Crushable Particles Research Articles

Particle crushing and critical state of volcanic pumice – 2018 Hokkaido Eastern Iburi Earthquake

Volcanic pumice, with special characteristics such as crushable particles and high water retention, is distributed throughout Japan and serves as the source layer for slope hazards characterised by post-failure gentle slope flows and long-distance flows. The aim of this study is to determine the relationship between the crushing characteristics and the mechanical properties of porous pumice, which often contributes to such disasters. As the porous pumice material, Ta-d pumice, which caused numerous slope disasters during the 2018 Hokkaido Eastern Iburi Earthquake in Japan, was collected and subjected to a series of triaxial compression tests. The grain size distribution of the pumice before all the tests was adjusted to be uniform, and the amount of crushing was quantified by measuring the grain size distribution after the tests. The results suggest that the critical state and isotropic consolidation of porous pumice can be systematically expressed in a three-dimensional space with the axes of the void ratio, mean effective stress, and degree of particle crushing. Furthermore, a gentle slope disaster with an inclination of less than 21°, that had occurred at the site from which the Ta-d pumice was collected, was discussed in terms of its flow potential, showing that the flow distance can be adequately explained.

3D DEM Analysis of Particle Breakage Effect on Direct Shear Tests of Coarse Sand.

This paper explores the effect of particle breakage on the mechanical behavior of coarse sand through 3D Discrete Element Method (DEM) simulations of direct shear tests (DST). The objective is to gain insights into the macro- and micro-mechanical behaviors of crushable coarse sand, with a particular focus on the stress-strain relationship, volumetric deformation, and evolution of grain crushing. The simulations involve a comparison between non-crushable and crushable particle models, where the crushable particles are implemented in the shear zone of the DST subjected to different high normal stresses. The findings indicate that the crushable particles experience partial crushing at peak shear stress, with further particle crushing leading to the production of finer particles at the shearing plane during shearing at the critical state. The migration of these finer particles under pressure and gravity generates their accumulation predominantly in the lower section of the simulation box. Importantly, the presence of crushing in the DST induces a decrease in the shear stress and an increase in the volumetric strain leading to contractive behavior instead of dilation, which gradually stabilizes the volumetric deformation at higher normal stresses.

Design of agglomerates using Weibull distribution to simulate crushable particles in the discrete element method

Design of agglomerates using Weibull distribution to simulate crushable particles in the discrete element method

Engineering characteristics of coral reef and site assessment of hydraulic reclamation in the South China Sea

Calcareous soil is a peculiar geotechnical medium which is utilized as land filler for engineering activities, such as the Maldives, Middle East, and South China Sea (SCS). Since the hydraulic dredged calcareous soil has a wide grain size distribution, irregular particle shape, crushable particles, high porosity, and poor uniformity, ground-improvement was conducted to improve the bearing capacity and reduce foundation settlement. In this paper, the geological conditions, material characteristics, and mechanical behavior of dredger fill site on a coral reef in the SCS were studied. The densification of soil using deep vibro-compaction combined with impact rolling was investigated. An innovative method of sprinkling water during impact rolling was put forward to improve rolling efficiency and soil compactness. A set of modified in-situ measurement and calculation methods suitable for the compactness of calcareous soil were put forward. Based on in-situ test results, the relationship between the soil bearing capacity and blow count of dynamic penetration test was established. It was suggested that the two-point parallel and arranged in equilateral triangles vibro-flotation method is effective for the reinforcement of hydraulic dredged calcareous soil, and after vibro-flotation, the loose soil on the ground surface should be compacted with a 25 kJ three-side impact roller for 20 passes and then compacted with 25 t smooth vibratory roller until there are no visible wheel tracks. In addition, the methods of ground improvement, in-situ test, and foundation evaluation were proposed to contribute to coral reef engineering designers. This paper also provides new insights for coral reef foundation filling and improvement around the world.

