Zhen-Yu Yin

An elasto-viscoplastic model for soft clay

The purpose of this paper is to present the development of an elasto-viscoplastic model for normally consolidated clay. A three dimensional constitutive equation was derived based on the framework of Perzynas overstress theory and Modified Cam-Clay model. It was implemented in a finite element program, and applied to analyze time-dependent behavior of soft clay. The influence of two viscous parameters, which can be determined by undrained triaxial compression tests, was studied. The proposed model was verified using observed laboratory responses of several natural clays. The results showed that the model can satisfactorily describe the stress-strain-rate behavior such as strain rate effect, creep and stress relaxation.

A finite element modeling of the impact of internal erosion on the stability of a dike

This paper presents a numerical model of internal erosion based on the approach of continuous porous media. The soil skeleton saturated by a pore fluid is treated as the superposition of four constituents in interaction: solid skeleton, erodible fines, fluidized particles, and fluid. The detachment and transport of fine particles are modeled by the mass exchange between solid and fluid phases. In order to take into account the influence of the change of void ratio induced by internal erosion on the soil skeleton, a critical state based constitutive model is used to calculate the effective stress-strain response of the soil skeleton. This coupled hydro-mechanical analysis is applied to study the impact of erosion on a dike. The numerical simulations show the progressive development of internal erosion within the foundation of the dyke. The effect of the mechanical degradation due to internal erosion is demonstrated by evaluating the factor of safety of the dyke slope by means of the shear strength reduction method.

Numerical study of crushable granular materials

The paper aims to develop an elasto-plastic model considering particle crushing. For this purpose, the relationship between the grain size distributions, the position of the critical state line and the plastic work is investigated using experimental results on Cambria sand. The constitutive equations describe two yield mechanisms (isotropic and deviatoric) accounting for particle crushing, which in return affects the position of the critical state line in the e – log(p) diagram. The model is then used to simulate triaxial tests under different loading conditions (drained compression and extension) with high stress levels. All comparisons between experimental results and simulations demonstrate that the model can reproduce with good accuracy the mechanical behavior of granular materials with particle crushing.

Multi-Scale Modeling of the Mechanical Behaviour of Clays

Abstract: Under a microstructural approach for stress-strain modeling, a set of microsystems (e.g. inter-particle contacts) is used to represent the material. Material properties are defined for each microsystem and the overall stress-strain relationship for the material is obtained from averaging the behaviors of this set of microsystems.

Behavior of Granular Materials Affected by Grain Breakage

Abstract: In geotechnical and particularly in dam engineering, observations in the field and, in some cases, dramatic failures of structures have often incited research and increased knowledge. For instance, the first rockfill dams were elevated without any prior knowledge of the mechanical capacities of the rockfill mass. As natural and local resources are usually preferred for economic and environmental issues, their non-optimal properties can lead to potential severe damage to the hydraulic structure. Similarly, a basic principle of construction of rockfill dams or embankments consists in sluicing rockfill after deposition and before compaction: this recommendation has been adopted for a long time. However, the beneficial effects of such a practice have been understood only in light of recent studies on unsaturated soil and multi-scale analyses.

Strain-Rate Dependency of Shear Strength for a Highly Overconsolidated Clay

The paper summarizes the results from undrained triaxial compression tests on Merville clay, a highly overconsolidated clay at Merville in the north of France. The specimens were prepared in four different overconsolidation ratios (OCR=7, 14, 28, 56), and for each OCR, undrained shear tests were performed using three or four axial strain rates varying from 0.26 %/h to 25.89 %/h. The strain-rate dependency of the undrained stress level q/qf, the pore pressure, the effective stress path and the stress ratio q/p were investigated. The undrained shear strength was found to increase with the strain rate. The shear band formation during loading was observed for all strain rates. For each OCR test series, a linear relationship between the normalized peak deviatoric stress and the strain rate was obtained in the semi- logarithm plane. As OCR increasing, the slope of best-fitting line for the linear relationship increases. Therefore, the strain rate influence on the normalized undrained shear strength is greater for higher OCRs.

Clay Subjected to Cyclic Loading: Constitutive Model and Time Homogenization Technique

Numerical modeling of the behavior of geotechnical structures subjected to cyclic loading requires the consideration of two main aspects: the constitutive relationship and numerical strategy. The constitutive model has to be representative of the clay behavior, whereas the numerical strategy should be efficient in order to reduce the computation time. To simulate undrained triaxial cyclic tests on clay, a method of time homogenization is applied to a bounding surface plasticity model. This method of homogenization is based upon splitting time into two separate scales. The first scale relates to the period of cyclic loading and the second to the characteristic time of the material. Simulations of undrained triaxial cyclic tests on normally consolidated clay under one-way cyclic loading are carried out. The performance of time homogenization is numerically validated. The sensitivity of the time increment is also investigated.