Imperial College London

ProfessorCatherineO'Sullivan

Faculty of EngineeringDepartment of Civil and Environmental Engineering

Professor of Particulate Soil Mechanics
 
 
 
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Contact

 

+44 (0)20 7594 6117cath.osullivan Website

 
 
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Location

 

501Skempton BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

191 results found

Bandera S, Angioletti-Uberti S, Tangney P, O'Sullivan Cet al., 2021, Coarse-grained molecular dynamics simulations of clay compression, Computers and Geotechnics, Vol: 138, Pages: 1-18, ISSN: 0266-352X

This paper outlines a framework for using molecular dynamics to simulate the compression of kaolinite saturated at alkaline pH (=8) in a low (1 mM) concentration solution. The particles are modelled as flat (3D) ellipsoids and their interactions are described by a modified form of the Gay-Berne potential, calibrated against DLVO theory. The LAMMPS software was used to generate monodisperse and slightly polydisperse samples, and to simulate isotropic compression to 100 kPa. The influences of sample size and strain rate on the void ratio and the arrangement of particles within the samples were investigated via parametric studies. It is useful to consider the extent to which the system temperature (a measure of the average kinetic energy) is controlled when assessing whether the applied strain rate is appropriate. It is found that the number of particles that can be considered a representative element volume is orders of magnitude larger than the number simulated in earlier studies and that larger number of particles are required in polydisperse samples than in the monodisperse case. A comparison between the results obtained and those of published experimental studies show that the methodology proposed can deliver sensible results for the material considered.

Journal article

Liu D, O'Sullivan C, Carraro JAH, 2021, The influence of particle size distribution on the stress distribution in granular materials, Géotechnique, Pages: 1-37, ISSN: 0016-8505

Journal article

Zhao B, O'Sullivan C, 2021, Fluid Particle Interaction in Packings of Monodisperse Angular Particles, Powder Technology, ISSN: 0032-5910

Journal article

O'Sullivan C, Cheng H, Zhao J, 2021, Use of DEM in geomechanics: Special issue associated with the DEM 8 conference, Computers and Geotechnics, Vol: 137, Pages: 1-4, ISSN: 0266-352X

Journal article

Yu M, Reddyhoff T, Dini D, Holmes A, O'Sullivan Cet al., 2021, Using ultrasonic reflection resonance to probe stress wave velocity in assemblies of spherical particles, IEEE Sensors Journal, ISSN: 1530-437X

A high-sensitivity method to measure acousticwave speed in soils by analyzing the reflected ultrasonic signalfrom a resonating layered interface is proposed here.Specifically, an ultrasonic transducer which can be used to bothtransmit and receive signals is installed on a low-high acousticimpedance layered structure of hard PVC and steel, which in turnis placed in contact with the soil deposit of interest. The acousticimpedance of the soil (the product of density and wave velocity)is deduced from analysis of the waves reflected back to thetransducer. A system configuration design is enabled bydeveloping an analytical model that correlates the objectivewave speed with the measurable reflection coefficient spectrum.The physical viability of this testing approach is demonstratedby means of a one-dimensional compression device that probesthe stress-dependence of compression wave velocity of differentsizes of glass ballotini particles. Provided the ratio of thewavelength of the generated wave to the soil particle size issufficiently large the data generated are in agreement with dataobtained using conventional time-of-flight measurements. Inprinciple, this high-sensitivity approach avoids the need for thewave to travel a long distance between multiple transmitterreceiver sensors as is typically the case in geophysical testingof soil. Therefore it is particularly suited to in-situ observation ofsoil properties in a highly compact setup, where only a single transducer is required. Furthermore, high spatialresolution of local measurements can be achieved, and the data are unaffected by wave attenuation as transmitted insoil.

Journal article

Kalderon M, Smith E, O'Sullivan C, 2021, Comparative analysis of porosity coarse-graining techniques for discrete element simulations of dense particulate systems, Computational Particle Mechanics, ISSN: 2196-4378

