A tensor-type capillary stress, instead of a scalar suction, has been proposed to serve as a stress-like state variable to capture the effects of capillarity in the mechanics of unsaturated granular soils. However, the influence of water content on the evolution of capillary stress in such soils remains insufficiently understood. This study performs numerical simulations of unsaturated granular soils in the pendular regime using the Discrete Element Method (DEM) involving a volume-controlled capillary bridge model. In these simulations, water content is maintained constant by redistributing the water from ruptured capillary bridges to adjacent ones. The evolution of capillary stress with varying water contents during triaxial and biaxial loading conditions is systematically examined. The DEM simulation results show that, under both loading conditions, the mean component of the capillary stress generally decreases, while its deviatoricity gradually develops. These changes are observed to become less significant as the initial degree of saturation increases. At low saturation levels, capillary bridges between non-contacting particle pairs rupture due to soil deformations, and the water from these ruptured bridges redistributes to existing contacts. This redistribution leads to an anisotropic distribution of pore water aligned with the contact network. At higher saturation levels, non-contacting capillary bridges persist due to their ability to sustain large relative displacements between particles, allowing the spatial distribution of pore fluids to remain less constrained by the solid contact network. Additionally, at higher water contents, relative sliding and particle rearrangement are the primary factors influencing the directional distribution of capillary bridges.

The effects of confining pressure and particle breakage on the mechanical behavior of tailings were investigated using the discrete-element method to simulate conventional triaxial tests. The particle breakage was simulated using the octahedral shear stress breakage criterion and 14 Apollonian fragments replacement method. The macroscopic behavior of tailings revealed that the peak shear stress ratio is sensitive to confining pressure and the critical shear stress ratio is less sensitive to particle breakage. Confining pressure and particle breakage affect shear expansion, leading to changes in shear damage patterns. The quantitative study shows that particle breakage is the main factor influencing the nonlinear variation of the tailing strength. However, the influence proportion of particle breakage is gradually decreasing with the increase in the confining pressure. Microscopic analysis reveals a positive correlation between the overall anisotropy and the shear stress ratio, with the anisotropy of the normal contact force distribution contributing the most. The variation of the overall anisotropy is caused by the variation of the contact state, in which the sliding contact state is the main influencing factor.

Cumin (Cuminum cyminum L. cv.' Xin Ziran 1 '), classified within an agricultural crop, necessitates uprooting as a critical harvesting process. In this paper, we tried to study the force dynamics behind direct cumin uprooting by developing mechanical models for field uprooting and taproot-soil friction. A mechanical model for cumin uprooting and a friction model between the cumin taproot and sandy loam soil were built. The coefficient of static friction was determined using laboratory experiments. Pull-out, tensile force, and field uprooting experiments were conducted to validate the model. The physical and mechanical properties of the taproot were also measured. DEM simulation was employed for pull-out analysis. The static coefficient of friction between the cumin taproot and sandy loam soil was found to be approximately 0.766. The mechanical model showed high precision (0.4% and 5% error rates). Measured taproot properties included 80.91% moisture content, 0.40 Poisson's ratio, 15.95 MPa elastic modulus, 5.70 MPa shear modulus, and 3.49 MPa bending strength. A DEM simulation revealed agreement with experimental observations for maximum frictional resistance at pull-out. The minimum resistance was noted at the extraction angle of 60 degrees. The developed mechanical model for cumin uprooting was satisfactory in accuracy. Overcoming initial soil resistance is the primary factor affecting pull-out force magnitude. The optimized extraction angle had the potential to decrease uprooting resistance, improving harvesting efficiency.

