Publications
224 results found
O'Sullivan C, O'Donovan J, Ibraim E, et al., 2016, Micromechanics of seismic wave propagation in granular materials, Granular Matter, Vol: 18, ISSN: 1434-7636
In this study experimental data on a model soil in a cubical cell are compared with both discrete element (DEM) simulations and continuum analyses. The experiments and simulations used point source transmitters and receivers to evaluate the shear and compression wave velocities of the samples, from which some of the elastic moduli can be deduced. Complex responses to perturbations generated by the bender/extender piezoceramic elements in the experiments were compared to those found by the controlled movement of the particles in the DEM simulations. The generally satisfactory agreement between experimental observations and DEM simulations can be seen as a validation and support the use of DEM to investigate the influence of grain interaction on wave propagation. Frequency domain analyses that considered filtering of the higher frequency components of the inserted signal, the ratio of the input and received signals in the frequency domain and sample resonance provided useful insight into the system response. Frequency domain analysis and analytical continuum solutions for cube vibration show that the testing configuration excited some, but not all, of the system’s resonant frequencies. The particle scale data available from DEM enabled analysis of the energy dissipation during propagation of the wave. Frequency domain analysis at the particle scale revealed that the higher frequency content reduces with increasing distance from the point of excitation.
Shire T, O'Sullivan C, Hanley KJ, 2016, The influence of fines content and size-ratio on the micro-scale properties of dense bimodal materials, Granular Matter, Vol: 18, ISSN: 1434-7636
This paper considers factors influencing the fabric of bimodal or gap-graded soils. Discrete element method simulations were carried out in which the volumetric fines content and the size ratio between coarse and fine particles were systematically varied. Frictionless particles were used during isotropic compression to create dense samples; the coefficient of friction was then set to match that of spherical glass beads. The particle-scale data generated in the simulations revealed key size ratios and fines contents at which transitions in soil fabric occur. These transitions are identified from changes in the contact distributions and stress-transfer characteristics of the soils and by changes in the size of the void space between the coarse particles. The results are broadly in agreement with available experimental data on minimum void ratio and contact distributions. The results have implications for engineering applications including assessment of the internal stability of gap-graded soils in embankment dams and flood embankments.
Taylor HF, O'sullivan C, Sim WW, 2016, Geometric and hydraulic void constrictions in granular media, Journal of Geotechnical and Geoenvironmental Engineering, Vol: 142, ISSN: 1943-5606
Constrictions in the void space between soil particles govern hydraulic conductivity, internal stability, and filtration performance of sands and gravels. Various analytical, numerical, and image-based methods have been proposed to measure void constrictions based solely on analysis of particle and void geometry. These geometric constrictions are increasingly being used in models to predict hydraulic conductivity or filtration performance. However, both of these phenomena depend not only on the void geometry, but also on the directions and magnitudes of fluid velocities within the void space. This paper presents computational fluid dynamics (CFD) simulations performed on microcomputed tomography (microCT) images of voids in real sands, as well as idealized materials generated by discrete element modeling (DEM). Laminar flow conditions are considered and an alternative definition of a void constriction is presented, the hydraulic constriction, which is based on fluid velocities rather than void geometry. The data show that for laminar flow, where Darcy’s law is applicable, the position, size, and orientation of hydraulic and geometric constrictions share many similarities, but there are measurable differences, which should be considered in hydraulic conductivity and filtration analyses.
Bernhardt ML, Biscontin G, O'Sullivan C, 2016, Experimental Validation Study of 3D Direct Simple Shear DEM Simulations, Soils and Foundations, Vol: 56, Pages: 336-347, ISSN: 0038-0806
Simple shear element tests can be used to examine numerous geotechnical problems; however, the cylindrical sample (NGI-type) direct simple shear (DSS) devices have been criticized for an inability to apply uniform stresses and strains, as well as the inability to fully define the stress state of the soil during shearing. Discrete element method (DEM) simulations offer researchers a means to explore the fundamental mechanisms driving the overall behavior of granular soil in simple shear, as well as improve understanding of the DSS device itself. Here three-dimensional DEM simulations of laminar NGI-type direct simple shear element tests and equivalent physical tests are compared to validate the numerical model. This study examines the sensitivity of the DEM simulation results to sample size, contact model and stiffness inputs, and ring wall boundary effects. Sample inhomogeneities are also considered by examining radial and vertical void ratio distributions throughout the sample. Both the physical experiments and the DEM simulations presented indicate that the observed material response is highly sensitive to the particle size relative to the sample dimensions. The results show that samples with a small number of relatively large particles are very sensitive to small changes in packing, and thus an exact match with the DEM simulation data cannot be expected. While increasing the number of particles greatly improved the agreement of the volumetric and stress-strain responses, the dense DEM samples are still initially much stiffer than the experimental results. This is most likely due to the fact that the inter-particle friction was artificially lowered during sample preparation for the DEM simulations to increase the sample density.