The Discrete Element Method (DEM) is a well-established approach to study granular materials in numerous fields of application; modelling each granular particle individually to predict the overall behavior. This behavior can be then extracted by averaging, or coarse graining, the sample using a suitable method. The choice of appropriate coarse-graining method entails a compromise between accuracy and computational cost, especially in the large-scale simulation typically required by industry. A number of coarse-graining methods have been proposed in the literature, these are reviewed and categorized in this work. Within this contribution two novel porosity coarse-graining strategies are proposed including a Voxel method where a secondary dense grid of “pixel-cells” is implemented adopting a binary logic for the coarse graining and a Hybrid method where both analytical formulas and pixels are utilized. The proposed methods are compared with four coarse- graining schemes that have been documented in the literature, including the Particle Centroid Method (PCM), an Analytical method, a method which solves the diffusion equation and an approach which employs averaging using kernels. The novel methods are validated for problems in both two and three dimensions through comparison with the “accurate” Analytical method. It is shown that, once validated, both the proposed schemes can approximate the exact solutions quite accurately, however there is a high computational cost associated with the Voxel method. The accuracy of both methods can be adjusted allowing the user to decide between accuracy and computational time. A detailed comparison is then presented for all six schemes considering “accuracy”, “smoothness” and “computational cost”. Optimal parameters are obtained for all six methods and recommendations for coarse graining DEM samples are discussed.

Journal article

Sanvitale N, Zhao B, Bowman E, O'Sullivan Cet al., 2021, Particle-scale observation of seepage flow in granular soils using PIV and CFD, Geotechnique: international journal of soil mechanics, ISSN: 0016-8505

Seepage-induced instabilities pose a challenge in many geotechnical applications. Particle-scale mechanisms govern the initiation of instability. However, current understanding is based on a macro-scale perspective that draws on continuum mechanics. Recent developments in imaging and numerical analysis can provide the particle-scale fundamental perspective needed to develop a comprehensive insight. This contribution demonstrates the value of combining particle-scale experimental and numerical studies. The experiments consider transparent soil samples created using refractive image matching and monitored by particle image velocimetry (PIV). Three-dimensional pore topology is extracted from a series of 2D images and imported into computational fluid dynamics (CFD)simulations. Permeability is estimated by three distinct approaches: using flow rate, PIV-and CFD-generated data. The flow fields obtained from PIV and CFD are in good agreement considering both flow rate contour plots and flow rate distributions; this demonstrates the successful reconstruction of three-dimensional pore structure and flow-field analysis. The comparison also reveals that the side boundary effects in CFD simulations are constrained within a limited region. The multi-plane results characterize the variance of flow velocity with the three-dimensional pore topology. Finally, the fluid-particle interactions obtained from CFD results show a larger variance in the angular particle packings.

Journal article

Bernhardt-Barry M, Biscontin G, O'Sullivan C, 2021, Analysis of the stress distribution in a laminar direct simple shear device and implications for test data interpretation, Granular Matter, Vol: 23, ISSN: 1434-5021

Direct simple shear (DSS) testing allows observation of load-deformation response under rotation of the major principal stress plane, which is descriptive of many actual field problems. While the simplicity of the test configuration makes its use popular in research and industry, key uncertainties still remain regarding the interpretation of the laboratory data. This study uses laboratory validated discrete element method (DEM) models to examine the stress transmission in laminar-type direct simple shear devices under drained constant effective stress conditions. The DEM models (comprised of spheres) closely replicate physical specimens of precision chrome steel ball bearings for which the properties (e.g., shape, surface friction, and stiffness) were measured directly. The DEM models were also validated using experimental tests, so that conclusions regarding the system response can be derived with confidence from the available DEM data. The testing program included both loose and dense specimens, allowing for a comparison of the influence of density on stress state which has not been examined in previous simple shear DEM studies. Differences were observed between vertical effective stresses and shear stresses derived from boundary measurements (as commonly carried out in experimental programs) and those derived from force measurements within the DEM specimens. The failure state of the material in simple shear was also examined through Mohr’s circles of stress. The evolution of stresses on both the horizontally and vertically oriented planes were considered so that established methods of direct simple shear interpretation could be critically assessed. For the loose specimens, the angle of shearing resistance can be confidently estimated considering the maximum shear stress acting on the horizontal plane, which is easily inferred from measurements of the shear force during the physical test. This was true considering both internal and boundary calculated stress

Journal article

Altuhafi FN, O'Sullivan C, Sammonds P, Su T-C, Gourlay Cet al., 2021, Triaxial compression on semi-solid alloys, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, Vol: 52, Pages: 2010-2023, ISSN: 1073-5623

Multi-axial compression of the mushy zone occurs in various pressurized casting processes. Here, we present a drained triaxial compression apparatus for semi-solid alloys that allow liquid to be drawn into or expelled from the sample in response to isotropic or triaxial compression. The rig is used to measure the pressure-dependent flow stress and volumetric response during isothermal triaxial compression of globular semi-solid Al-15 wt pct Cu at 70 to 85 vol pct solid. Analysis of the stress paths and the stress–volume data show that the combination of the solid fraction and mean effective pressure determines whether the material undergoes shear-induced dilation or contraction. The results are compared with the critical state soil mechanics (CSSM) framework and the similarities and differences in behavior between equiaxed semi-solid alloys and soils are discussed.