Evaluating cyclic liquefaction of soil from the perspective of energy dissipation provides a more comprehensive insight into its liquefaction mechanism. This study conducted a series of undrained cyclic triaxial tests using discrete element method to investigate the influence of plastic fines content (FC) on the dynamic characteristics of sand-clay mixtures. A new evaluation index, the Viscous Energy Dissipation Ratio (VEDR), is introduced to assess the energy dissipation performance of sand-clay mixtures. Macroscopically, it is shown that when FC 30 %, the trend reverses. In terms of energy dissipation, as the fines content increases, VEDR gradually transitions from the sand-like to the clay-like mode, exhibiting a unique transitional mode when FC = 50 %. Microscopically, the development of bond breakage is highly similar to that of VEDR. The bond breakage facilitates particle sliding and rolling, which is the fundamental factor causing the differences of energy dissipation between pure sand and sand-clay mixtures. This paper contributes to the mechanistic study of liquefaction criteria based on energy theory by establishing the connection between microscopic particle behavior and macroscopic energy dissipation during the cyclic liquefaction process.

The depth of seed burial and impact damage are critical indicators of sowing quality in wheat accelerated seeding technology. To investigate the factors influencing seed burial depth and impact damage, a simulation model of wheat seed impact and soil penetration was developed using EDEM (2018) software, and the motion of wheat seed impact into soil was simulated and analyzed to identify the main influencing factors of wheat seed impact into soil. Seeding velocity, wheat seed equivalent diameter, and soil surface energy were selected as experimental factors, while burial depth and maximum impact force were chosen as response indicators. Both single-factor tests and three-factor, three-level orthogonal tests were conducted. Single-factor simulations showed that burial depth increased with seeding velocity and seed diameter, but decreased with soil surface energy. In contrast, maximum impact force increased with velocity and diameter, peaking at low soil surface energy before declining beyond a threshold. The orthogonal test results indicated that a maximum burial depth of 26.37 mm and a maximum impact force of 0.0704 N were achieved when the wheat seed diameter was 4 mm, the seeding velocity was 65 m/s, and the soil surface energy was 0.5 J/m2. Bench tests were conducted to validate the simulation results further. The results of the bench tests were consistent with the simulation results, with relative deviations of less than 5%, indicating the reliability of the simulation outcomes. This experimental study has provided data and a theoretical basis for the selection of technical parameters and the design and application of accelerated sowing technology for wheat.

Liquefaction and dynamic response of granular materials under dynamic loading has been studied intensively in field and laboratory tests. However, theoretical modeling and analytical solutions on liquefaction are still lagging and investigations are mostly restricted to laboratory observations. To investigate undrained liquefaction shear deformation and fluidity of granular material, the updated state evolution model is proposed by introducing an excess pore water pressure ratio parameter. A series of undrained cyclic triaxial tests and DEM simulations are conducted to verify the proposed model. The result indicates that the liquefaction behavior of granular materials can be captured by the updated state evolution model both at constant and varying loading frequency. Furthermore, the state parameter based on the deviatoric strain and excess pore water pressure ratio is determined to quantify assess the fluidity of granular materials. It facilitates the refinement of the discriminative criteria for cyclic liquefaction of granular materials. This parameter increases slowly at the beginning of loading, followed by a rapid and fluctuating rise, and reaches the peak before the initial liquefaction. Another significant finding is that the turning point of the state parameter range from 0.89 to 0.95 in the theta - t/t0 plane and between 0.84 and 0.94 in the theta - ruplane, as affected by the cyclic loading conditions.

The discrete element method (DEM) is used to simulate the behavior of a model sand under cyclic stress. Two approaches are employed in the contact model to account for the effect of anisotropic particle shape: (1) spheres with a rolling resistance moment and (2) clumps of spheres. Model parameters are calibrated using experimental results from drained monotonic triaxial tests on NE34 sand. Then, a series of cyclic triaxial tests is done on a homogeneous elementary volume sample with varying density index (ID\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$I_D$$\end{document}) and cyclic stress ratio (CSR). Both macroscopic and micromechanical characteristics of the material are examined under cyclic loads. In particular, the evolution of Young's modulus (E) and the damping ratio (D) with strain amplitude are evaluated at varying ID\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$I_D$$\end{document} and compared with values from the literature. An analysis of the coordination number (Z), orientation of strong and weak contact forces, friction mobilization, sliding contacts and fabric evolution links the observed macroscopic behavior of energy dissipation to the phenomenon of frictional sliding at the grain scale.

