Effect of Particle Shape on the Mechanical Behaviour of Granular Media : Discrete Element Simulations

Abstract

Granular materials are characterized by its discrete nature which makes their behaviour very complex to understand when subjected to various loading situations. Comparing other materials, the understanding of granular materials is poor. This is because experimental analysis provides the macroscopic responses of the considered assembly whereas the discrete nature of the particles point to the fact of understanding the micro scale details and correlating it with the overall behaviour. Among the various modeling tools viz. analytical, physical or numerical, Discrete Element Method (DEM) a numerical technique, originally developed by Cundall (1971, 1974) and modified by Cundall and Strack (1979a, 1979b) is widely used for granular materials. Later a thorough description of DEM was given by Cundall (1988) and Hart et al (1988). Moreover Cundall & Hart (1992) reported discrete element code as one which allows finite displacements and rotations of discrete bodies along with recognition of new contacts as the calculation progresses which is followed in particle flow code and is used for this study. Generally the discrete particles are modeled as discs or spheres in 2-D and 3-D simulations respectively. The discs or spheres were considered as it is very easy to characterize the grain interactions and the contact detection. Even though the significance of particle shape has been reported in literature, a comprehensive 3-D study of the effect of particle shape on the various aspects of soil behaviour is lacking and is not reported. Particle shape is generally defined in terms of elongation, roundness and texture. Elongation is an indication of the particle aspect ratio whereas roundness measures the sharpness or angularity of particle’s edges and corners. Texture is related to the roughness of the surface. Particle gradation also plays a role in the mechanical behaviour. The influence of each of these factors on the mechanical behaviour of the assembly is important. Hence the major factors like elongation, texture etc which are used to define the particle shape are incorporated in this study. Evaluating the particle shape is another hurdle. In this study, the particle shape is analyzed using a 3D laser scanner which helps to identify the major and minor axis lengths of the particle. Additionally, the effective use of 3D DEM on large scale real life applications incorporating the particle shape effect is also not dealt with very extensively. Hence in this research, a comprehensive study on the calibration of DEM using glass beads, effect of particle shape on drained and undrained monotonic behaviour, liquefaction, post liquefaction and dynamic properties and the application of DEM to a grain polishing machine and an underground tunnel assembly is presented. In the present study, a set of drained triaxial tests were done on glass bead assembly using a laboratory triaxial set up. The glass beads used for the test were spherical and ellipsoidal in shape. The shape of glass beads was characterized through a sophisticated method of 3D laser scanning. In this scanning, the shape of the image of the glass bead is captured through an array of digitized points. These images obtained as unstructured 3D triangular meshes on processing will render the long and short axes of the particle which can be used for proper modeling of the particle shape. After obtaining the long and short axes for the particles, the same is used for the numerical modeling of the glass beads. The numerical simulation results have shown that the assembly modeled with clumped particles gives results qualitatively and quantitatively similar to the observed experimental macro responses. Hence this is used to demonstrate the power of DEM to realistically model the granular behaviour by incorporating the particle shape effect. In addition, undrained simulation of granular material has been numerically predicted from drained triaxial tests which are similar to the approach proposed by Norris et al (1997). An excellent correlation between undrained results predicted from drained triaxial test and undrained test (performed under constant volume conditions) has been observed. This further underlines the fact that the constant volume simulations are equivalent to undrained tests. Having validated the DEM results to the values obtained from the experiments on glass beads, a series of monotonic drained and undrained triaxial tests were performed on cylindrical assemblies of height to diameter ratio 2:1. Four different sets of assemblies were prepared which consists of particles of different aspect ratio to study the influence of particle shape. The behaviour of these assemblies under drained shearing indicates that the strength of the clumped assemblies is higher than that of the spherical assembly at all confining pressures. This has been explained from the magnitude of the anisotropic vi coefficients associated with the fabric and normal contact force tensors. It is also noted that eventhough both assemblies reach the peak strength at the same axial strain, the strain softening associated with the clumped assembly is very rapid which is due to the fact that clumps try to push each other apart as it offers more resistance to rotation resulting in dilation. Another significant observation is that a general increase in aspect ratio will not keep on increasing the strength. As the aspect ratio increases, the particles have a tendency to orient along their larger dimensions. This helps them to attain the lowest potential energy leading to a stable equilibrium and resulting in inherent fabric anisotropy. But when the particles try to align along the larger dimension, the formation of strong contact forces along the direction of loading is hindered. In addition, the lower strength associated with the higher aspect ratio particle assembly can also be attributed to the formation of unexpected void spaces when