Journal article

Che H, O'Sullivan C, Sufian A, Smith Eet al., 2021, A novel CFD-DEM coarse-graining method based on the Voronoi tessellation, Powder Technology, Vol: 384, Pages: 479-493, ISSN: 0032-5910

In unresolved flow CFD-DEM simulations, the porosity values for each CFD cell are determined using a coarse-graining algorithm. While this approach enables coupled simulations of representative numbers of particles, the influence of the porosity local to the particles on the fluid-particle interaction force is not captured. This contribution considers a two-grid coarse-graining method that determines a local porosity for each particle using a radical Voronoi tessellation of the system. A relatively fine, regular point cloud is used to map these local porosity data to the CFD cells. The method is evaluated using two different cases considering both disperse and tightly packed particles. The data show that the method conserves porosity data, is reasonably grid-independent and can generate a relatively smooth porosity field. The new method can more accurately predict the fluid-particle interactive force for polydisperse particle system than alternative methods that have been implemented in unresolved CFD-DEM codes.

Journal article

Sufian A, Artigaut M, Shire T, O'Sullivan Cet al., 2021, Influence of Fabric on Stress Distribution in Gap-Graded Soil, Journal of Geotechnical and Geoenvironmental Engineering, Vol: 147, ISSN: 1090-0241

The combined influence of density and stress-induced fabric anisotropy on the nature of stress transmission in gap-graded soils with cohesionless fines has been explored using the discrete element method (DEM). Various particle size ratios and fines contents were considered in simulations of constant mean stress triaxial compression. Analysis of the available particle-scale data focused on understanding how stress was distributed among and between the finer and coarser particles. While the study confirms that stress is transferred from the coarser to the finer fraction with increasing fines content, the concept of a threshold fines content at which there is a definitive transition in the nature of stress transmission is not supported. Rather, there is a gradual evolution of the distribution of stresses between the two size fractions with increasing fines content, and the relationship between fines content and stress in the finer fraction depends on the size ratio, density, and fabric anisotropy. For the denser samples considered, the stress transmitted by the finer fraction systematically reduced during shearing. An alternate definition of granular void ratio is introduced, which accounts for the nonactive fine and coarse particles and is formulated in a consistent manner to capture both the intergranular and interfine void ratios commonly found in the literature, along with the equivalent granular void ratio. The anisotropy of the network of contacts formed by the interactions of coarse particles was observed to be the dominant contributor to fabric anisotropy.

Journal article

Liu D, O'Sullivan C, Harb Carraro JA, 2021, Influence of particle size distribution on the proportion of stress-transmitting particles and implications for measures of soil state, Journal of Geotechnical and Geoenvironmental Engineering, Vol: 147, Pages: 04020182-1-04020182-14, ISSN: 0733-9410

It is generally accepted that the use of void ratio and bulk density as measures of soil8state have limitations in the case of gap-graded soils as the finer grains may not 9transmit stress. However, hitherto no one has systematically explored whether this 10issue also emerges for soils with continuous gradings. Building on a number of experimental and discrete element method (DEM) studies that have considered the idea of an effective void ratio for gap-graded or bi-modal soils, this contribution extends consideration of this concept to a broader range of particle size distributions. By exploiting high performance computers, this study considers a range of ideal isotropically compressed samples of spherical particles with linear, fractal and gap-graded (bimodal and trimodal) particle size distributions. The materials’ initial packing densities are controlled by varying the inter-particle coefficient of friction. The results show that even for soils with continuous particle size distributions, a significant proportion of the finer particles may not transmit stress and be inactive. Drawing on ideas put forward in relation to gap-graded soils, both a mechanical void ratio and mechanical bulk density that consider the inactive grains as part of the void space are determined. Even for the linear and fractal gradings considered here, the difference between the conventional measures and the mechanical measures is finite and density dependent. The difference is measurably larger in the looser samples considered. These data highlight a conceptual/fundamental limitation of using the global void ratio26as a measure of state in expressions to predict granular material behaviour

Journal article

Otsubo M, Kuwano R, O'Sullivan C, Shire Tet al., 2021, Using geophysical data to quantify stress-transmission in gap-graded granular materials, Geotechnique: international journal of soil mechanics, ISSN: 0016-8505