Crushing behavior of recycled waste materials: Experimental analysis and DEM simulation

Sustainable usage of construction and demolition (C&D) wastes in pavement and railway structures has attracted attention globally. However, the high probability of C&D particle breakage can impact the performance of the material and calls for careful investigation and assessment. This study aims to investigate the particle shape effect on the breakage behavior of C&D aggregates under one-dimensional compression testing using the discrete element method (DEM). Crushable particles with realistic particle shapes were modeled by agglomerates of bonded sub-particles based on 3D laser scanning technique. The DEM model was calibrated and validated with a series of single-particle crushing experiments. Results of single crushing tests indicated that the particle shape had a considerable effect on the crushing strength of the tested materials. Comparable macro-mechanical results were observed between the DEM simulation of the one-dimensional compression test and the laboratory experiment. The results revealed that the proposed model is able to accurately capture the macro and micro-mechanical responses of the tested samples. The percentage of broken bonds, number of agglomerates and volumetric size distribution of recycled concrete aggregate (RCA) materials during compression is shown to be significantly affected by the particle shapes. A hyperbolic equation is furthermore proposed to capture the correlation between the dissipated energy and the volumetric breakage index.

Numerical investigation of the time size effect of high rockfill geostructures

The creep parameters of rockfill are usually derived from creep test in the laboratory, in which the loading period before the creep stage is much shorter than the construction period in the field. This difference might cause underestimation of the long-term settlement of high rockfill geostructures if an elastic-viscoplastic model based on isotach assumption is adopted. This paper aims to investigate this hypothesis. A time-dependent strain hardening model is proposed to simulate the creep behavior of rockfill based on the relationship between the macro- and micro-responses of crushable particles during triaxial creep test revealed by previous DEM study. The results of triaxial creep tests on various granular materials are used to validate the model. The time size effect is highlighted by comparing the predicted long-term deformation of rockfill using the proposed model and an existing isotach model. The results show that the isotach model tends to underestimate the long-term deformation of high rockfill geostructures in the field compared with that of the proposed model, even when both models are calibrated against the same laboratory test results.

DEM Modeling of Crushable Grain Material under Different Loading Conditions

This paper deals with the effect of contact conditions on the crushing mechanisms and the strength of granular materials. The computation of crushable grain material under different loading conditions is performed using 3D model of discrete element method (DEM). The crushable macro-grain is generated from a large number of identical spherical micro-grains which are connected according to the bonded particle model. First, the parameters of the proposed DEM model are calibrated to match the force-displacement curve obtained from Brazilian Tests performed on cylinders made of artificially crushable material. The damage profile right at the point when the force-displacement curve reaches its maximum is seen to replicate the same crack patterns observed in Brazilian test experiments. Then, parametric investigations are performed by varying the coordination number, the contact location distribution, and the contact area. The results show that these parameters play a significant role in determining the critical contact force and fracture mechanism of crushable particles compared to a traditional macro-grain crushing test. Increasing distribution and coordination number of the macro-grain increases particle strength when large area contact is permitted. However, for linear contact area, the effect of increasing coordination number on particle strength is marginal.

Exploration of the Survival Probability and Shape Evolution of Crushable Particles during One-Dimensional Compression Using Dyed Gypsum Particles

AbstractObserving the fragmentation of individual particles within granular assemblies is a subject of evident theoretical and practical importance. A new technique using dyed gypsum particles (DGP...

The effects of particle shape on the yielding behaviour of crushable sand

Discrete element method simulations are used to investigate the effects of particle shape on the plastic behaviour of sand. This is achieved by using four different shapes of crushable particles. The behaviours of the different particles when subjected to isotropic compression, triaxial shearing, and a range of stress path tests are compared, revealing the important role of particle shape on key aspects of behaviour. Yield surfaces are established and analysed with respect to different modes of deformation, and are also shown to depend on the particle shape. A simple state boundary surface is then obtained, which is confirmed by a performing an additional stress path test on a sample that had been sheared to a critical state.

