Xiaoqiang Gu

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.

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.

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.

Discrete Element Analysis of the K0 of Granular Soil and Its Relation to Small Strain Shear Stiffness

AbstractThe discrete element method (DEM) was used to investigate the coefficient of earth pressure at rest, K0, of granular soils. The results indicate that K0 decreases as the void ratio decrease...

Theoretical Prediction of Strain Localization in Anisotropic Sand by Non-coaxial Elasto-Plasticity

Due to the interaction of stress state and soil micro-structure, the onset of strain localization in anisotropic sand is different from that in isotropic sand. In order to accurately predict the onset of strain localization in inherent anisotropic sands under multi-dimensional stress condition, a state-dependent critical state constitutive model was proposed. The anisotropic critical state line was modified by incorporating micro-structure information, termed as fabric anisotropy. The model was shown to be able to capture influences of loading direction and intermediate principal stress ratio on stress-strain relationships and volumetric characteristics. Through the integration of the rate-form stress-strain relationship, bifurcation analysis was employed to predict the onset of strain tests. The results showed that the major principal strain at the bifurcation points increases with the deposition angle, while the stress ratio decreases with the angle varying from 0° to around 60° and increases afterwards in plain strain tests. Overall, the predicted shear stress at bifurcation points compare well with the stress peak in experiments.

Laboratory Measurements of the Dynamic Properties of Shanghai Clay

It is well agreed that the dynamic properties including the dynamic shear modulus and damping ratio play essential role in many dynamic geotechnical problems. In this paper, the dynamic properties of the typical Shanghai clay were determined by resonant column tests on 38 undisturbed samples obtained from 9 site investigations in Shanghai. The results showed that the dynamic small strain shear modulus G 0 of the soil significantly depends on the void ratio and the effective confining pressure. Meanwhile, the ratio of shear modulus at a certain shear strain level to the small strain shear modulus G/G 0 (i.e. modulus reduction curve) decreases and the damping ratio increases continuously as the shear strain increases, indicating the nonlinear behavior of clay under dynamic loading. Meanwhile, the modulus reduction curve and the damping curve are generally independent of the void ratio and the effective stress. Empirical equations were provided to predict the small strain shear modulus, the modulus reduction curve and damping ratio of Shanghai clay in practical engineering.

Influence of Inherent Anisotropy on the Soil Behavior in Simple Shear Tests Using DEM

The discrete element method was adopted in this paper to numerically investigate the macroscopic behaviors of granular material with initial anisotropic fabric in simple shear tests. The evolutions of the stress-strain relationship, the non-coaxiality referring to the difference of principle directions between the strain rate and the stress increment, and the fabric anisotropy were analyzed during the tests. The results indicated that inherent anisotropy affects the shear strength. The directions of principal strain rate and principal stress are non-coaxial at the start of the shear loading and they become coaxial gradually as the shear strain increases. Both the principle directions of particle orientation and contact normal change towards 45°, but the later is much faster than the other.

Micromechanics-Based Constitutive Modeling and DEM Simulation of Localized Failure in Soil

To account for the fabric effect on the localized failure, the fabric tensor describing the soil fabric is related to the stress tensor and a micromechanics-based isotropic-kinematic model is developed in this study. With this model, the effect of fabric anisotropy on the onset of localization and the angle of shear band, is investigated and compared with the experimental results. Meanwhile, discrete element method (DEM) simulation is carried out to study the localized failure of granular soil and the evolution of fabric anisotropy during shearing. The numerical results are compared with the theoretical predictions by the micromechanics-based model. This study successfully illustrates the importance of considering the fabric anisotropy in constitutive modeling of the localized failure in granular soil.