Cosserat Elastoplastic Finite-Element Model Incorporating Structural Properties of Natural Soil and Case Study of the Saint-Alban Embankment

Natural soils often exhibit significant structural characteristics such as heterogeneity, anisotropy, and strain softening, which can severely affect soil strength and are crucial for the bearing capacity and stability issues in geotechnical engineering. However, due to the complexity of soil structural properties and the difficulties in achieving convergence when incorporating strain-softening effects into numerical simulations, currently few finite-element (FE) models could comprehensively account for soil structural characteristics while being suitable for practical engineering applications. As a new contribution, this article addresses this challenge by developing a new soil strength formula that logically couples the heterogeneity of shear strength with depth, stress-state-dependent anisotropy, and nonlinear strain softening associated with plastic deformation. For practical applications, an approximate expression is proposed for clay under undrained saturated conditions, allowing for the direct use of commonly employed engineering parameters, such as triaxial compressive strength and vane shear strength. Given the stability of the Cosserat continuum theory in numerical simulations of strain-softening soils, the proposed strength expression is incorporated into a Cosserat FE framework with a Mohr–Coulomb-matched Drucker–Prager yield criterion, resulting in the development of the Cosserat continuum heterogeneous, anisotropic, and strain-softening Drucker–Prager (CHAS-DP) plastic model. Subsequently, FE analyses are conducted on the Saint-Alban test embankment as a case study, explaining the determination method for each strength-related model parameter in practical engineering applications. The numerical results demonstrate that the CHAS-DP model accurately predicts critical fill heights and sliding surface locations at instability. Further, the model effectively captures the evolution of the mobilized undrained shear strength related to the structural effects of soil during the progressive instability process of the foundation caused by the embankment fill. The CHAS-DP model also addresses common issues, such as poor convergence and mesh dependency, typically encountered in traditional FE simulations of strain localization and soil strain softening, confirming its viability in practical engineering applications.