Particle Shape-Driven Stiffness Anisotropy in Calcareous Sand and the Underlying Mechanism

The angular shape and breakage of particles for calcareous sand significantly influence its mechanical behavior and the safety of the engineering. Although previous studies have explored the impact of particle shape on the mechanical properties of calcareous sand, the effects of shape-induced stiffness anisotropy and particle breakage remain insufficiently investigated. This study employs the Yade open-source 3D discrete element platform to conduct a series of numerical simulations of isotropic compression and simple shear tests on calcareous sand, examining stiffness, deformation characteristics, microscopic behavior, anisotropic properties, and the influence of different particle breakage rates. The results reveal that particle shape-driven stiffness anisotropy in calcareous sand is obvious. The horizontal shear modulus is different from the vertical modulus by up to 15% under confining pressures of 50 kPa to 1200 kPa. Irregularly shaped particles tend to align in a layered fabric under gravitational deposition, resulting in spatial anisotropy in the distribution of contact normals. Strong contact forces concentrate in the direction of gravitational deposition (i.e., the vertical direction), leading to significant anisotropy in shear modulus, with the horizontal shear modulus being notably greater than the vertical one. The values of horizontal shear modulus ranging from 40 MPa for chunky particles to 120 MPa under high confining pressure. While increasing confining pressure generally enhances the shear modulus of calcareous sand, the concentration of strong contact forces in the vertical direction due to particle shape causes differential increments in shear modulus across directions, thereby altering anisotropy. Particle breakage under high confining pressure (10%) disrupts the concentration of strong contact forces in the vertical direction and triggers a “surrounding particle compensation” mechanism (accounting for >95% of cases), leading to homogenization of contact force distribution. This significantly reduces the shear modulus and diminishes the degree of anisotropy by up to 50% at breakage rates of 10%. The cross-scale relationship between particle morphology, breakage, and fabric evolution is quantified.