The behaviour of gap-graded granular materials, i.e. mixtures of coarse and cohesionless finer grains having a measurable difference in particle size, does not always confirm to established frameworks of sand behaviour. Prior research has revealed that the role of the finer particles on the stress-strain response, liquefaction resistance, and internal stability of non-cohesive gap-graded soils is significant and complex, and highly dependent on both the volumetric proportion of finer particles in the material and the coarse-particle to finer-particle size ratio. Quantifying the participation of the finer particles on the stress transmission and overall behaviour is central to understanding the behaviour of these materials. However, no experimental technique that can directly quantify the contribution of finer particles to the overall behaviour has hitherto been proposed. This paper explores to what extent the participation of finer particles can be assessed using laboratory geophysics, recognizing that granular materials act as a filter to remove the high frequency components of applied seismic / sound waves. Discrete element method simulations are performed to understand the link between particle-scale stress transmission and the overall response observed during shear wave propagation. When the proportion of finer particles is increased systematically both the shear wave velocity (VS) and low-pass frequency (flp) increase sharply once a significant amount of the applied stress is transferred via the finer particles. This trend is also observed in equivalent laboratory experiments. Consequently, the flp–VS relationship can provide useful insights to assess whether the finer particles contribute to stress transmission and hence the mechanical behaviour of the gap-graded materials.

Journal article

Dutta TT, Otsubo M, Kuwano R, O'Sullivan Cet al., 2020, Evolution of shear wave velocity during triaxial compression, Soils and Foundations, Vol: 60, Pages: 1357-1370, ISSN: 0038-0806

Accurate design of geotechnical structures requires precise estimation of the shear wave velocity (Vs) and the small-strain shear modulus. However, the interpretation of Vs data measured in deformed/sheared soil has not been extensively considered. This study used a triaxial apparatus equipped with planar piezoelectric transducers to monitor the evolution of Vs during triaxial compression of cohesionless soils. Recognizing that the grain shape and surface characteristics affect the overall mechanical response of granular materials, various natural sands and glass bead samples were considered. Discrete element method (DEM) simulations using spherical particles were carried out to compute particle-scale responses that cannot be measured in the laboratory. The experimental results revealed that the Vs values for samples with different initial densities tend to approach one another and have similar values (merge) at large axial strains. This merging occurs at a lower strain level for spherical particles in comparison with non-spherical particles. The linear Vs-void ratio relationship, which is often developed and used for homogeneous and isotropic stress states, is no longer applicable during shearing. It is the mean coordination number that dictates the evolution of Vs during triaxial compression. Furthermore, the axial strain at which the peak Vs is achieved is found to be comparable to the axial strain at which specimen dilation takes place.

Journal article

Otsubo M, OSullivan C, Ackerley S, Parker D, Otsubo M, O'Sullivan C, Ackerley S, Parker Det al., 2020, Selecting an appropriate shear plate configuration to measure elastic wave velocities, Geotechnical Testing Journal, Vol: 43, ISSN: 0149-6115

The (small-strain) elastic moduli of soil can be determined from stress wave velocity measurements. Bender/extender elements are widely used in laboratory experiments; however, discussion on how to accurately determine wave velocities using this method continues. Planar piezoelectric transducers (sometimes called shear plates) are a relatively new technology, whose use is not yet widely established, that appear to offer some advantages in comparison with bender/extender elements for laboratory geophysics tests. This contribution critically assesses the use of planar piezoelectric elements experimentally and using discrete element method (DEM) simulations. Planar piezoelectric elements capable of generating and receiving either shear or compression waves were placed in the top and base caps of a triaxial apparatus. Samples of glass ballotini were used so that stress wave propagation simulations could be performed on equivalent virtual samples using DEM. The appropriate shear plate configuration to effectively measure the shear wave velocity is explored. Considering both time- and frequency-domain responses, it is revealed that shear plate signals are sensitive to the surface area and thickness of the piezoelectric elements and to the lateral boundary conditions. Using a shear plate with the widest possible surface area exposed to the soil specimen is recommended to increase the signal-to-noise ratio and to produce more planar shear waves, resulting in a more accurate measurement of shear wave velocity.

