Constitutive model for rammed earth materials based on a modified Mohr–Coulomb criterion

• Smooth MMC yield model for shear and tensile behavior (C2 & Wilkins smoothing). • ABAQUS UMAT (CPPM algorithm) enabling robust simulation of rammed earth behavior. • Captures moisture content effects on UCS and UTS of rammed earth. • Validated by triaxial and UCS/UTS tests on Pingyao rammed earth. • Supports stability assessment of earthen heritage sites (e.g., Pingyao). Rammed earth is a key construction material in earthen heritage structures, and its mechanical behavior directly governs the long-term stability of these sites. The classical Mohr–Coulomb (MC) yield criterion is widely used in numerical analyses of rammed earth structures, but the singularities on the yield surface lead to poor numerical convergence, and it systematically overestimates the tensile strength of the material. To address these issues, this study adopts second-derivative continuous smoothing scheme to round the hexagonal corners and Wilkins’ piecewise smoothing method to smoothly merge the shear yield surface with the tensile yield surface, thereby defining a globally smooth and convex composite yield function and constructing a modified Mohr–Coulomb (MMC) model that accounts for tensile yielding. A user-defined material subroutine is implemented on the basis of the closest point projection method. The computational accuracy and convergence performance of the MMC-based subroutine are verified through conventional triaxial compression and triaxial extension examples, and further through numerical simulations of the unconfined compressive strength (UCS) and uniaxial tensile strength (UTS) of rammed earth from the Pingyao ancient city at different water contents. The results show that the MMC criterion can markedly reduce the overestimation of tensile strength inherent in the traditional MC criterion while maintaining computational stability, and can reasonably capture the effects of water content on the UCS, UTS and failure patterns of rammed earth, thereby providing a reliable constitutive model for numerical evaluation of the stability of earthen heritage sites.