Vulnerability of volcanic loose soils having cementation and crushable particles

In Japan, a multitude of slope disasters occur annually, resulting in often severe loss of human life. On occasion, disasters that occur at very gentle slopes and disasters that flow long distance, associated with destructive energy, are observed. In such cases, the trigger layer often consists of volcanic soils with extremely high void ratios and external triggers, including earthquake or heavy rainfall, increase the risk of slope disasters. These volcanic soils are often characterised by an extremely loose structure and are either retained mainly by weak cementation or are composed of crushable particles having intra voids. Although the cause of destructive long-distance flow has been explained by liquefaction, there are some observed disasters in which the trigger layer was not fully saturated, i.e. liquefaction should not be considered as a major cause. Focusing on the fact that long-distance flow or gentle slope flow, disasters are triggered by volcanic soil layers of extremely loose structure, artificial loose soil samples were prepared in the laboratory. These samples were prepared with cementation between particles and with crushable particles. In this study, deformation and strength characteristics of the extremely loose soils were investigated by conducting CD triaxial tests. It is revealed that such extremely loose soils can be vulnerable after the weakly cemented structure collapses as a result of external factors such as earthquakes.

Maximum shear modulus of calcareous sand in Dejebel Dahar, Tunisia and its dependency on applied stress

The present experimental investigation aims at investigating the small strain stiffness of calcareous sand as a function of applied stress. The calcareous sand was sampled at Tunisia's Dejebel Dahar region, and the shear wave velocity (Vs) of calcareous sand was measured using modified oedometer cell equipped with bender elements. The results of this study demonstrate that the Vs of the tested calcareous sand is smaller than that of silica sand with minimal crushable particles at relatively low applied stress (σ); however, Vs of calcareous sand is greater than that of silica sand at high σ, reflecting strong dependency of calcareous sand on σ. The applied stress dependency of soils can be expressed as a power function of applied stress (Vs = α (σ / 1 kPa)β, where α = Vs at σ = 1 kPa and β = stress exponent). Generally, the single α-β can capture the dependency of Vs on σ, and the typical β value for sand is around 0.25. The measured β of tested silica sand was around 0.20; while, Tunisia calcareous sand shows β of greater than 0.32, and the dependency of Vs on σ cannot be captured by single α-β. This can be attributed to the fact that the variation of Vs of tested calcareous sand with increasing σ reflects not only fabric change but also particle crushing.

Design of crushable particles in DEM based on single grain compression

Grain crushing controls the response in shear of granular materials and has an impact on the collapse mechanism of geo-structures. To simulate the effects of grain crushing on the macroscopic response, the Discrete Element Method (DEM) appears to be a viable option considering its proven capability of capturing key aspects of the response of granular materials. One of the approaches to simulate particle crushing via DEM consists in creating a particle by assembling smaller sub-particles ‘bonded’ together by adhesion forces. When the force at the contact between two sub-particles exceeds the adhesion, the bonding is broken irreversibly. A critical aspect in this approach is the characterization of i) the ‘random’ distribution of the adhesion between the sub-particles, and ii) the initial rotation angle of the particles being assembled initially. The tensile strength of a single grain, as measured experimentally by diametral compression between flat platens, typically follows the Weibull distribution. This is proposed to be the criterion to validate any probabilistic distribution selected for the adhesion and the initial rotation angle. The effect of a normal distribution of adhesion and initial rotation on compressive strength was investigated by compressing either a regular ‘cylindrical’ particle or an irregular particle. It is shown that a normal probabilistic distribution of both the adhesion and the rotation angle returns a Weibull distribution of the compressive strength. It is also shown that the rotation angle plays a very critical role and this should be taken into account when initially assembling the ‘crushable’ particles. The rotation angle relocates the contact points, which in turn control the stress at which the particle breaks.

