Load reduction on high-filled cut-and-cover tunnel using discrete element method

SUMMARYUnderstanding the extent to which discrete element method (DEM) simulations can capture the critical state characteristics of granular materials is important to legitimize the use of DEM in geomechanics. This paper documents a DEM study that considered the sensitivity of the critical state response characteristics to the coefficient of interparticle friction (μ) using samples with gradings that are representative of a real soil. Most of the features that are typically associated with sand behaviour at the critical state were seen to emerge from the DEM simulation data. An important deviation occurs when high μ values (μ ≥ 0.5) are used, as has been the case in a number of prior DEM studies. While there is a systematic variation in the critical state behaviour with μ for μ < 0.5, when μ ≥ 0.5, the behaviour at the critical state seems to be insensitive to further increases in μ. In contrast to observations of conventional soil response, when μ ≥ 0.5, the void ratio at the critical state initially increases with increasing mean effective stress (p′). Analysis of the DEM data and use of simple models of isolated force chains enabled some key observations. When ‘floating’ particles that do not transmit stress are eliminated from the void ratio calculation, the void ratio at the critical state decreases consistently with increasing p′. There is a transition from sliding to rolling behaviour at the contact points as μ increases. Beyond a limiting value of μ, further increases in μ do not increase the buckling resistance of individual strong force chains. Copyright © 2014 John Wiley & Sons, Ltd.

A DEM study on microstructural behaviour of soluble granular materials subjected to chemo-mechanical loading

Coupling effects of interfacial friction and layer thickness on soil desiccation cracking behavior

Experimental and DEM studies on the distribution of voidage in the random filling bed of ellipsoidal particles

DEM study on effect of particle roundness on biaxial shearing of sand

This paper presents the discrete element method (DEM) simulations on the instability behaviour of granular materials during constant shear drained condition (CSD). CSD condition was implemented by decreasing mean effective stress on an assembly of particles under strain controlled loading. In this study, the instability condition was predicted at the particle scale level using particle second order work increment (Nicot et al. in Int J Solids Struct 49(10):1252–1258, 2012). The DEM contact parameters have been calibrated to capture the macroscopic responses and the instability behaviour consistently with the laboratory experiments. The effect of different range of initial states at the beginning of CSD condition such as different initial mean effective stress ( $$p_{0}^{^{\prime}}$$ ), void ratio ( $$e_{0}$$ ) and deviatoric stress ( $$q$$ ) on the instability behaviour were analysed. In addition, the micromechanical parameters such as coordination number, anisotropic coefficients (geometric, mechanical) have been extracted to assist in characterising the instability behaviour during CSD condition. The initial stress state of the soil (i.e. at the onset of CSD) has shown a significant influence on the evolution of anisotropic coefficients. An increase in geometric anisotropy and a decrease in mechanical anisotropy with time was observed after the instability.

Effects of particle shape, physical properties and particle size distribution on the small-strain stiffness of granular materials: A DEM study

Experimental and DEM studies on the particle mixing performance in rotating drums: Effect of area ratio

Bioinspired lightweight Primitive and Gyroid structures are two well-known TPMS (triply periodic minimal surface) cellular structures. These structures have been shown to possess high specific strength and energy absorption capacity, and thus hold great promise for a range of prefabricated civil engineering applications. As a result, concrete cellular TPMS structures have garnered the attention of researchers. However, prior to this study, no investigation has been carried out on the mechanical properties and failure patterns of concrete Primitive and Gyroid structures under coupled compressive and torsional loads. This lack of knowledge on the behaviour of these structures can lead to safety concerns in construction projects. To better understand the mechanical behaviour of Primitive and Gyroid structures under combined compression and torsion, the discrete element method (DEM) is adopted to simulate the TPMS structures. Both linear contact and nonlinear softbond contact models are utilized to simulate the brittle mechanical behaviour of concrete material. After validating the DEM parameters using published experimental data, the DEM models are subjected to coupled compressive and torsional loads to study their compressive and torsional bearing capacity and cracking patterns. The results indicate that the Primitive based structures outperform the Gyroid based structures in terms of both compressive and torsional resistance. The study is the world's first to reveal that a compressive load enhances the ultimate torsional bearing capacity of TPMS structures, but a torsional load reduces their compressive bearing capacity. Additionally, the loading conditions have little impact on the cracking patterns of the four TPMS structures.