Hanley KJ, O'Sullivan C, 2016, Analytical study of the accuracy of discrete element simulations, International Journal for Numerical Methods in Engineering, Vol: 109, Pages: 29-51, ISSN: 1097-0207
The numerical errors in idealised discrete element method (DEM) simulations are investigated analyticallyby comparing energy balances applied at the beginning and end of one time-step. This study focuses onthe second-order velocity-Verlet integration scheme due to its widespread implementation in DEM codes.The commercial DEM software PFC2D was used to verify the correctness of key results. The truncationerrors, which are larger than the round-off errors by orders of magnitude, have a superlinear relationshipwith both the simulation time-step and the interparticle collision speed. This remains the case regardlessof simulation details including the chosen contact model, particle size distribution, particle density orstiffness. Hence, the total errors can usually be reduced by choosing a smaller time-step. Increasing thepolydispersity in a simulation by including smaller particles necessitates choosing a smaller time-step tomaintain simulation stability and reduces the truncation errors in most cases. The truncation errors areincreased by the dissipation of energy by frictional sliding or by the inclusion of damping in the system.The number of contacts affects the accuracy and one can deduce that because 2D simulations containfewer interparticle contacts than the equivalent 3D simulations, they therefore have lower accrued simulationerrors.
Perez JCL, Kwok CY, O'Sullivan C, et al., 2016, Assessing the quasi-static conditions for shearing in granular media within the critical state soil mechanics framework, SOILS AND FOUNDATIONS, Vol: 56, Pages: 152-159, ISSN: 0038-0806
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- Citations: 74
Huang X, O'Sullivan C, Hanley KJ, et al., 2016, Partition of the contact force network obtained in discrete element simulations of element tests, Computational Particle Mechanics, Vol: 4, Pages: 145-152, ISSN: 2196-4378
The transmission of stress within a granular material composed of rigid spheres is explored using the discrete element method. The contribution of contacts to both deviatoric stress and structural anisotropy is investigated. The influences of five factors are considered: inter-particle friction coefficient, loading regime, packing density, contact model, and boundary conditions. The data generated indicate that using the above-average normal contact force criterion to decompose the contact force network into two subsets with distinct contributions to stress transmission and structural anisotropy is not robust. The characteristic normal contact forces marking the transition from negative to positive contribution to the overall deviatoric stress and structural anisotropy are not unique values but vary during shearing. Once the critical state is attained (i.e., once shearing continues at a constant deviator stress and solid fraction), the characteristic normal contact force remains approximately constant and this critical state characteristic normal force is observed to decrease with increasing inter-particle friction. The characteristic normal contact force considering the contribution to deviatoric stress has a power-law relationship with the mean effective stress at the critical state.
Gens A, Arroyo M, Butlanska J, et al., 2016, Simulation of the cone penetration test: Discrete and continuum approaches, Pages: 125-134
The paper presents the modelling of the cone penetration test using two procedures: a discrete approach and a continuum approach. The discrete approach is based on the Discrete Element Method where a granular material is represented by an assembly of separate particles. Cone penetration has been simulated for both uncrushable and crushable sands. For the continuum approach, the Particle Finite Element Method has been adapted in order to overcome the difficulties posed by the occurrence of large displacements as well as by the geometrical, material and contact nonlinearities of the problem. Both single phase and two-phase (coupled hydromechanical) formulations have been developed and applied. Although not exempt of problems, both approaches yield realistic results leading to the possibility of a closer examination and an enhanced understanding of the mechanisms underlying cone penetration.
