This study presents a hierarchical multiscale approach that combines the finite-element method (FEM) and the discrete-element method (DEM) to investigate tunneling-induced ground responses in coarse-grained soils. The approach considers both particle-scale physical characteristics and engineering-scale boundary value problems (BVPs) simultaneously, accurately reproducing typical tunneling-induced mechanical responses in coarsegrained soils, including soil arching and ground movement characteristics observed in laboratory tests and engineering practice. The study also unveils particle-scale mechanisms responsible for the evolution of soil arching through the underlying DEM-based RVEs. The results show that the rearrangement of microstructures and the deflection of strong contact force chains drive the rotation of macroscopic principal stress and the formation of soil arch. The microscopic fabric anisotropy direction can serve as a quantitative indicator for characterizing soil arching zones. Moreover, the effects of particle size distributions (PSD) and soil densities on ground deformation patterns are interpreted based on the stress-strain responses and contact network characteristics of DEM RVEs. These multiscale insights enrich the knowledge of tunneling-induced ground responses and the same approach can be applied to other geotechnical engineering analyses in coarse-grained soils.

Frozen mixed soils are widely distributed in the strata and slopes of permafrost regions. This paper aims to study the strength criterion and elastoplastic constitutive model for frozen mixed soils from micro to macro scales. Based on the knowledge of mathematical set theory and limit analysis theory, the support function of frozen soils matrix is derived. The concept of local equivalent strain is proposed to solve the problem of nonuniform deformation caused by rigid inclusions in frozen mixed soils. According to the nonlinear homogenization theory and the Mori-Tanaka method in micromechanics, the strength criterion of frozen mixed soils is established, which can consider coarse particle contents. By introducing the concepts of equivalent yield stress and equivalent plastic deformation, the elastoplastic constitutive model is proposed by the associated flow rule, which can also consider the influence of coarse particle contents. Finally, using the data in the literature, the proposed strength criterion and elastoplastic constitutive model for frozen mixed soil are verified, respectively. The effects of coarse particle contents on the mechanical properties of frozen mixed soils are discussed.

This paper investigates the mechanical response of coral sand under particle breakage using a hierarchical multiscale model combining the discrete element method (DEM) and the finite element method (FEM). This DEM-FEM model links the microscopic interaction mechanisms to macroscopic phenomena such as strain localization and failure. A cohesive contact model was first utilized to simulate compaction bands in the DEM and construct a cohesive assembly with smaller particles distributed around a larger particle to better simulate the grinding and angular breakage of coral sand. A representative volume element (RVE) that includes particle breakage was then constructed and analyzed under periodic boundary conditions. DEM analysis was performed, and the results were compared with triaxial compression test data obtained from the literature, demonstrating that the constructed RVE effectively represents the mechanical properties of coral sand. The constructed RVE was used for hierarchical multiscale simulations, which showed good agreement with existing triaxial testing of coral sand. Finally, by setting a larger cohesive force, the constructed coral sand particles were prevented from breakage, and comparative analysis revealed that particle breakage weakens the mechanical properties of coral sand. Furthermore, different shapes of coral sand particles were constructed, and RVE and hierarchical multiscale simulations of triaxial tests were performed. The results indicated that the triaxial tests of long strip-shaped coral sand particles exhibit higher peak values compared to spherical coral sand particles. Additionally, a double porosity model of coral sand was constructed to analyze the impact of internal porosity on soil mechanical properties. The results showed that the presence of internal porosity significantly weakened the mechanical properties of coral sand. These findings highlight the significant impact of particle breakage and shape on the mechanical behavior of coral sand, offering important insights for engineering applications.

We present a novel multiscale framework that integrates the single -point multiphase material point method (MPM) and the discrete element method (DEM) to model the complex freeze -thaw behavior of ice -bonded granular media. The proposed numerical framework is featured by (a) employing the continuum -based MPM to solve the macroscopic governing equations for granular systems involving thermo-hydro-mechanical (THM) coupling and phase transitions, and (b) using the grain -scale discontinuum-based DEM to capture the thermodynamically sensitive mechanical behaviors of ice -bonded granular media. The multiscale framework is constructed by attaching a DEM-based representative volume element (RVE) at each material point in MPM. This RVE serves as a live sample of each material point to track the state -dependent effective stress with respect to the local deformation and thermodynamic conditions like ice saturation, bridging the macroscopic phenomena and the underlying microstructural evolution. In particular, we implement a semiimplicit staggered integration scheme for the macroscale THM-coupled MPM to boost computational efficiency and enhance numerical stability. We also propose an innovative ice saturation -dependent bond contact to effectively reproduce the thermodynamically sensitive mechanical behaviors. The new multiscale framework is first benchmarked against analytical solutions for 1D non -isothermal consolidation problems. We then demonstrate its exceptional capability in simulating intricate freeze -thaw behavior of granular media through a boundary value problem involving cyclic freeze -thaw actions. Further cross -scale analyses reveal its potential in capturing key loading- and state -dependent THM responses with explainable microstructural mechanisms during complex freezing and thawing loading conditions.