Constitutive modeling in granular materials has historically been based on macroscopic experimental observations that, while being usually effective at predicting the bulk behavior of these type of materials, suffer important limitations when it comes to understanding the physics behind grain-to-grain interactions that induce the material to macroscopically behave in a given way when subjected to certain boundary conditions. The advent of the discrete element method (DEM) in the late 1970s helped scientists and engineers to gain a deeper insight into some of the most fundamental mechanisms furnishing the grain scale. However, one of the most critical limitations of classical DEM schemes has been their inability to account for complex grain morphologies. Instead, simplified geometries such as discs, spheres, and polyhedra have typically been used. Fortunately, in the last fifteen years, there has been an increasing development of new computational as well as experimental techniques, such as non-uniform rational basis splines (NURBS) and 3D X-ray Computed Tomography (3DXRCT), which are contributing to create new tools that enable the inclusion of complex grain morphologies into DEM schemes. Yet, as the scientific community is still developing these new tools, there is still a gap in thoroughly understanding the physical relations connecting grain and continuum scales as well as in the development of discrete techniques that can predict the emergent behavior of granular materials without resorting to phenomenology, but rather can directly unravel the micro-mechanical origin of macroscopic behavior. In order to contribute towards closing the aforementioned gap, we have developed a micro-mechanical analysis of macroscopic peak strength, critical state, and residual strength in two-dimensional non-cohesive granular media, where typical continuum constitutive quantities such as frictional strength and dilation angle are explicitly related to their corresponding grain-scale counterparts (e.g., inter-particle contact forces, fabric, particle displacements, and velocities), providing an across-the-scale basis for better understanding and modeling granular media. In the same way, we utilize a new DEM scheme (LS-DEM) that takes advantage of a mathematical technique called level set (LS) to enable the inclusion of real grain shapes into a classical discrete element method. After calibrating LS-DEM with respect to real experimental results, we exploit part of its potential to study the dependency of critical state (CS) parameters such as the critical state line (CSL) slope, CSL intercept, and CS friction angle on the grain's morphology, i.e., sphericity, roundness, and regularity. Finally, we introduce a first computational algorithm to ``clone'' the grain morphologies of a sample of real digital grains. This cloning algorithm allows us to generate an arbitrary number of cloned grains that satisfy the same morphological features (e.g., roundness and aspect ratio) displayed by their real parents and can be included into a DEM simulation of a given mechanical phenomenon. In turn, this will help with the development of discrete techniques that can directly predict the engineering scale behavior of granular media without resorting to phenomenology.