Taylor HF, O'Sullivan C, Sim WW, 2016, Interpreting filtration-based suffusion criteria using micro-computed tomography and laboratory filter tests, 8th International Conference on Scour and Erosion (ICSE), Publisher: CRC PRESS-BALKEMA, Pages: 515-521
Shire T, O'Sullivan C, Taylor HF, et al., 2016, Measurement of constriction size distributions using three grain-scale methods, 8th International Conference on Scour and Erosion (ICSE), Publisher: CRC PRESS-BALKEMA, Pages: 1067-1073
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- Citations: 4
Kawano K, O'Sullivan C, Shire T, 2016, Using DEM to assess the influence of stress and fabric inhomogeneity and anisotropy on susceptibility to suffusion, 8th International Conference on Scour and Erosion (ICSE), Publisher: CRC PRESS-BALKEMA, Pages: 85-94
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- Citations: 4
Shire T, O'Sullivan C, Hanley K, et al., 2015, Closure to "Fabric and Effective Stress Distribution in Internally Unstable Soils" by T. Shire, C. O'Sullivan, K. J. Hanley, and R. J. Fannin, JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING, Vol: 141, ISSN: 1090-0241
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- Citations: 1
O'Sullivan C, Hanley KJ, Huang X, 2015, Particle-scale mechanics of sand crushing in compression and shearing using DEM, Soils and Foundations, Vol: 55, Pages: 1100-1112, ISSN: 0038-0806
In this paper, the discrete element method is used to explore why differing amounts of breakage, quantified using Hardin's relative breakage parameter (Br), are associated with the critical state line (CSL) and the normal compression line (NCL) at similar stress levels. Virtual samples, initially containing more than 20,000 spherical particles, were isotropically compressed to a range of confining pressures up to 56 MPa and subjected to triaxial compression, both considering and disregarding particle crushing. A particle crushing model was developed for these simulations which is both computationally tractable and gives macro-scale results qualitatively in agreement with laboratory tests. The CSLs are both linear in q–p' space. A curved peak envelope, corresponding to a curved Mohr–Coulomb envelope, is obtained for the crushing simulations which is absent when crushing is disabled. Consideration of particle crushing reduces the peak stress, and the volumetric response is much more contractive with crushing at high p'. These simulations capture the behaviour in Br–p' space expected from published laboratory tests. The difference in behaviour along the NCL and CSL is explained by the larger fluctuations in contact force during triaxial shearing than during isotropic compression which was quantified using a newly-defined measure, the contact number ratio. Particle crushing continues after the critical state is attained, contributing to counteract the dilation induced by particle rearrangement.
Lopera Perez JC, Kwok CY, O'Sullivan C, et al., 2015, Numerical study of one-dimensional compression in granular materials, Geotechnique Letters, Vol: 5, Pages: 96-103
The discrete element method has been employed to simulate vertical one-dimensional compression of an idealised soil. Direct measurement of the full stress tensor was possible and the results show that K0 (the ratio of horizontal to vertical effective stresses) increases with void ratio, which is consistent with previous experimental studies. The anisotropic fabric induced during compression was quantified by considering the orientations and magnitudes of the normal contact forces. For the denser samples there was a definite bias towards more vertically oriented contacts, resulting in lower stresses being transmitted in the horizontal direction for a given vertical stress. In contrast, the contacts were oriented more isotropically in the looser samples, allowing more similar stresses to be transmitted in the horizontal and vertical directions.
O'Donovan J, O'Sullivan C, Marketos G, et al., 2015, Anisotropic stress and shear wave velocity: DEM studies of a crystalline granular material, Geotechnique Letters, Vol: 5, Pages: 224-230, ISSN: 2045-2543
Discrete element modelling (DEM) of a face-centred cubic assembly of spherical particles hasbeen used to study the influence of anisotropic stress states on the shear wave velocity of a granularmaterial. The shear waves were generated and detected in a way equivalent to the use of benderelements in laboratory testing. Comparisons are presented between the discrete element simulationsand analytical and empirically derived methods of relating stiffness to the degree of confiningstress anisotropy. The results confirm previous empirical observations that wave velocity is stronglyinfluenced by the stresses in the direction of propagation and in the direction of oscillation of the shearwave. The wave velocity is, however, largely independent of the stress orthogonal to the planecontaining the wave motion.