Journal article

Bowles AJ, Fowler GD, O'Sullivan C, Parker Ket al., 2020, Sustainable rubber recycling from waste tyres by waterjet: A novel mechanistic and practical analysis, Sustainable Materials and Technologies, Vol: 25, Pages: 1-15, ISSN: 2214-9937

Production and disposal of car tyres are major contributors to environmental damage. The first stage in tyre rubber recycling is granulation to smaller particle sizes. The sub-optimal physical, mechanical and chemical properties of mechanically ground tyre rubber (GTR) when incorporated into recycled blends are major obstacles to wider use of this potentially sustainable, recovered resource. Consequently, newly manufactured tyres contain less than 5% recycled material. This study compares two types of GTR product: mechanically ground crumb (MGC) and ultrahigh pressure waterjet-produced rubber crumb (WJC). A novel image analysis method showed that when the two particle types were compared, MGC was associated with both greater convexity and sphericity: the geometric mean ratio of MGC/WJC sphericity was 1.67. When part-recycled rubber blends comprising 30% crumb of particle size < 300 μm were compared to virgin polymer, the WJC blend exhibited superior mechanical properties to the MGC blend. These results can be explained by the higher surface area to volume ratio of WJC when compared to MGC which results in strong bonding in new blends using WJC. Further analysis by scanning electron microscopy (SEM) elucidated significant shape and textural variation within the WJC sample, allowing grouping into two sub-categories: “W1” which comprises particles with complex geometries, and “W2” particles which have a relatively simple topology that is similar to MGC. Maximising the W1:W2 particle ratio is likely to be crucial to the optimisation of output quality in the WJC process, and so a composite model is proposed that unifies three well-established fluid effects: brittle fracturing, impact cratering and cavitation. Impact cratering and cavitation effects should be maximised by altering process parameters with the aim of producing a higher proportion of crumb with a more irregular surface morphology to achieve better bonding properties in recycled

Journal article

Su TC, O'Sullivan C, Yasuda H, Gourlay CMet al., 2020, Rheological transitions in semi-solid alloys: in-situ imaging and LBM-DEM simulations, Acta Materialia, Vol: 191, Pages: 24-42, ISSN: 1359-6454

Rheological transitions from suspension flow to granular deformation and shear cracking are investigated in equiaxed-globular semi-solid alloys by combining synchrotron radiography experiments with coupled lattice Boltzmann method, discrete element method (LBM-DEM) simulations. The experiments enabled a deformation mechanism map to be plotted as a function of solid fraction and shear rate, including a rate dependence for the transition from net-contraction to net-dilation, and for the initiation of shear cracking. The LBM-DEM simulations are in quantitative agreement with the experiments, both in terms of the strain fields in individual experiments and the deformation mechanism map from all experiments. The simulations are used to explore the factors affecting the shear rate dependence of the volumetric strain and transitions. The simulations further show that shear cracking is caused by a local liquid pressure drop due to unfed dilatancy, and the cracking location and its solid fraction and shear rate dependence were reproduced in the simulations using a criterion that cracking occurs when the local liquid pressure drops below a critical value.

Journal article

Smith E, Trevelyan D, Ramos-Fernandez E, Sufian A, O'Sullivan C, Dini Det al., 2020, CPL library - a minimal framework for coupled particle and continuum simulation, Computer Physics Communications, Vol: 250, Pages: 1-11, ISSN: 0010-4655

We present an open-source library for coupling particle codes, such as molecular dynamics (MD) or the discrete element method (DEM), and grid based computational fluid dynamics (CFD). The application is focused on domain decomposition coupling, where a particle and continuum software model different parts of a single simulation domain with information exchange. This focus allows a simple library to be developed, with core mapping and communication handled by just four functions. Emphasis is on scaling on supercomputers, a tested cross-language library, deployment with containers and well-documented simple examples. Building on this core, a template is provided to facilitate the user development of common features for coupling, such as averaging routines and functions to apply constraint forces. The interface code for LAMMPS and OpenFOAM is provided to both include molecular detail in a continuum solver and model fluids flowing through a granular system. Two novel development features are highlighted which will be useful in the development of the next generation of multi-scale software: (i) The division of coupled code into a smaller blocks with testing over a range of processor topologies. (ii) The use of coupled mocking to facilitate coverage of various parts of the code and allow rapid prototyping. These two features aim to help users develop coupled models in a test-driven manner and focus on the physics of the problem instead of just software development. All presented code is open-source with detailed documentation on the dedicated website (cpl-library.org) permitting useful aspects to be evaluated and adopted in other projects.

