Constitutive models of sands play an essential role in analysing the foundation responses to cyclic loads, such as seismic, traffic and wave loads. In general, sands exhibit distinctly different mechanical behaviours under monotonic, regular and irregular cyclic loads. To describe these complex mechanical behaviours of sands, it is necessary to establish appropriate constitutive models. This study first analyses the features of hysteretic stressstrain relation of sands in some detail. It is found that there exists a largest hysteretic loop when sands are sufficiently sheared in two opposite directions, and the shear stiffness at a stress-reversal point primarily depends on the degree of stiffness degradation in the last loading or unloading process. Secondly, a stress-reversal method is proposed to effectively reproduce these features. This method provides a new formulation of the hysteretic stress-strain curves, and employs a newly defined scalar quantity, called the small strain stiffness factor, to determine the shear stiffness at an arbitrary stress-reversal state. Thirdly, within the frameworks of elastoplastic theory and the critical state soil mechanics, an elastoplastic stress-reversal surface model is developed for sands. For a monotonic loading process, a double-parameter hardening rule is proposed to account for the coupled compression-shear hardening mechanism. For a cyclic loading process, a new kinematic hardening rule of the loading surface is elaborately designed in stress space, which can be conveniently incorporated with the stressreversal method. Finally, the stress-reversal surface model is used to simulate some laboratory triaxial tests on two sands, including monotonic loading tests along conventional and special stress paths, as well as drained cyclic tests with regular and irregular shearing amplitudes. A more systematic comparison between the model simulations and relevant test data validates the rationality and capability of the model, demonstrating its distinctive performance under irregular cyclic loading condition.

The majority of existing effective stress-based constitutive models approach thermal effects through the temperature dependency of surface tension and its effects on the soil-water retention curve (SWRC) and effective stress. Experimental tests and theoretical studies, however, suggest that the temperature effect on surface tension alone is not sufficient to properly explain thermal-induced changes in the effective stress and SWRC. This study focuses on the temperature-dependent elastoplastic behavior of low plasticity unsaturated soils by developing a set of constitutive-level relations that incorporate temperature-dependent SWRC and effective stress models. These models account for the effect of temperature on the enthalpy, contact angle, and surface tension. The application of the presented constitutive relations was demonstrated and validated for low plasticity soils, specifically incorporating temperature effects into the hardening modulus, specific volume change, yield stress of the modified Cam-Clay model, and stress-strain relationships. The proposed relationships are incorporated in any effective stress-based constitutive model for modeling temperature dependency of elastoplastic response in low plasticity unsaturated soils. Employing these relationships can enhance the numerical simulation of low plasticity unsaturated soils under thermo-mechanical or other coupled processes involving temperature-dependent conditions.

Numerical challenges, incorporating non-uniqueness, non-convexity, undefined gradients, and high curvature, of the positive level sets of yield function F > 0 are encountered in stress integration when utilizing the return-mapping algorithm family. These phenomena are illustrated by an assessment of four typical yield functions: modified spatially mobilized plane criterion, Lade criterion, Bigoni-Piccolroaz criterion, and micromechanics-based upscaled Drucker-Prager criterion. One remedy to these issues, named the Hop-to-Hug (H2H) algorithm, is proposed via a convexification enhancement upon the classical cutting-plane algorithm (CPA). The improved robustness of the H2H algorithm is demonstrated through a series of integration tests in one single material point. Furthermore, a constitutive model is implemented with the H2H algorithm into the Abaqus/Standard finite-element platform. Element-level and structure-level analyses are carried out to validate the effectiveness of the H2H algorithm in convergence. All validation analyses manifest that the proposed H2H algorithm can offer enhanced stability over the classical CPA method while maintaining the ease of implementation, in which evaluations of the second-order derivatives of yield function and plastic potential function are circumvented. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

The stress-strain behavior of granular soils is related largely to their grain-size distributions and density. If sand particles are crushed under load, some basic physical and mechanical properties of sands will be changed to a certain extent, and the traditionally defined critical state is not unique in this crushing process. To describe the complex effects of particle breakage, the volumetric and shearing responses of crushable sands to compression-shear loading are attributed to the rearranging and crushing state (RCS) of sand particles. The interrelation between rearrangement and breakage of sand particles was analyzed phenomenologically as the physical basis of this study. To characterize the RCS over a large stress range, a special curve named the RCS curve is defined in the e-ln p plane and quantified through a specific loading path. A new breakage model is proposed to correlate the crushing stress with the breakage index and to control the evolution of the RCS-curve. To account for the state-dependent dilatancy of sand particles, a new state parameter called the RCS parameter is introduced into the plastic potential function. An elastoplastic model for crushable sands was established based on the evolution of the RCS and verified by relevant triaxial test data of three representative crushable sands with an initial confining pressure ranging from 50 kPa to 68.9 MPa. The stress-strain behavior, excess pore-water pressure, and accumulated particle breakage of these crushable sands were simulated satisfactorily. In addition, procedures for calibrating the model parameters are suggested to make the established model more reliable.

Soil -structure interfaces in shallow foundations, embankments and other geo-systems are usually unsaturated, which has great influences to the performance of geo-structures. However, researches on unsaturated soilstructure interfaces are still kept in very limited numbers, particularly for the theoretical part. This paper proposes a state -dependent model for unsaturated soil -structure interfaces based on two independent stress state variables: net stress and suction. To consider the effects of initial state, stress level, and suction, a state -dependent dilatancy and suction -induced relocation of critical state line is introduced, and a rigorous, exhaustive and schematic calibration procedure of the proposed model is presented. Thereafter, simulations of direct shear tests on sand -steel, sand-geotextile, silt -steel and soil-cement interfaces with different initial states, boundary conditions and suctions are carried out. Results show that, shear strengths of interfaces increase with initial density, normal stiffness and suction, and the strain softening and dilatancy behavior are significantly enhanced by initial density and suction, while normal stiffness makes a contrary contribution. More importantly, calculations of the proposed model are fairly consistent with measurements, indicating such features of strain softening, stateand suction -dependent dilatancy, as well as the improved peak and critical shear strengths, are well captured by the model.