Otsubo M, O'sullivan C, Sim W, et al., 2015, Quantitative assessment of the influence of surface roughness on soil stiffness, Geotechnique, Vol: 65, Pages: 694-700, ISSN: 1751-7656
The nature of soil stiffness at small strains remains poorly understood. The relationship between soilstiffness (e.g. shear stiffness, G0) and isotropic confining pressure ( p′) can be described using a powerfunction with exponent (b), that is, G0¼A ( p′=pr)b, where A is a constant and pr is an arbitrary referencepressure. Experimentally determined values of b are usually around 0·5 and these are higher than thevalue of 0·33 that can be analytically determined using Hertzian theory. Hertzian theory considerscontact between two smooth, elastic spheres; however, in reality, inter-particle contacts in soil arecomplex with particle shape and surface roughness affecting the interaction. Thus Hertzian theory isnot directly applicable to predict real soil stiffness. It has, however, provided a useful basis to develop ananalytical framework to consider the influence of particle surface roughness on small-strain soilstiffness. Here, earlier contributions using this framework are extended and improved by payingparticular attention to roughness and the tangential contact stiffness. Stiffness values calculated usingthe newly derived analytical expressions were compared with the results of bender element tests onsamples of borosilicate glass beads (ballotini) whose surface roughness was quantified using an opticalinterferometer. The analytical expression captures the experimentally observed sensitivity of the smallstrainshear modulus to surface roughness
Perez JCL, Kwok CY, O'Sullivan C, et al., 2015, Numerical study of one-dimensional compression in granular materials, Geotechnique Letters, Vol: 5, Pages: 96-103, ISSN: 2049-825X
The discrete element method has been employed to simulate vertical one-dimensional compression ofan idealised soil. Direct measurement of the full stress tensor was possible and the results show that K0(the ratio of horizontal to vertical effective stresses) increases with void ratio, which is consistent withprevious experimental studies. The anisotropic fabric induced during compression was quantified byconsidering the orientations and magnitudes of the normal contact forces. For the denser samplesthere was a definite bias towards more vertically oriented contacts, resulting in lower stresses beingtransmitted in the horizontal direction for a given vertical stress. In contrast, the contacts were orientedmore isotropically in the looser samples, allowing more similar stresses to be transmitted in thehorizontal and vertical directions.
Taylor H, O'Sullivan C, Sim W, 2015, A new method to identify void constrictions in micro-CT images of sand, Computers and Geotechnics, Vol: 69, Pages: 279-290, ISSN: 0266-352X
Micro-Computed Tomography (micro-CT) provides 3D images of the internal structure of sands, allowingquantitative measurements of internal features, including void topology. Methods have already been proposedto measure constriction sizes from idealised particle arrangements or from micro-CT data, howeverthe 3D geometry of constrictions in sands is extremely complex and can be difficult to interpret usingexisting methods. This paper outlines a new method to measure and visualise void constrictions in sandsusing micro-CT data, with a view to assessing performance of granular filters. The method is based onwatershed segmentation of the void space. Synthetic data obtained from DEM simulations are used tovalidate the new algorithm and its performance against existing image-based methods is assessed byconsidering micro-CT data for a representative filter material.
Hanley KJ, O'Sullivan C, Wadee MA, et al., 2015, Use of elastic stability analysis to explain the stress-dependent nature of soil strength, Royal Society Open Science, Vol: 2, ISSN: 2054-5703
The peak and critical state strengths of sands are linearly related to the stress level, just as the frictional resistance to sliding along an interface is related to the normal force. The analogy with frictional sliding has led to the use of a ‘friction angle’ to describe the relationship between strength and stress for soils. The term ‘friction angle’ implies that the underlying mechanism is frictional resistance at the particle contacts. However, experiments and discrete element simulations indicate that the material friction angle is not simply related to the friction angle at the particle contacts. Experiments and particle-scale simulations of model sands have also revealed the presence of strong force chains, aligned with the major principal stress. Buckling of these strong force chains has been proposed as an alternative to the frictional-sliding failure mechanism. Here, using an idealized abstraction of a strong force chain, the resistance is shown to be linearly proportional to the magnitude of the lateral forces supporting the force chain. Considering a triaxial stress state, and drawing an analogy between the lateral forces and the confining pressure in a triaxial test, a linear relationship between stress level and strength is seen to emerge from the failure-by-buckling hypothesis.
