert the traditional two‐dimensional (2D) constitutive models on the basis of the Von‐Mises criterion to the general three‐dimensional (3D) models considering cross‐anisotropy. Comparing the predicted and the tested results of strength and stress‐strain relationship tests for geomaterials under the true triaxial loading condition, the validity and the applicability of the proposed TS method with related criterion can be ensured.