On the packing and crushing of granular materials

This paper is a study of the dependence of the volume of voids in a granular material on the particle size distribution. It has previously been proposed that the volume of voids is proportional to the volume of the smallest particles. In a particle size distribution which is progressively becoming wider (e.g. as occurs due to crushing during the compression of sand), the smallest size of particle decreases, yet there are only ever a few of these particles out of many thousands or millions. This paper attempts to identify which particles govern the overall density of a granular material, and a new definition of the ‘smallest particles’ is proposed. These particles are shown to govern the void space in a range of simulations of spherical and non-spherical crushable particles. The theory also applies to idealised Apollonian sphere packings.

Crushing and oedometer compression of rockfill using DEM

A new methodology was developed to perform computer simulations of crushable rockfill particles using the discrete element method. Rockfill was modelled using a clump logic, responsible for providing a statistical and spatial variability in the strength and shape of the particles. Clumps have a major advantage of severely decreasing the number of contact equations to be solved in the model, resulting in less computer time. A series of single-particle crushing tests and an oedometer test were simulated using crushable particles and were in agreement with experimental data. A comprehensive study of the brittle failure of single-particle crushing tests is presented.

Rheology of granular materials composed of crushable particles.

We investigate sheared granular materials composed of crushable particles by means of contact dynamics simulations and the bonded-cell model for particle breakage. Each particle is paved by irregular cells interacting via cohesive forces. In each simulation, the ratio of the internal cohesion of particles to the confining pressure, the relative cohesion, is kept constant and the packing is subjected to biaxial shearing. The particles can break into two or more fragments when the internal cohesive forces are overcome by the action of compressive force chains between particles. The particle size distribution evolves during shear as the particles continue to break. We find that the breakage process is highly inhomogeneous both in the fragment sizes and their locations inside the packing. In particular, a number of large particles never break whereas a large number of particles are fully shattered. As a result, the packing keeps the memory of its initial particle size distribution, whereas a power-law distribution is observed for particles of intermediate size due to consecutive fragmentation events whereby the memory of the initial state is lost. Due to growing polydispersity, dense shear bands are formed inside the packings and the usual dilatant behavior is reduced or cancelled. Hence, the stress-strain curve no longer passes through a peak stress, and a progressive monotonic evolution towards a pseudo-steady state is observed instead. We find that the crushing rate is controlled by the confining pressure. We also show that the shear strength of the packing is well expressed in terms of contact anisotropies and force anisotropies. The force anisotropy increases while the contact orientation anisotropy declines for increasing internal cohesion of the particles. These two effects compensate each other so that the shear strength is nearly independent of the internal cohesion of particles.

Modeling the influence of particle morphology on the fracture behavior of silica sand using a 3D discrete element method

The constitutive behavior and deformation characteristics of uncemented granular materials are to a large extent derived from the fabric or geometry of the particle structure and the interparticle friction resulting from normal forces acting on particles or groups of particles. Granular materials consist of discrete particles with a fabric (microstructure) that changes under loading. Synchrotron micro-computed tomography (SMT) has emerged as a powerful non-destructive 3D scanning technique to study geomaterials. In this paper, SMT was used to acquire in situ scans of the oedometry test of a column of three silica sand particles. The sand is known as ASTM 20–30 Ottawa sand, and has a grain size between US sieves #20 (0.841 mm) and #30 (0.595 mm). The characteristics and evolution of particle fracture in sand were examined using SMT images, and a 3D discrete element method (DEM) was used to model the fracture behavior of sand particles. It adopts the bonded particle model to generate a crushable agglomerate that consists of a large number of small spherical sub-particles. The agglomerate shape matches the 3D physical shape of the tested sand particles by mapping the particle morphology from the SMT images. The paper investigates and discusses the influence of agglomerate packing (i.e., the number and size distribution of spherical sub-particles that constitute the agglomerate) and agglomerate shape on the fracture behavior of crushable particles.

