Soil-rock mixtures with large particle size variations are often used as fill materials for expressway construction in mountainous areas. Conventional testing methods do not enable fast and nondestructive monitoring of real-time changes in the compaction quality of soil-rock filled subgrades. Selecting an appropriate evaluation method is the key to controlling the compaction quality of a soil-rock filled subgrade. In this study, three-dimensional DEM models of subgrade materials were reconstructed by a spherical harmonic series whose harmonization degree was fixed at 15. The macroscopic and mesoscopic behaviours and characteristics of the subgrade under vibratory rolling were analysed. The results showed that the porosity, contact force and coordination number of the subgrades tended to be stable in the last two passes. The subgrades with 4 filler combinations presented the similar mechanical anisotropy and meso-mechanical states. On-site monitoring of subgrades under vibratory rolling and settlement after construction was performed, and the results were considered. An evaluation method and criterion to control the compaction quality of the SRM subgrade was proposed, i.e., whether the average value of the vibration compaction value from the second to last pass differed by more than 2% from the average value in the last pass.

The shape of particles significantly influences their mechanical properties, making accurate shape modeling crucial in numerical simulations. This paper proposes a framework for generating particles by applying improved spherical harmonic reconstructions to convex hull surfaces. The framework integrates mesh refinement tech- niques to enhance mesh resolution, enabling the generation of finer surface details than 3D laser scanning. Three parameters are introduced: Delta K1, which controls roundness; Delta K2, which governs roughness; and Rd, which represents the boundary between roundness and roughness in spherical harmonic reconstructions. Introducing these parameters not only allows independent control over the three levels of shape (form, roundness, and roughness) but also enhances the flexibility of the method, enabling the generation of various particle shapes. Granular assemblies with varying roundness and roughness distributions are generated and applied in discrete element method (DEM) simulations of triaxial shear. The results show that roundness is negatively correlated with the peak friction angle, while roughness is positively correlated. The proposed method enhances the ability to generate complex particle shapes, offering a practical tool for modeling and simulating granular materials.

This paper investigates the effects of particle morphology (PM) and particle size distribution (PSD) on the micro-macro mechanical behaviours of granular soils through a novel X-ray micro-computed tomography (mu CT)-based discrete element method (DEM) technique. This technique contains the grain-scale property extraction by the X-ray mu CT, DEM parameter calibration by the one-to-one mapping technique, and the massive derivative DEM simulations. In total, 25 DEM samples were generated with a consideration of six PSDs and four PMs. The effects of PSD and PM on the micro-macro mechanical behaviours were carefully investigated, and the coupled effects were highlighted. It is found that (a) PM plays a significant role in the micro-macro mechanical responses of granular soils under triaxial shear; (b) the PSD uniformity can enhance the particle morphology effect in dictating the peak deviatoric stress, maximum volumetric strain, contact-based coordination number, fabric evolution, and shear band formation, while showing limited influences in the maximum dilation angle and particle-based coordination number; (c) with the same PSD uniformity and PM degree, the mean particle volume shows minimal effects on the macro-micro mechanical behaviours of granular soils as well as the particle morphology effects.

This study proposes a resolved framework coupling computational fluid dynamics (CFD) with discrete element method (DEM) to simulate the sedimentation of granular sand. Realistic sand particles were reconstructed by spherical harmonic representation combined with the multi-sphere clump method, and a fictitious domain method for irregular clumps was further developed to solve the fluid-particle interaction. This resolved CFD-DEM offers a direct and robust approach for computing real fluid forces on irregular-shaped granular sands, without relying on any empirical drag force models. Initially, the effectiveness and accuracy of the proposed CFD-DEM were validated through a series of single-particle free settling simulations for various ideal-shaped particles. Critical fluid-particle interacting behaviors in terms of drag force and wake structure were mainly investigated and corroborated with experimental data. The study subsequently progressed to simulate the sedimentation processes of various granular sand assemblies composed of realistic-shaped sand particles, utilizing the proposed CFD-DEM. Detailed numerical analyses concentrated on particle-scale mechanics during sedimentation, including settling trajectories and velocities of particles, as well as the coordination and anisotropy of inter-particle contacts. The results and findings gained from this study provide a novel insight into the micro-mechanisms underlying the sedimentation and accumulation process of granular soils in geological environments.

Spatial distribution orientations of blocks can cause significant errors in the discrete element model (DEM) calculation of soil-rock mixture (SRM). To avoid this error, spherical harmonic (SH) series whose harmonization degrees fixed at 15 were proposed for block reconstruction. This research refers to the case-history of a deep excavation rift valley spanning from the Mabian to Zhaojue of the Leshan-Xichang Expressway, mainly containing moderately-weathered silty mudstone, in the Leshan City, Sichuan Province, China. The appropriate degree of finite-term SH series is selected by the volume, surface area. 100 blocks were scanned on site, and sphericity and angularity of the blocks were calculated. The sphericity and angularity of 50 reconstructed blocks were considered for the error analysis of SH method. Moreover, stochastic polyhedron method was considered for comparing different block reconstructions. The maximum block placement angle was defined to control the spatial distribution orientations of the blocks. Large scale direct tests were carried. Numerical simulations of large-scale direct shear tests were conducted to study the influence of the spatial distribution orientations of the blocks on the mechanical properties of the SRMs. The results revealed that the finite-term SH series fixed at 15 accurately reflected the shape characteristics and mechanical behaviors of actual blocks. The spatial distribution orientations of the blocks had a minimal impact on the friction angle and cohesion of SRM constructed through the SH method. The SRMs developed via the SH method exhibited marginal variations in contact force and anisotropy index of contact across diverse block placement strategies. The evolution of coordination number was closer when employing the SH method under varied block placement methods. Blocks reconstructed by the SH method, could mitigate errors in DEM calculation caused by the spatial distribution orientations of the blocks.