these longer particles bridge gaps over the underlying grains. The critical state studies indicate that the clumped assembly is having a higher residual strength compared to that of the spherical assembly. In the case of clumped assemblies also, irrespective of the initial loose or dense state of the assembly and the confining pressure applied, the samples reached the same critical state which underlines that the critical state is unique for a granular material. Moreover, the critical state line is non-linear for both the spherical and clumped assemblies. The studies conducted on the liquefaction behaviour indicates that at lower confining pressures the assemblies with particles consisting of lower aspect ratios loses its strength at less number of cycles which can be attributed to the interlocking of non-spherical particles resulting in higher number of contacts per particle. Moreover, during the initial cycles of loading, it is seen that the strong contacts are predominantly in the vertical direction or more precisely along the direction of maximum axial strain. But the plots extracted at higher cycles indicated that the strong contacts along the vertical direction have diminished considerably. This reduction in contact force magnitude and force chain indicates a drop in the number of contacts and is clearly visible in the gradual decrease of average coordination number. Another significant observation is that as the confining stresses increases, the rate of pore pressure generation of the assembly vii consisting of only spherical particles is less compared to the other two samples. Furthermore, at higher confining pressures, when the load direction reverses, the fabric of the clumped assemblies fails to change to a new orientation immediately. But to retain equilibrium the force anisotropy will quickly adjust itself. This mismatch results in losing the contacts and resulting in lower strength and less resistance to liquefaction at higher stresses for assemblies consisting of clumped particles. The post liquefaction study of the numerically liquefied samples shows that the assembly consisting of clumped shaped particles gained strength at a much faster rate compared to the assembly consisting of only spheres. This may be attributed to the ability of the clumps to rearrange themselves on a faster rate compared to that of the spherical particles. The rate of development of average coordination number is very significant as it explains the ability of the assembly to build up the deviatoric stress from a complete collapsed structure. As the contacts develop, the average coordination number as well as the deviatoric stress starts increasing with both the values higher for the assembly consisting of clumped particles. The evaluation of the dynamic properties viz. shear modulus and damping ratio showed a trend similar to the experimental observations on real granular materials. It is observed that the normalized shear modulus reduces with an increase in shear strain and the rate of reduction is very high at low strains for all the samples. It can be seen that as the confining pressure increases, the normalized shear modulus value also increases and the rate of increment is higher for the assemblies consisting of non-spherical particles. Furthermore, for all the samples the threshold shear strain is about 0.001 up to which the behaviour is elastic. Beyond the threshold shear strain, the variation of the normalized shear modulus ratio is non-linear. At small shear strains, the energy dissipation is low resulting in smaller values of damping. As the strains increase, the non-linearity in the constitutive behaviour results in higher material damping leading to high damping value. The simulations of the food polishing machine helps to understand the pattern of hitting of clumped grains on the wall with due importance to the velocity of hit, angle of hit, force of hit, and the number of grains hitting the wall. The modeling and subsequent extraction of the data helped to identify that the wear and tear of the machine was not uniform and was clustered to specific regions due to the non-uniform distribution of the considered parameters. This helped to design a more sophisticated system such that the parts which are subjected to more deterioration are provided with additional support. To bring out the effect of the particle shape, simulations are performed using spherical particles and the results show that the pattern of variation is same, but the magnitudes are different owing to the less surface area associated with the spherical particles. The 3-D simulations of an underground tunnel assembly in a weak weathered rock helped to understand the variation in the stability of the system with and without lining. It was observed that the introduction of lining resulted in a more stable configuration and the circumferential stresses were found to be distributed uniformly around the tunnel. FEM simulations also show a similar trend of stress and strain variations but were unable to capture the ground loosening around the tunnel and the formation of the ground arch whereas DEM could realistically capture all these phenomena. It was observed that as the shape changes from sphere to non-spherical particles, circumferential stresses around the tunnels increased. In addition, as the distance from the tunnel face increases, the strains are reduced. The maximum vertical strain near the crown of the tunnel is observed for the assembly consisting of spherical particles. In short, this research is focused on a comprehensive understanding of the particle shape effect on the mechanical behaviour of granular mass. Numerical simulations incorporating the shape effect has been done on drained and undrained monotonic shear tests, critical state, liquefaction, post liquefaction and dynamic properties. Besides, the granular dynamics simulation of the movement of long food grains in a food polishing machine and the behaviour of an underground tunnel in a granular assembly is also reported.