Piles are critical structural elements that often face lateral loads from various sources, including moving vehicles on bridges, wind, waves, slope movement, and seismic activities. These lateral loads can cause substantial bending moments and lateral deflections in piles, which can compromise the structural integrity of the foundation. The point of fixity is a widely used concept in the design of laterally loaded piles, providing the necessary rigidity to withstand these forces and minimize deflections. While several analytical methods have been developed to determine the point of fixity in single-layered soils, their effectiveness and applicability in multilayered soils remain less certain. This study investigates the validity of two commonly used analytical methods, Broms method and Kocsis method for determining the point of fixity in multilayered soils. The depth of fixity was estimated using average soil properties and compared against results obtained from more advanced approaches, including finite element modelling and p-y curve analysis, which consider distinct properties of individual soil layers. A comprehensive parametric study was also conducted to examine the influence of various factors, such as soil type, soil layer thickness, pile diameter, magnitude of lateral and axial loads, and pile embedment length, on the point of fixity. The comparison revealed significant differences between the analytical methods, particularly in the context of predominantly cohesive soils. The Broms and Kocsis methods estimated the depth of fixity to be between 1.0 and 2.0 times the pile diameter below the ground surface, while the p-y curve and FEA methods yielded slightly more conservative values, ranging from 1.0 to 1.5 times the pile diameter. In predominantly cohesionless soils, the estimated fixity depths were more consistent across all methods, varying between 1.0 and 1.5 times the pile diameter below the ground surface. These findings highlight potential limitations in the applicability of traditional analytical methods to multilayered soils and underscore the importance of refining these approaches for more accurate and reliable design. The parametric study further revealed that for long (flexible) piles, the depth of fixity is generally not significantly affected by factors such as surrounding soil type, layer thickness, axial load, and pile length. However, pile diameter and lateral load were found to have a substantial impact on the depth of fixity, with the effects varying depending on the analytical method employed and the soil's cohesive or cohesionless nature. Interestingly, the study also found that, even in single-layered soils, the Broms and Kocsis methods could yield results that significantly deviate from those obtained via p-y curve analysis. This discrepancy underscores the necessity for further detailed studies, including experimental work with instrumented pile tests, to enhance the accuracy and reliability of these methods. Overall, the study suggests that the point of fixity in predominantly cohesive soils ranges from 0.5 to 2.5 times the pile diameter below the ground surface, whereas in predominantly cohesionless soils, it ranges from 0.5 to 2.0 times the pile diameter.

A framework for evaluating deformation-based basal heave stability is proposed in order to distinguish between the different responses under freely developed and prohibited basal heave failures. In the case of freely developed basal heave failure, the maximum deformation values occur at the center point of pit bottom, whereas this is not the case for the prohibited basal heave failure. The critical thickness of soft soil layer between the end of supporting structures and the top of hard stratum is about 0.3B (B = excavation width), beyond which the freely developed basal heave failure arises. In situations otherwise, the prohibited basal heave failure occurs. The failure probability of basal heave failure at the center point increases significantly as B ranges within a limited value; then, it begins to decrease or to vary slightly at a certain value under a given thickness of soft soil layer. If the thickness of soft soil layer is so sufficiently large that freely developed basal heave failure occurs for any of B, the failure probability of basal heave failure at the center point increases as B increases. The selection of the optimum monitoring points for basal heave stability is recommended to account for the weights in the contribution to the basal heave deformations of the influencing factors such as excavation width and thickness of soft soil layer. The proposed framework is applicable to basal heave reliability analysis for braced excavations where deformation values are focused.

Influence of particle shape on liner wear in tumbling mills: A DEM study

On Geldart Group A behaviour in fluidized beds with and without cohesive interparticle forces: A DEM study

This study investigates the microscale mechanisms underlying the compressibility of biochar-amended soils through combined discrete element method (DEM) simulations and laboratory consolidation tests. A three-dimensional discrete element model was established based on the MatDEM platform, accounting for the particle crushing process of biochar particles and its impact on soil mechanical properties. The biochar agglomerate particles generated in the simulation exhibit irregular morphology, and particles within different size ranges were selected for investigation. According to the model and experimental results, the average relative error is about 7%. Results demonstrate that moderate biochar content effectively reduces soil compressibility by enhancing load transfer through stable force chains formed by biochar particles, which exhibit larger contact areas and higher stiffness compared to native soil particles. However, when the biochar content exceeds approximately 40%, particle crushing intensifies, particularly under high initial void ratios, leading to increased soil compressibility. Furthermore, a larger initial void ratio weakens interparticle confinement, promotes microcrack propagation, and thereby exacerbates compressive deformation. Biochar fragmentation progresses through three stress-dependent stages: initial compaction (<100 kPa), skeletal damage (100–800 kPa), and crushing saturation (>800 kPa). Increased biochar particle size correlates with higher fragmentation rates, refined particle gradation, and reduced coordination numbers, collectively weakening the soil skeleton and promoting deformation. These findings underscore the importance of optimizing biochar content and applying graded loading strategies to balance enhanced soil performance with material integrity. These findings emphasize the necessity of optimizing biochar application rates to balance enhanced soil performance with resource efficiency, providing critical insights for sustainable geotechnical practices.