Jiangu Qian

A micromechanics-based model for estimating localized failure with effects of fabric anisotropy

The shear strength of cohesionless granular materials is g enerally attributed to the com- pactness or anisotropy of their microstructure. An open issue is how such compact or anisotropic microstructures, and thus the shear strength, depend on the particle properties. We first recall the role of fabric and force anisotropies with respect to the cri tical-state shear stress. Then, a model of accessible geometrical states in terms of particle connect ivity and contact anisotropy is presented. This model incorporates in a simple way the fact that, due to steric exclusions, the highest levels of connectivity and anisotropy cannot be reached simultaneously, a property that affects seriously the shear strength. We also analyze the force anisotropy in the light of the specific role of weak forces in sustaining strong force chains and thus the main mechanism that underlies anisotropic force patterns. Finally, we briefly discuss the effect of int erparticle friction, particle shape, size polydispersity and adhesion. keyword: granular media; shear strength; fabric anisotropy; weak and strong forces.

The influence of traffic moving speed on shakedown limits of flexible pavements

ABSTRACT Shakedown theory is often used to analyse elastic–plastic responses of structures subjected to variable or repeated loads. In pavement engineering, it has been used to predict the maximum admissible load (termed as ‘shakedown limit’) against excessive rutting in flexible pavements. Pavement shakedown analysis, which involves the calculation of the pavement shakedown limit, usually utilised elastic stresses induced by a static wheel load and therefore neglected any possible dynamic responses. This paper will, for the first time, evaluate the dynamic effect of the moving load on the shakedown limit of flexible pavement. A numerical approach is developed based on a recent lower bound method for solving the pavement shakedown problem. The dynamic responses of elastic stresses to the moving traffic loads are computed using finite element method in which infinite elements are used for boundaries. Shakedown limits for a single subgrade layer and a pavement-subgrade system under traffic loads at various speeds are investigated. It is found that the shakedown limit is consistent with the static solution when the load moving speed is very low and it is reduced as the load moving speed is changed towards the speed of Rayleigh wave propagation. In the layered system, the shakedown limit increases with rising strength ratio and layer thickness until a maximum value is reached. The rise of stiffness modulus ratio can either increase or decrease the shakedown limit of the first layer due to the negative effect of stress redistribution and the positive effect of rising Rayleigh speed. When the stiffness modulus ratio is relatively small, the latter effect overwhelms the former one, or vice versa.

Dynamic Stress Responses to Traffic Moving Loading in the Saturated Poroelastic Ground

This work aims to gain an insight into the dynamic stress responses of subsoil subjected to traffic loading. By introducing scalar potential functions, Helmholtz decomposition theorem, Fourier transform and inverse Fourier techniques are used to achieve an analytical solution to the three-dimensional dynamic stress in the saturated poroelastic ground, subjected to a harmonic rectangular moving load. The stress responses to dynamic characteristic of moving loading - including frequency and moving velocity - are investigated. It was found that both loading velocity and loading frequency produce significant influence on the dynamic stress. Some critical velocity has been observed and tends to result in the strongest dynamic response of stress.

Discrete numerical modeling of granular materials considering crushability

The aim of this study is to numerically investigate the influence of particle breakage on the mechanical behavior of granular materials using a discrete element method (DEM). To enable particle crushing, non-crushable elementary particles are boned together to represents the granular aggregates which can be crushed when the external force exceeds its strength. The flaw of the aggregate was also modeled by randomly distributed void. Single particle crushing tests were carried out to determine the distribution of particle strength. The results of single particle crushing tests illustrate that the simulated single particle fracture strength and pattern agree well with the Weibull’s distribution equation. Conventional oedometer tests, drained monotonic and cyclic triaxial tests were also carried out to investigate the crushing of the aggregates and the associated mechanical behaviors. The effect of confining pressure on the crushing of aggregates and the mechanical behavior was also analyzed. It was found that the peak stress and dilation decrease significantly when particle crushing was considered. The deformation behavior of the specimen is essentially controlled by two factors: particle rearrangement - induced dilation and particle crushing - induced contraction. The increase of permanent strain and the reduction of dilation were observed during cyclic loading and they tend to reach a stable state after a certain number of cycles. The crushing of aggregate is most significant in the first two cycles. The results also indicated that for the same axial strain the volumetric strain and the bound breakage in the cyclic loading tests are significantly larger than that in the monotonic loading tests, especially at high cyclic stress ratio.

Experimental identification of plastic shakedown behaviors of saturated clay subjected to traffic loading

ABSTRACT: Traffic moving loading causes the rotation of principal stress axis and leads to a heart-shaped stress path in the deviatoric stress space. Using Dynamic Hollow Cylinder Apparatus (DHCA), a series of heart-shaped and triaxial cyclic undrained tests were performed on Shanghai saturated soft clay. The axial strain and its rate, energy dissipation are used to classify three cyclic characteristic behaviors, i.e., plastic shakedown, cyclic plastic creep and cyclic incremental failure. For the plastic shakedown response, the permanent axial strain becomes stable and the hysteretic loop area tends to reach a low constant level. For the cyclic plastic creep, axial strain rate and energy dissipation decrease with a small increment of axial strain, but the axial strain increment is tend to be constant after a large number of cycles. For the incremental failure response, the strain increment is growing and the energy dissipation increment increases after a limited number of cycles.