Journal article

O'Sullivan C, Ciantia M, 2020, Calculating the state parameter in crushable sands, International Journal of Geomechanics, Vol: 20, Pages: 04020095-1-04020095-10, ISSN: 1532-3641

The state parameter (y) measures the distance from the current state to the critical state line (CSL) in thecompression plane. The existence of a correlation between both the peak angle of shearing resistance (�#$ )and peak dilatancy and y is central to many constitutive models used to predict granular soil behaviour. Thesecorrelations do not explicitly consider particle crushing. Crushing induced evolution of the particle sizedistribution influences the CSL position and recent research supports used of a critical state plane (CSP) toaccount for changes in grading. This contribution evaluates the whether the CSP can be used to calculate yand thus enable prediction of the peak angle of �#$ and peak dilatancy where crushing takes place. The dataconsidered were generated from a validated DEM model of Fontainebleau sand that considers particlecrushing. It is shown that where y is calculated by considering the CSL of the original uncrushed material therecan be in a significant error in predicting the material response. Where the CSP is used there is a significantimprovement in our ability to predict behaviour whether the CSP is accurately determined using a largenumber of tests or approximated using crushing yield envelopes. It is shown that the state parametercalculated using the previously available definition can give a false sense of security when assessingliquefaction potential of potentially crushable soils. The contribution also highlights the stress-pathdependency of the relationship between �#$ and y whichever approach is used to determine y

Journal article

Knight C, O'Sullivan C, Dini D, Van Wachem Bet al., 2020, Computing drag and interactions between fluid and polydisperse particles in saturated granular materials, Computers and Geotechnics, Vol: 117, Pages: 1-16, ISSN: 0266-352X

Fundamental numerical studies of seepage induced geotechnical instabilities and filtration processes depends on accurate prediction of the forces imparted on the soil grains by the permeating fluid. Hitherto coupled Discrete Element Method (DEM) simulations documented in geomechanics have most often simulated the fluid flow using computational fluid dynamics (CFD) models employing fluid cells that contain a number of particles. Empirical drag models are used to predict the fluid-particle interaction forces using the flow Reynolds number and fluid cell porosity. Experimental verification of the forces predicted by these models at the particle-scale is non-trivial. This contribution uses a high resolution immersed boundary method to model the fluid flow within individual voids in polydisperse samples of spheres to accurately determine the fluid-particle interaction forces. The existing drag models are shown to poorly capture the forces on individual particles in the samples for flow with low Reynolds number values. An alternative approach is proposed in which a radical Voronoi tesselation is applied to estimate a local solids volume fraction for each particle; this local solids fraction can be adopted in combination with existing expressions to estimate the drag force. This tessellation-based approach gives a more accurate prediction of the fluid particle interaction forces.

Journal article

Liu D, O'Sullivan C, Carraro JAH, 2020, Stress inhomogeneity in gap-graded cohesionless soils - A contact based perspective, Geo-Congress 2020, Pages: 341-348, ISSN: 0895-0563

Gap-graded cohesionless soils, comprising mixtures of fine and coarse grains, pose a particular challenge in soil mechanics. Reasoning and experimental data indicate that some of the finer grains may exist in the void space without transmitting any stress. A number of authors have proposed considering at least some of the volume of these particles along with the void space when calculating the void ratio in the case of low fines contents. The concept of a transitional fines content has been proposed, i.e., a fines content delineating materials whose behavior is dominated by the coarser grains and materials whose behavior is determined by the finer grains. This contribution uses discrete element method (DEM) simulations to explore the nature of stress transmission in gap-graded materials comprised of spherical particles. Partitioning the stress tensor by considering the contributions of the contacts between coarse particles, the contacts between coarse and fine particles, and the contacts between fine particles is shown to provide useful insight into the contribution of each type of particle to the overall stress transmission. In general, for the mixtures considered here, the coarse-coarse contacts transmit a greater range of forces and a greater average force. For the mixture with size ratio of 3.7, the range of contact force magnitudes transmitted by each contact type reduces with increasing fines content and increasing sample density. This sensitivity is more evident for the lower fines contents studied.

Conference paper

Nadimi S, Otsubo M, Fonseca J, O'Sullivan Cet al., 2019, Numerical modelling of rough particle contacts subject to normal and tangential loading, Soils and Foundations, Vol: 21, ISSN: 0038-0806

Our understanding of the mechanics of contact behaviour for interacting particles has been developed mostly assumingthat surfaces are smooth. However,real particlesof interest inengineering science are generally rough. While recent studies have considered the influence of roughness on the normal force-displacement relationship, surface roughness was quantified using only a single scalar measure, disregardingthe topology of the surface. There are some conflicting arguments concerning the effect of roughness on the tangential or shear force-displacement relationship. In this study,optical interferometry data are used to generate the surface topology for input into a 3D finite element model. This model is used to investigate the sensitivity of the normal force-displacement response to thesurfacetopology by considering differentsurfaces with similar overall roughness values. The effect of surface roughness on the tangential force-displacement relationshipand the influence of loading history are also explored. The results indicate that quantifying roughness using a single value, such as the root mean square height of roughness, Sq, is insufficient to predict the effect of roughness upon stiffness. It is also shown that in the absenceof interlocking,rough particle surfaces exhibit a lower frictional resistance in comparison with equivalent smooth surfaces.

