The breakage phenomenon has gained attention from geotechnical and mining engineers primarily due to its pivotal influence on the mechanical response of granular soils. Numerous researchers performed laboratory tests on crushable soils and incorporated the corresponding effects into numerical simulations. A systematic review of various studies is crucial for gaining insight into the current state of knowledge and for illuminating the required developments for upcoming research projects. The current state-of-the-art study summarizes both experimental evidence and numerical approaches, particularly focusing on discrete element simulations and constitutive models used to describe the behavior of crushable soils. The review begins by exploring particle breakage quantification, delving into experimental evidence to elucidate its influence on the mechanical behavior of granular soils, and examining the factors that affect the breakage phenomenon. In this context, the accuracy of various indices in estimating the extent of breakage has been assessed through ten series of experiments conducted on different crushable soils. Furthermore, alternative breakage indices are suggested for constitutive models to track the evolution of particle crushing under continuous shearing. Regarding numerical modeling, the review covers different approaches using the discrete element method (DEM) for simulating the behavior of crushable particulate media, discussing the advantages and disadvantages of each approach. Additionally, different families of constitutive models, including elastoplasticity, hypoplasticity, and thermodynamically based approaches, are analyzed. The performance of one model from each group is evaluated in simulating the response of Tacheng rockfill material under drained triaxial tests. Finally, new insights into the development of constitutive models and areas requiring further investigation utilizing DEM have been highlighted.

This study investigates the simultaneous influence of particle shape and initial suction on the hydromechanical behavior of unsaturated sandy soils. Anisotropic loading-unloading tests at constant water content conditions were conducted on three sands with distinct shapes (Firoozkooh-most angular, Babolsar-Subangular, and Mesr-roundest) using a direct shear apparatus. Particle shapes were quantified in terms of sphericity, roundness, and regularity using the results of scanning electron microscopy (SEM) tests. In addition, a coupled hydromechanical model based on elasto-viscoplasticity was developed and validated against the experimental results first. The model was then employed to conduct a parametric study (compressibility, pore water pressure, and permeability) with an emphasis on the role of particle morphology and shape. The findings revealed rounder particles (higher regularity) experienced higher volumetric strain (epsilon v) under lower suction but less epsilon vwith increasing suction compared to angular sands. Moreover, the rate of permeability reduction during loading in Mesr sand was 1.5 times and 2.4 times higher than that of Babolsar and Firoozkooh sands at near-saturation condition. However, this amount decreased with increasing suction. Pore water pressure (PWP) generation was highest in the most angular sand due to its retention characteristics. The relationship between void ratio and PWP was independent of loading cycles and exhibited a linear dependence. Particle shape significantly impacted this relationship, with rounder sands showing a higher rate of void ratio change per unit change in PWP.

The weakening of shear strength in granular soils under various vibrations is a common phenomenon, though its underlying mechanisms remain unclear. In this study, the macroscopic and microscopic shear behaviors of granular soils under vibration are investigated using the discrete element method (DEM). Specifically, the effects of vibrational acceleration a, vibrational frequency f and confining pressure sigma(n) on the dynamic mechanical properties of granular soils are examined. Dense and loose specimens composed of spherical particles are subjected to triaxial compression tests until the critical state is reached. At the macroscopic level, the results show that the degree of shear strength weakening and reduction in void fraction exhibit an approximately linear relationship under different vibration conditions. On the microscopic scale, anisotropic analysis sheds light on the mechanisms behind shear strength weakening from two perspectives. First, the weakening is primarily driven by reductions in the contact normal anisotropy a(n), normal contact force anisotropy a(c), and tangential contact force anisotropy a(t), with their contribution follows a(n) > a(c) approximate to a(t). Second, a linear relationship is observed between the stress ratio q/p' and contact normal anisotropy within the strong and non-sliding contacts a(c)(sn) during vibration phase (i.e., q/p' = 0.62a(c)(sn) ). Thus, the decrease of shear strength due to vibration is fundamentally linked to the reduction of a(c)(sn) .

The physical and mechanical properties of granular soils are strongly related to the overlying stresses to which they are subjected. In particular, during the engineering construction phase, which involves activities like foundation stacking and building construction, the applied loads on the soil increase continuously over time. Unfortunately, current stress-controlled compression geotechnical tests have not adequately considered this situation. Therefore, this study aims to examine the effects of various factors, including void ratio, confining stress, stress loading rate, and particle shape, on both macroscopic shear properties and microscopic characteristics of granular soils under conditions of increasing axial stress in biaxial compression numerical simulations. The results show that: (1) In stress-controlled tests on granular soils, samples exhibit three different shear behaviors as the void ratio varies; (2) the confining stress and particle shape will change the magnitude of the deviatoric stresses and axial strains in the peak state of the sample, but not their trends; (3) the stress loading rate does not affect the strength of the samples. Therefore, the loading rate can be increased appropriately to improve the computational efficiency of the numerical model. These findings will enhance understanding of the time-dependent behavior of granular soils and provide valuable insights for engineering applications, particularly in soil mechanics, foundation treatment, and slope stability.