Dynamic shakedown limits for flexible pavement with cross-anisotropic materials

Shakedown solution is widely used to analyse the elastic-plastic behaviours of structures in pavement design. Previous studies only concerned the shakedown theorem under traffic loads for an isotropic material, although the material is usually anisotropic. This paper firstly proposed a numerical analysis for anisotropic materials under moving traffic load, based on Melan’s lower-bound shakedown theory. An anisotropic Finite Element–Infinite Element (FE–IE) model is used to calculate the dynamic stresses in the anisotropic material subjected to moving traffic loads with different speeds. Then, the shakedown limits for an anisotropic half-space and a two-layered pavement system are determined, respectively. It is found that the Rayleigh wave speed of the soil has a significant effect on the shakedown limits. For a cross-anisotropic half-space, the shakedown limits mainly depend on the stiffness ratio of the two layers and the Poisson’s ratio only has a small effect, although both of them significantly affect the Rayleigh wave speed. Furthermore, the shakedown limit increases with increasing cohesion ratio until it reaches a maximum value, and it gets rid of the control of shakedown condition when the moving speed exceeds the Rayleigh wave speed. For a two-layered anisotropic system, the results are similar to those in an isotropic system. Failure tends to occur on the top of the second layer instead of the first layer when the speed of moving load or the anisotropic Young’s modulus ratio increases, together with the decrease of shakedown limit.

Dynamic Shakedown of Cohesive-Frictional Materials Under Moving Traffic Load

Shakedown theory provides a rational tool for prediction of the long-term plastic behavior of pavement subjected to variable or repeated loads. A dynamic lower-bound shakedown solution has been proposed to estimate the critical shakedown limit load, over which plastic collapse or excessive permanent deformation of the pavement takes place. However, dynamic effects on the shakedown limit remains unexplored, particularly when rolling and sliding contact between vehicle and pavement are involved. In this paper, a finite-infinite (FE-IF) dynamic numerical method is presented to calculate the dynamic elastic stresses resulting from rolling and sliding contact at different moving speed for computing the shakedown limit. It is found that the shakedown limit decreases with the increasing moving speed initially and then turns to increase when the moving speed exceeds the Rayleigh wave speed of the pavement system. This dynamic effect is more profound as the horizontal force component reduces. The influence of frictional coefficient on shakedown limit is also discussed.

Investigation on Stress-Fabric Relation for Anisotropic Granular Material

In order to build a proper micro-mechanics-based constitutive model, the stress-fabric should be investigated in detail. A micromechanics-based constitutive framework is presented in this work first to predict the mechanics characteristic of anisotropic granular material. From the micromechanical aspect, the origin of deviatoric stress is decomposed into two components: contact force anisotropy and contact normal anisotropy. Based on past research, the rate of the reduced stress tensor is related with the rate of the fabric tensor. In this work, the relationship between them is verified by the DEM method. After derivation, the back stress is also related to the fabric tensor. Then the back stress is deduced from the stress-fabric-force relationship and determined with reference to the deviation of the principal directions between the reduced stress and its rate. The DEM simulation in this paper refers to Oda’s experiment to calibrate parameters. A series of three-dimensional simple shear tests simulated by DEM is first analyzed to study the stress-fabric relationship. The influence of fabric anisotropy on granular materials is specifically investigated.

Discrete element modelling of the influence of inherent anisotropy on the shear behaviour of granular soils

Abstract The laboratory tests had revealed that the fabric anisotropy plays an important role in the mechanical behaviour of the granular materials. However, the fabric anisotropy includes two aspects, the degree and the principle direction, and their contributions to the mechanical behaviour cannot be separated in the laboratory tests. In this study, granular soil specimens with different degrees and principle directions of inherent anisotropy were successfully produced. The mechanical behaviours of these specimens were numerically investigated by drained biaxial tests using discrete element method (DEM). Meanwhile, the evolutions of the microstructure, including coordination number, anisotropy of the contact normal and the clump orientation, were monitored during the shearing. It was found that not only the principal direction of inherent anisotropy can significantly influence the strength and the deformation characteristics, but also the degree of inherent anisotropy. The peak shear strength first decreases as the bedding angle increases and reaches the minimum value at a bedding angle of 60–75°. Meanwhile, the peak shear strength increases with increasing fabric anisotropy when the bedding angle is small, but decreases when the bedding angle is large.

The deformation of granular materials under repeated traffic load by discrete element modelling

Abstract The deformation of the gravel soil is an important component of the total settlement of the pavement under traffic load. The traffic load induces a heart-shaped stress path in the deviatoric stress space characterised by the rotation of principal stress. However, cyclic triaxial tests are usually carried out to investigate the deformation of gravel soil, which cannot account for the effect of principal stress rotation. In this study, discrete element method (DEM) is used to investigate the response of granular soil subjected to a heart-shaped stress path, and the results are compared with cyclic simple shear tests and biaxial tests. The results indicate that the accumulated volumetric strain of the soil increases with increasing cyclic stress ratio and initial confining pressure. Meanwhile, the accumulated volumetric strain changes from contractive to dilative when the cyclic stress ratio increases from 0.3 to 0.7. At the particulate level, the degree of fabric anisotropy increases and the principal direction of the fabric rotates accordingly in the cyclic loading. The results also show that the accumulated volumetric strain under heart-shaped stress path is much larger than that in cyclic biaxial tests, indicating the significance of principal stress rotation in soil deformation.