Pile-supported embankments are typically composed of soil-rock mixtures. within these structures, while the soil arching effect is crucial for effective load transfer, it remains incompletely understood, particularly when the impact of various loading conditions needs to be considered. This study investigates this problem using a 1 g physical experimental modeling approach. Subsequently, a DEM model for a full-scale pile-supported embankment of high-speed railways, accounting for multiple pile interactions, is established with proper model calibration. Numerical simulations are conducted to explore the load transfer mechanism and soil arching processes under self-weight, embankment preloading, and train-induced dynamics influences. The findings indicate that under self-weight, fully developed soil arching structures can be achieved with a sufficiently high embankment height, although they can diminish as the soil-pile relative displacement increases. However, during embankment preloading processes, represented by static loading, pressure can be transferred from pile caps to subsoil regions, potentially compromising the integrity of soil arching structures. Train-induced dynamics effects are modeled as cyclic loading inputs, revealing that an increase in loading frequency leads to weakened dynamic pressure fluctuation for both pile caps and subsoil regions, with a limited impact on the valley values of the pressures. Additionally, a higher loading frequency corresponds to smaller accumulated loading plate settlements.

The penetration of ballast in ballast track significantly affects subgrade performance. A unit specimen was designed with ballast on top and subgrade soil below. Laboratory dynamic triaxial tests and discrete element method (DEM) simulations were used to study the macroscopic deformation behavior and local deformation characteristics of crushed ballast penetration into soil subgrade under dynamic loads. The results indicate that, under train-induced dynamic loads, the ballast and subgrade soil only transmit stress through a limited number of discrete contacts at the interface. As the dynamic stress amplitude increases, the depth of ballast penetration into subgrade soil also increases, exhibiting an exponential relationship with the dynamic stress. The deformation process of the ballast penetration specimens can be divided into three stages: localized compression, shear band formation, and shear band development. Under train-induced loads, ballast penetration significantly increases the porosity of soil samples near the ballast-subgrade interface, and causes significant lateral deformation at the contact interface. Saturated specimens with higher porosity can experience mud pumping under relatively low dynamic stress. The increase in subgrade surface porosity caused by ballast penetration is a significant factor contributing to mud pumping in existing railways. Prevention of mud pumping should focus on preventing the local increase in subgrade porosity caused by ballast penetration. The findings deepen our understanding of the ballast penetration phenomenon and the resulting deformation behavior of the subgrade surface.

Traditional methods for harvesting medicinal materials with long roots, like Astragalus membranaceus, require extensive soil excavation, leading to problems like inefficient soil separation, low stemming rates, and blockages in conveyor chains. To address these challenges, this study introduces a prototype machine capable of digging, separating soil, crushing soil, and collecting the medicinal materials in one continuous process. The paper focuses on the machine's design and working principle, with theoretical analysis and calculations for key components like the digging shovel, multi-stage conveyor, and soil-crushing device. Specific structural parameters were determined, and the screening efficiency of the roller screen was analyzed using EDEM 2020 software, comparing scenarios with and without rollers. A motion model for the medicinal materials during conveyance was established, allowing for the determination of optimal linear velocity and mounting angle for the conveyor. Additionally, a motion model for the second-stage conveyor chain and rear soil-crushing device was used to optimize their placement, ensuring efficient soil crushing without affecting the thrown Astragalus. Compared to traditional Chinese medicine diggers, this machine boasts superior resistance reduction and soil-crushing capabilities. Compared with traditional harvesters, the drag-reducing and soil-crushing device of this machine is more efficient, reducing the damage to Astragalus during the harvesting process, reducing the labor intensity of farmers, and improving the quality and efficiency of Astragalus harvesting. Field experiments have shown that when the operating speed of the prototype is 1.0 m/s and the roller-screen speed is 130 similar to 150 rpm, the operating performance is optimal, and comparative experiments can be conducted under the optimal parameters. From the experimental results, it can be seen that the improved equipment has increased the bright-stem rate by about 4%, the digging and loosening rate by 97.42%, and the damage rate by 2.44%. The equipment design meets the overall design requirements, and all experimental indicators meet national and industry standards. This provides a reference for the optimization and improvement of the soil-crushing device and the structure of the Astragalus membranaceus harvester.