The progressive accumulation of secondary deformation, occurring incrementally under lowamplitude, high-cycle loading in soils, can lead to significant displacement of foundations. This study has developed a novel phenomenological model to describe the shakedown accumulation behavior of secondary deformation in granular soils subjected to low-amplitude, high-cycle loading. Firstly, gradual densification of granular packing yields an average volume strain that obeys a logarithmic law as the cyclic loading persists. A log-hyperbolic function, constrained by a limit, is reasonable, considering that the strain will reach a steady state of finite value as the cycle number approaches infinity. Secondly, cyclic loadings with average stress induce the accumulation of strain in the direction of average stress as the cycle number increases. This has been incorporated into the well-known modified Cam-clay model. Lastly, the proposed model has been calibrated using data obtained from undrained and drained cyclic triaxial tests conducted on uniformly fine-grained sands. The results suggest that the model effectively exhibits important features of the accumulation of both volumetric and deviatoric deformation induced by drained cyclic loading over a large number of cycles.

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.

Fujian River sand (FJS) is a complex mixture of minerals and rock fragments shaped by the dynamic geological history of Fujian province, China. The macro-micro mechanical responses of FJS under triaxial shear were carefully investigated through the X-ray tomography-based in situ triaxial test. By utilising the particle tracking strategy with the signature of histograms of orientation, both intact and crushed FJS particles can be successfully recognised and tracked at different stages of axial strain. It is found that (a) smaller particles are more likely to crush than larger ones, and the crushed particles have more irregular particle shapes than the original set of particles; (b) the coordination number, fabric anisotropy, 3D rose map, and particle displacement are found to highly correlate to the phase transition point from volumetric contraction to dilation; (c) the sample deformation is found to be uniform at the early stage, and then it starts to spread from the boundaries to the inner part and finally develops into an inclined shear band; (d) locations of particle breakage within the granular assemblage show an overall sporadic and irregular pattern throughout the shearing process, which is not strongly correlated with the shear band that has developed, even at large strains.

The fabric anisotropy in granular soils is a very important character in soil mechanics that may directly affect many geotechnical engineering properties. The principal objective of this study is to develop an efficient approach for assessing the degree of fabric anisotropy as a function of grading, particles shape and weighting specifications. By assuming cross-anisotropy, the anisotropic shear stiffness values of 1042 implemented tests on 200 various sandy and gravelly soil specimens from 43 different soil types were collected from the literature. Those were combined with their corresponding void ratios, stress conditions, grading parameters, particles shape and weighting attributes to generate a global database of anisotropic shear moduli in terms of testing conditions. The magnitudes of fabric anisotropy ratio were obtained using a well-known empirical equation, and they were plotted against the relevant soil grading and particles information to examine the dependency level of this ratio to the particularities. A series of multiple regression analyses were carried out to develop a global correlation for evaluating fabric anisotropy ratio in granular soils from grading, particles shape and weighting characteristic. The results showed that reliable quantities of fabric anisotropy ratio can be estimated using the surface appearance soil specifications. The findings may serve as an appropriate technique to yield good approximations for fabric and shear stiffness anisotropies using soil grading and particle properties.

Coarse-grained granular soils are critical construction and building materials. The particle shape of granular soils significantly affects their macro mechanical properties. The particle shape can be quantified by sphericity, roundness, and roughness at three different scales. At each scale, researchers have proposed various descriptors for shape characterizations. Before the advent of computers and digital cameras, the two-dimensional (2D) maximum particle projections were utilized to determine particle shape descriptors through either manual measurements or visual comparisons. In the last three decades, image-based techniques have been utilized to automate particle shape analysis. However, there is a lack of comprehensive review of existing particle shape definitions and image-based techniques for particle shape analysis. As such, this study first investigates the particle shape descriptions and image acquisition system. The image-based methods for computed-aided particle shape analysis and influence factors are also reviewed. Further, the review emphasizes empirical relationship between particle shape and mechanical properties of granular soil for engineering applications. The results indicate that different descriptors yield inconsistent outcomes. Therefore, researchers should choose appropriate particle shape descriptors and evaluation techniques based on the specific problems that need to be addressed.

This study employs the Discrete Element Method (DEM) to investigate the influence of initial fabric anisotropy on the cyclic liquefaction behavior of granular soils. Static and cyclic biaxial compression tests under undrained condition are simulated using two-dimensional elongated sharp-angled particles. Initial fabric anisotropy is introduced by considering a pre-defined inclined angle of elongated particles inside the sample. Results from the simulations reveal that varying fabric anisotropy affects the stress paths, resulting in a significant decrease in the maximum internal friction angle; however, the critical state internal friction angle is less affected. When subjected to cyclic loading, anisotropic samples exhibit distinct behavior influenced by initial fabric anisotropy. Comparison of the results with those of limited experiments in the literature confirms the simulations validity. The effective confining stress diminishes, leading to progressive liquefaction. The number of cycles required for initial liquefaction varies due to inherent anisotropy, and fabric anisotropy causes a shift in the concentration of compression or extension strains within the samples. Lower values of cyclic stress ratio amplifies the influence of inherent anisotropy on excess pore water pressure ratios. In addition to stress approach, the strain-based liquefaction resistance is also investigated by defining double amplitude strain values. It is found that when the double strain level is relatively small, the impact of inherent anisotropy becomes more noticeable. This study enhances the understanding of the role of initial fabric anisotropy in cyclic liquefaction behavior and provides insights for engineering design and mitigation strategies in seismic-prone areas.