Conventional plasticity assumes that a yield surface exists and the direction of plastic strain increment (DPSI) is uniquely dependent on the current stress state. Triaxial stress probing tests of yield and plastic flow of sand have been conducted using discrete-element modelling with polyhedral particles resembling the shapes of Toyoura sand. It is found that a yield surface does not exist, but a memory surface (MS) separating two types of distinct sand behaviour can be established. Within the MS, the DPSI is primarily controlled by the stress increment, and the magnitude of plastic strain increment is insensitive to the stress increment direction. When the stress state is on or outside the MS, a much larger plastic strain increment is observed if the stress increment points outside the MS, and the DPSI is dependent on both the current stress state and stress increment. The shape and size of the MS, which can be modelled by the SANISAND yield function, are dependent on the soil density and evolve with plastic strain.

Consolidated-drained true triaxial tests with constant b\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$b$$\end{document} values were performed on normally consolidated cross-anisotropic kaolin clay. Isotropic stress probes were incorporated into these true triaxial tests to study the orientations of plastic strain increment vectors and positioning of the plastic potential surface at different levels of shearing. An isotropic compression test was also performed to characterize the cross-anisotropic response of the clay. Pronounced cross-anisotropy was observed in the K0\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$K_{0}$$\end{document} consolidated kaolin clay during shear, particularly when the major and minor principal stresses were perpendicular and parallel to the axis of material symmetry, respectively. A simple rotational kinematic hardening mechanism incorporated into the single hardening constitutive model for soil has been found to fairly accurately simulate the evolution of anisotropy in the form of expansion and rotation of the yield and plastic potential surfaces during true triaxial shearing.

This study offers a novel investigation into the incremental behavior of granular materials by focusing on the effects of particle elongation on mechanical properties and microstructural evolution. Through a series of Discrete Element Method (DEM) simulations, samples with varying elongation coefficients (n) are systematically analyzed using two strain decomposition methods: the energy dissipation constraint method and the loading cycle method. The results show that as n increases, the strain envelope size decreases, indicating greater stiffness. A 'memory effect' is observed in the elastic strain envelope, suggesting internal rearrangement and partial microstructural recovery in later stages. The plastic strain envelope exhibits distinct patterns that vary with loading conditions, with magnitude decreasing as n increases. Despite identical initial stress states, the orientation of the plastic strain envelope shifts significantly, highlighting the impact of loading history and anisotropy. Notably, the misalignment between the normal direction of the yield surface and the incremental plastic flow direction indicates a non-associated flow rule for the generated granular materials. This misalignment varies with n and loading conditions. The study also reveals a transition from contraction to dilation behavior across different probing states, with increasing n leading to a denser packing of particles with a lower void ratio. As n increases, the anisotropy within the granular assembly becomes more pronounced, leading to a stronger directional dependence of the mechanical response.

BackgroundThe theory of stress distribution in soil based on continuum mechanics introduces a stress concentration factor (xi) of 3 for a purely elastic soil and larger than 3 for an elastic-plastic soil material. However, the experimental estimation of xi as a function of loading geometry and soil properties is a challenge. Furthermore, the insertion of a stress probe into the soil exacerbates the stress concentration due to the arching effect.ObjectiveThe aim of this study is to model xi under circular surface (uniform) loading as a function of soil strength, loading area, and depth using finite element method.Materials and MethodsThe simulations were performed using a model of stress propagation under circular uniform loading in two parts: with and without a stress probe. Simulations were carried out for combinations of 21 soil properties of varying water content and cone index (CI), surface loading radius (R), soil depth (z), and surface stress (q). For each combination, the stress at a given depth (sigma z) and the resulting concentration factor (xi) were analyzed.ResultsA total of 1512 values were obtained for xi from simulations. Regression models were developed and validated for with-probe and without-probe xi as a function of CI, R, z, soil yield stress (sigma yield), and vertical stress (sigma z). Experimental data of stress measurements under plate sinkage loading for samples of a clay loam soil at two levels of water content each at two levels of bulk density were used to validate the with-probe regression model.ConclusionThe values obtained from the model and those from the experimental tests showed a relatively good correlation with R2 of 0.7. xi varied between 3.5 and 14 which is much larger than the values obtained for the without-probe model or reported in the literature.

Machinery traffic is associated with the application of stress onto the soil surface and is the main reason for agricultural soil compaction. Currently, probes are used for studying the stress propagation in soil and measuring soil stress. However, because of the physical presence of a probe, the measured stress may differ from the actual stress, i.e. the stress induced in the soil under machinery traffic in the absence of a probe. Hence, we need to model the soil -stress probe interaction to study the difference in stress caused by the probe under varying loading geometries, loading time, depth, and soil properties to find correction factors for probe -measured stress. This study aims to simulate the soil -stress probe interaction under a moving rigid wheel using finite element method (FEM) to investigate the agreement between the simulated with -probe stress and the experimental measurements and to compare the resulting ratio of with/without probe stress with previous studies. The soil was modeled as an elastic -perfectly plastic material whose properties were calibrated with the simulation of cone penetration and wheel sinkage into the soil. The results showed an average 28% overestimation of FEM-simulated probe stress as compared to the experimental stress measured under the wheel loadings of 600 and 1,200 N. The average simulated ratio of with/without probe stress was found to be 1.22 for the two tests which is significantly smaller than that of plate sinkage loading (1.9). The simulation of wheel speed on soil stress showed a minor increase in stress. The stress over -estimation ratio (i.e. the ratio of with/without probe stress) noticeably increased with depth but increased slightly with speed for depths below 0.2 m.