Journal article

Ciantia M, Arroyo M, O'Sullivan C, Gens Aet al., 2019, Micromechanical inspection of incremental behaviour of crushable soils, Acta Geotechnica, Vol: 14, Pages: 1337-1356, ISSN: 1861-1125

In granular soils grain crushing reduces dilatancy and stress obliquity enhances crushability. These are well-supported specimen-scale experimental observations. In principle those observations should reflect some peculiar micromechanism associated with crushing, but which is it? To answer that question the nature of crushing-induced particle-scale interactions is here investigated using an efficient DEM model of crushable soil. Microstructural measures such as the mechanical coordination number and fabric are examined while performing systematic stress probing on the triaxial plane. Numerical techniques such as parallel and the newly introduced sequential probing enable clear separation of the micromechanical mechanisms associated with crushing. Particle crushing is shown to reduce fabric anisotropy during incremental loading and to slow fabric change during continuous shearing. On the other hand, increased fabric anisotropy does take more particles closer to breakage. Shear enhanced breakage appears then to be a natural consequence of shear-enhanced fabric anisotropy. The particle crushing model employed here makes crushing dependent only on particle and contact properties, without any pre-established influence of particle connectivity. That influence does not emerge and it is shown how particle connectivity, per se, is not a good indicator of crushing likelihood.

Journal article

Dutta TT, Otsubo M, Kuwano R, O'Sullivan Cet al., 2019, Estimating stress wave velocity in granular materials: Apparent particle size dependency and appropriate excitation frequency range, Geotechnique Letters, Vol: 9, Pages: 1-26, ISSN: 2045-2543

There is a lack of consensus in the literature on the influence of the median particle size on stress wave velocityin cohesionless soils. For assemblies of spherical particles with Hertzian contacts, the stress wave velocitiesshould not depend on particle size. However, a link between particle size and stress wave velocity has beenreported in laboratory experiments. In this study, to identify the reasons for the discrepancies, wave velocitymeasurements were performed using planar piezoelectric transducers on four different sizes of alkaline glassbeads and natural silica sands. The experimental results indicate that shear and compression wave velocities areindependent of the median particle size. In accordance with dispersion theory, both the experiments and discreteelement simulations demonstrate that the maximum frequency that can propagate through a granular assembly(i.e., the lowpass frequency) reduces with increasing median particle size. The relationship between the lowpassfrequency and the input signal frequency determines the quality of the received signal and hence the accuracy ofthe interpreted stress wave velocity data. To accurately estimate shear wave velocities, the selected inputfrequencies should match those frequencies which exhibit the largest gain factors and input frequencies shouldnot exceed the half of lowpass frequency. To determine the compression wave velocity, it is suggested to adoptthe start to start method and to choose an input frequency which is slightly lower than the lowpass frequency.

Journal article

Otsubo M, Tanu Dutta T, Durgalian M, Kuwano R, O'Sullivan Cet al., 2019, Particle-scale insight into transitional behaviour of gap-graded materials – small-strain stiffness and frequency response, 7th International Symposium on Deformation Characteristics of Geomaterials, Publisher: EDP Sciences, Pages: 1-5

This study aims to develop a fundamental understanding of the role of fine particles on the small-strain stiffness of gap-graded granular soils. Stiffness was measured using cyclic triaxial probes, which give a measure of static stiffness, and dynamic wave propagation data, from which the dynamic stiffness can be measured. Assemblies of loosely packed spherical particles were considered. In the laboratory, local deformation transducers were used to measure the static stiffness, while the dynamic stiffness was calculated from stress wave velocities, measured using planar piezoelectric elements. To relate the particle-scale responses to the overall soil stiffness, complementary discrete element method (DEM) simulations were performed in which both static and dynamic stiffnesses were measured. Both the laboratory and the DEM data indicate that at low fines contents (< 30%) the stiffness decreases with increasing fines content. When the fines content increases from 30% to 35% there is a sharp increase in stiffness with increasing fines content; this is understood to mark the transition point at which the fines start to contribute significantly to the overall behaviour. Analyses of the frequency domain response of shear wave signals revealed that the lowpass frequency increases significantly at this transition point. This observation can be used to develop experimental interpretation protocols to assess to what extent fines are contributing to the overall soil stiffness.

