Y. P. Cheng

Micromechanical Study on Shear Wave Velocity of Granular Materials Using Discrete Element Methods

Shear wave velocity is a key parameter to evaluate the engineering properties of granular materials, like sand. It is generally obtained using piezoelectric transducers in various laboratory tests. The micromechanical study on shear wave velocity has become increasingly important to understand the microscopic mechanism of variation of the mechanical properties, as well as to bridge its micro- and macro-scale mechanisms. In this paper, both shear vibration and torsional vibration for shear wave propagation in DEM are implemented by applying a velocity pulse to the transmitter in a certain direction and monitoring the corresponding average velocity of the receiver. The cross-correlation analysis is adopted due to its superiority of both determining the travel time and identifying similarities between two signals. The shear wave velocity is calculated using the wave travel time and the distance of the travel path, in exactly the same way as in laboratory tests. On the basis of systematic parametric studies, reasonable values for the main parameters are proposed, including excitation frequency, excitation amplitude, size of transmitter and receiver as well as damp. For example, it is found that the appropriate excitation amplitude should be chosen on the basis of avoiding the generation of frictional work. The numerical methods are verified through the outcomes of even-particle assemblies for both regular and random packing, and also compared with the results from various homogenization techniques.

Discrete Element Simulation of Granular Column Collapse

Many natural hazards and industrial processes involve the collapse of granular particles onto a horizontal plane. Recent researches have studied the fundamental physics of the collapse of granular columns in experimental and numerical approaches. This paper presents a three‐dimensional discrete element simulation of the axisymmetric spreading of initially vertical granular columns, in which the runout of the grains and their dynamic motion are continuously monitored during the course of collapse. Using a polar coordinate method to quantify the spread of the grains, the numerical results are in good agreement with previous research findings. The collapse dynamics is shown to be dependent on the initial geometry of the cylindrical column and independent of the inter‐granular friction. Two distinct flow regimes are observed: a linear scaling and a power‐law scaling are derived for the final runout distances of the columns. The problem is further explored by studying the effects of coefficient of restitution ...

The influence of void ratio on small strain shear modulus of granular materials: A micromechanical perspective

The small strain shear modulus Gmax of granular materials is highly dependent on their current void ratio and stress state, generally expressed as the famous Hardin and Richart equation. Various forms of void ratio functions have been proposed, either based on experimental or theoretical research. It is noted that each of them can be applied for a certain soil within a limited void ratio range. Micromechanical studies on the influence of void ratio on Gmax are conducted in this paper, using Discrete Element Method. After each sample being isotropically consolidated, shear wave velocity is measured by applying a velocity pulse to the transmitter in a certain direction, and monitoring the corresponding average velocity of the receiver. The capabilities of various existing void ratio functions are examined, together with the relationship between coordination number and void ratio, distribution of coordination number, as well as the contact force network. The void ratio effect on Gmax is further explained in ...

Micro-characteristics of monodisperse and best-packing mixture samples under one dimensional compression

One dimensional compression behavior of poorly graded soils and their mixtures have been investigated using the discrete element method PFC3D software package with spherical particles and a size-related linear elastic contact model. Simulation results show that, for soil samples of the same grading that were compressed from different initial solid fractions, there exists a unique relationship between solid fraction and vertical stress at high stress levels. Contact force distributions of these different samples fall on a similar distribution curve in the same stress range. There is also a convergence for the size distributions of particles involved in the transmission of greater-than-average (strong) contact forces in each system. The results are mainly presented for a sand-like monodisperse material and a best packing efficiency mixture material. Cumulative contributions of individual contact forces to the deviator stress were calculated and the results show that the deviator stress is primarily attribut...

Mechanical Behaviour of Methane Hydrate Soil Sediments Using Discrete Element Method: Pore-filling Hydrate Distribution

Methane hydrate bearing soil is usually found under deep seabed and permafrost regions. It attracts research interest as a possible energy resource, but it also has potential impacts on climate change and geotechnical issues during methane gas production. Due to the limitations of laboratory studies, in this research, Discrete Element Method (DEM) simulations were performed to provide unique insights into the mechanical behaviour of hydrate-bearing sediments with pore-filling hydrate distribution. A series of drained triaxial shearing tests were systematically conducted to study the effects of hydrate saturation on the hydrate-bearing samples. It is shown that the peak shear strength increased and dilation was enhanced as hydrate saturation increased, especially when the hydrate saturation was above 20%. However, the critical state shear strength reduced slightly when hydrate saturation increased from 20%, with the dilation being reduced to zero in the critical state. The hardening effect of hydrate at the peak strength and the softening behaviour of samples in the critical state also reflected the peak and critical state friction angles. The strength of samples was enhanced with increasing confining pressure. It is found that for pore-filling hydrate distribution, the hydrate contribution to the strength of the sediments is of a frictional nature.

Study on small-strain behaviours of methane hydrate sandy sediments using discrete element method

Methane hydrate bearing soil has attracted increasing interest as a potential energy resource where methane gas can be extracted from dissociating hydrate-bearing sediments. Seismic testing techniques have been applied extensively and in various ways, to detect the presence of hydrates, due to the fact that hydrates increase the stiffness of hydrate-bearing sediments. With the recognition of the limitations of laboratory and field tests, wave propagation modelling using Discrete Element Method (DEM) was conducted in this study in order to provide some particle-scale insights on the hydrate-bearing sandy sediment models with pore-filling and cementation hydrate distributions. The relationship between shear wave velocity and hydrate saturation was established by both DEM simulations and analytical solutions. Obvious differences were observed in the dependence of wave velocity on hydrate saturation for these two cases. From the shear wave velocity measurement and particle-scale analysis, it was found that the small-strain mechanical properties of hydrate-bearing sandy sediments are governed by both the hydrate distribution patterns and hydrate saturation.

Strong Force Network of Granular Mixtures Under One-dimensional Compression

We studied one dimensional compressive behaviour of gap-graded granular mixtures, using the numerical simulations of the three-dimensional Discrete Element Method (DEM). Spherical particles of different size ranges were generated inside a cubical box to numerically create granular samples with the particle size distributions of a sand-like material mixed with different proportions of a uniform silt-size material. The DEM samples were subjected to one-dimensional compression similar to the loading path of the oedometer test in soil mechanics. Micromechanical characteristics behind the compressibility of granular mixtures were investigated with an emphasis on the strong force network. Unlike monodisperse assemblies, the deviatoric stress is mainly attributed to contact forces with a magnitude greater than the mean contact force of the system, the deviatoric stress of these mixture materials is attributed to a combination of the different contact types between different particle sizes. There is a primary shear-strength related load-bearing contact network, and this depends on the relative proportion of the silt particles in the system. For systems with a very little amount of silt, mainly consisting of sand particles, the main contribution to deviatoric stress is due to the contacts between the sand particles. On the other hand, for systems that have the void space between sand particles efficiently occupied by silt particles, all contact types contribute to the deviatoric stress. If strong forces of a particulate system are those responsible for the deviatoric stress, the characteristic force - that separates strong and weak forces - depends on the contact-type, the particle size distribution and the stress level; the value of the overall characteristic force however is not far from the mean force value of the system.