DEM of triaxial tests on crushable cemented sand

Abstract Using the discrete element method, triaxial simulations of cemented sand consisting of crushable particles are presented. The triaxial model used features a flexible membrane, allowing realistic deformation to occur, and cementation is modelled using inter-particle bonds. The effects of particle crushing are explored, as is the influence of cementation on the behaviour of the soil. An insight to the effects that cementation has on the degree of crushing is presented.

Normalized shear modulus reduction and damping ratio curves of quartz sand and rhyolitic crushed rock

This paper presents a laboratory investigation of the strain-dependent dynamic properties of volcanic granular soils composed of a rhyolitic crushed rock along with additional experiments on quartz sand through a high-amplitude resonant column testing program. The sands were tested in a dry state in torsional mode of vibration and thus the degradation of the normalized shear modulus and the increase of damping ratio in shear as a function of the shear strain amplitude (γ) were examined. It was revealed that, for a given mean effective confining pressure (σm') and coefficient of uniformity (Cu), the volcanic sands showed higher linearity in comparison to the quartz sands and that this trend became more pronounced with decreasing σm' and increasing Cu. In contrast to the general trend observed in the quartz soils, the confining pressure and the grain-size characteristics hardly affected the rate of normalized modulus degradation and damping increase in the volcanic sands. These differences are possibly related to the micro-mechanisms that dominate at particle contacts in the range of small to medium shear strain amplitudes. For example, the possible more pronounced crushing of the asperities during the elevation of the confining pressure and during the dynamic loading along with the lower inter-particle friction angle and stiffness of the volcanic sands of crushable particles in comparison to the quartz sands of stronger particles might play an important role in the energy dissipation during the dynamic excitation and thus on the rate of damping increase or modulus degradation.

DEM simulations of stiff and soft materials with crushable particles: an application of expanded perlite as a soft granular material

The mechanical behavior of granular materials is largely affected by particle breakage. Physical and mechanical properties of granular materials, such as grain size distribution, deviatoric and volumetric behavior, compressibility and mobilized friction angle are affected by particle crushing. This paper focuses on the evolution of the above mentioned characteristics using the Discrete Element Method (DEM). Behaviors of stiff and soft materials are studied using well established crushing criteria. Results from simulations indicate that stiff materials, have a typical fractal distribution of particle size, which is dominant when confining pressure increases. The fractal characteristic parameter of grain size effect is discussed. Evolution of shear stresses and volumetric strains during shearing are also predicted and analyzed. Expanded perlite, selected as a soft material, is investigated in terms of shear and volumetric behavior. For perlite, triaxial compression tests and corresponding DEM simulations are also performed. Results show good agreement between experiments and simulations and support the fact that the DEM can be considered as a useful tool to predict the behavior of crushable granular materials.

Engineering characteristics of the calcareous sand in Nansha Islands, South China Sea

Calcareous sand is widespread on the coral reefs of Nansha Islands, South China Sea. It represents a special soil type due to its compressibility which results from high void ratio sediments and crushable particles. In oceanic engineering, calcareous sand is commonly used as building foundations and backfill material for road embankments or airport runways. Site investigations of coral reef geology, geomorphology and sedimentation on Yongshu Reef, Nansha Islands, South China Sea indicate that calcareous sand is the main composition of the upper reef flat, the only available site for constructions. Therefore, it is very important to estimate the engineering characteristics of calcareous sand. This paper presents the site geology of calcareous sand on Yongshu Reef and results of corresponding laboratory plate load tests. The bearing capacity and deformational behavior of calcareous sand are studied. Results of this study show that: (1) the bearing capacity of dry sand exceeds 500 kPa when the relative density is 68%, so calcareous sand can be compacted to achieve greater bearing capacity and reduce settlement; (2) plastic deformation is the main part of the ground settlement; (3) the bearing capacity of saturated calcareous sand is 50% less than that of the dry sand; (4) square foundation can provide greater bearing capacity than circular one with the same area; (5) calcareous sand ground is obviously stiffer than quartz sand ground; (6) the effective depth of load transmission is about 1.0 to 2.0 times of the load plate width and increases with the imposed load.