Conference paper

Sufian A, Knight C, O'Sullivan C, Van Wachem B, Dini Det al., 2019, Ability of a pore network model to predict fluid flow and drag in saturated granular materials, Computers and Geotechnics, Vol: 110, Pages: 344-366, ISSN: 0266-352X

The local flow field and seepage induced drag obtained from Pore Network Models (PNM) is compared to Immersed Boundary Method (IBM) simulations, for a range of linear graded and bimodal samples. PNM were generated using a weighted Delaunay Tessellation (DT), along with the Modified Delaunay Tessellation (MDT) which considers the merging of tetrahedral Delaunay cells. Two local conductivity models are compared in simulating fluid flow in the PNM. The local pressure field was very accurately captured, while the local flux (flow rate) exhibited more scatter and sensitivity to the choice of the local conductance model. PNM based on the MDT clearly provided a better correlation with the IBM. There was close similarity in the network shortest paths, indicating that the PNM captures dominant flow channels. Comparison of streamline profiles demonstrated that local pressure drops coincided with the pore constrictions. A rigorous validation was undertaken for the drag force calculated from the PNM by comparing with analytical solutions for ordered array of spheres. This method was subsequently applied to all samples, and the calculated force was compared with the IBM data. Linear graded samples were able to calculate the force with reasonable accuracy, while the bimodal samples exhibited slightly more scatter.

Journal article

Taylor HF, O'Sullivan C, Shire T, Moinet WWet al., 2019, Influence of the coefficient of uniformity on the size and frequency of constrictions in sand filters, Géotechnique, Vol: 69, Pages: 274-282, ISSN: 0016-8505

Constrictions between voids control the filtration and permeability properties of granular materials. This study uses high-resolution microcomputed tomography images and discrete-element modelling to analyse two important characteristics of constrictions in granular filters: (a) the constriction size distribution (CSD) and (b) the constriction density per unit volume. The results demonstrate the importance of the particle size distribution (PSD) and void ratio of the granular material in determining the constriction density, with more widely graded materials having more densely spaced constrictions. The PSD is shown to be the main determinant of the CSD, in agreement with previous studies. The data are used to examine proposed approaches to estimate constriction spacing or void size.

Journal article

Su T-C, O'Sullivan C, Nagira T, Yasuda H, Gourlay Cet al., 2019, Semi-solid deformation of Al-Cu alloys: a quantitative comparison between real-time imaging and coupled LBM-DEM simulations, Acta Materialia, Vol: 163, Pages: 208-225, ISSN: 1359-6454

Semi-solid alloys are deformed in a wide range of casting processes; an improved understanding and modelling capability is required to minimise defect formation and optimise productivity. Here we combine thin-sample in-situ X-ray radiography of semisolid Al-Cu alloy deformation at 40–70% solid with 2D coupled lattice Boltzmann method - discrete element method (LBM-DEM) simulations. The simulations quantitatively capture the key features of the in-situ experiments, including (i) the local contraction and dilation of the grain assembly during shear deformation; (ii) the heterogeneous strain fields and localisation features; (iii) increases in local liquid pressure in regions where liquid was expelled from the free surface in the experiment; and (iv) decreases in liquid pressure in regions where surface menisci are sucked-in in experiments. The verified DEM simulations provide new insights into the role of initial solid fraction on the stress-deformation response and support the hypothesis that the behaviour of semi-solid alloys can be described using critical state soil mechanics.

Journal article

Ciantia MO, O'Sullivan C, Arroyo M, Gens Aet al., 2019, Breakage and critical state via DEM

Discrete-element simulations are used to explore the relation between breakage-induced grading evolution and the critical state line position on the compression plane. An efficient model of particle breakage is applied to perform a large number of tests, in which grading evolution is continuously tracked using a grading index. Using both previous and new experimental results, the discrete element model is calibrated and validated to represent Fontainebleau sand. The results obtained show that, when breakage is present, the inclusion of a grading index in the description of critical states is advantageous. This can be simply done using the critical state plane concept.

Conference paper

Liu D, O'Sullivan C, Carraro A, 2019, STRESS DISTRIBUTION IN TRIMODAL SAMPLES, 6th International Conference on Particle-Based Methods (PARTICLES) - Fundamentals and Applications, Publisher: INT CENTER NUMERICAL METHODS ENGINEERING, Pages: 58-67

Conference paper

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