O'Donovan J, O'Sullivan C, Marketos G, et al., 2015, Analysis of bender element test interpretation using the discrete element method, GRANULAR MATTER, Vol: 17, Pages: 197-216, ISSN: 1434-5021
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- Citations: 31
O'Sullivan C, Bluthe J, Sejpar K, et al., 2015, Contact based void partitioning to assess filtration properties in DEM simulations, COMPUTERS AND GEOTECHNICS, Vol: 64, Pages: 120-131, ISSN: 0266-352X
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- Citations: 24
Lopera Perez JC, Kwok CY, O’Sullivan C, et al., 2015, Static liquefaction and instability in granular media subjected to monotonic loading—a micromechanical investigation, Pages: 207-212, ISSN: 1866-8755
Static liquefaction has caused a number of failures involving dam tailings, hydraulically placed fills and slopes. In order to understand the failure mechanisms that induce static liquefaction, the discrete element method (DEM) was used to study the behavior of a representative model of a granular sample at the micro level. Samples with different void ratios at same confining pressures were sheared under constant-volume conditions and the changes in the macro and micromechanical responses of the granular media were quantified. Characteristic states such as the instability state, quasi-steady state, phase transformation and critical state were identified in the simulations. The transitions between different characteristic states were related to micromechanical characteristics such as coordination number. Finally, the orientation of the weak contacts was seen to be dependent on the characteristic state, while the orientation of the strong contacts coincided with the major principal stress direction.
Shire T, O'Sullivan C, Fannin J, et al., 2015, Use of discrete element modelling to assess the internal instability of dam filters, Pages: 687-692
Internal stability describes the ability of the coarse fraction of a broadly or gap-graded cohesionless soil to prevent the erosion of the finer fraction under seepage. Two conditions for internal instability are that: (i) the fine particles carry a lower stress than the coarse particles (hydromechanical condition) and (ii) the fine particles should be small enough to pass through the void constrictions between the coarse particles (geometric condition). A complete understanding of each of these mechanisms requires consideration of the fundamental particle-scale mechanics. Discrete element modelling (DEM) uses simplified particle geometries and ideal contact models to enable systems of particles to be modelled. This paper discusses the use of DEM to look at both the hydromechanical and geometrical conditions. The discussion on the hydromechanical condition considers the hypothesis of Skempton and Brogan (1994), i.e. that a prerequisite for internal instability is a reduction of the effective stress in the finer fraction of the soil. Particular emphasis is placed on the influnce of relative density. To assess the geometric condition a constriction size distribution (CSD) must be derived from the DEM analysis results. The data support the use of characteristic particle diameters in filter design.
O'Donovan J, Marketos G, O'Sullivan C, 2015, Novel methods of bender element test analysis, 3rd International Symposium on Geomechanics from Micro to Macro, Publisher: CRC PRESS-TAYLOR & FRANCIS GROUP, Pages: 311-316
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- Citations: 2
Bernhardt ML, O'Sullivan C, Biscontin G, 2015, Effects of sample preparation methods in DEM, 3rd International Symposium on Geomechanics from Micro to Macro, Publisher: CRC PRESS-TAYLOR & FRANCIS GROUP, Pages: 97-102
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- Citations: 1
Hanley KJ, O'Sullivan C, Huang X, 2015, Investigation of Christensen's two-parameter failure criterion for brittle materials, 3rd International Symposium on Geomechanics from Micro to Macro, Publisher: CRC PRESS-TAYLOR & FRANCIS GROUP, Pages: 129-134
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- Citations: 1
Otsubo M, Sim WW, O'Sullivan C, 2015, Experimental assessment of the influence of load-induced deformation on interparticle contacts, 6th International Symposium on Deformation Characteristics of Geomaterials, Publisher: IOS PRESS, Pages: 535-542
Hanley KJ, Huang X, O'Sullivan C, 2015, USING DELAUNAY TRIANGULATIONS TO INVESTIGATE THE EFFECT OF INTERPARTICLE FRICTION ON CRITICAL-STATE DEM SIMULATIONS, IV International Conference on Particle-based Methods (PARTICLES 2015), Publisher: INT CENTER NUMERICAL METHODS ENGINEERING, Pages: 890-899
Perez JCL, Kwok CY, O'Sullivan C, et al., 2015, Instability in granular materials - a micromechanical investigation, 3rd International Symposium on Geomechanics from Micro to Macro, Publisher: CRC PRESS-TAYLOR & FRANCIS GROUP, Pages: 135-139
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- Citations: 1
O'Sullivan C, 2015, Advancing geomechanics using DEM, 3rd International Symposium on Geomechanics from Micro to Macro, Publisher: CRC PRESS-TAYLOR & FRANCIS GROUP, Pages: 21-32
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- Citations: 8
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