Instability of gradient dependent elasto-viscoplasticity for clay

A gradient-dependent viscoplastic constitutive model for water saturated clay is proposed to describe the strain localization phenomena and pattern formation during deformation. Second- and fourth-order gradients of volumetric viscoplastic strain are introduced into the constitutive equations to account for the non-local effects due to the motion of microstructures. A linear perturbation analysis is applied to this model. The instability of the government equations (i.e. the constitutive equations and the equations of motion for the clay skeleton and pore water) is discussed for both the one-dimensional and the two-dimensional situations. In addition, issues concerned with the formulation of boundary value problems by finite element analysis in relation to the formulation and the boundary conditions are presented. Copyright © 1999 John Wiley & Sons, Ltd.     

Identification of non‐linear stress–strain–time relationship of soils using genetic algorithm

Recognition of non‐linear constitutive rock/soil model from experimental results is often multi‐modal in the large parameter space. A genetic evolution algorithm is thus proposed for its recognition, including that of structure of the model and coefficients in the model. The structure of the model can be firstly determined according to mechanical mechanism if the mechanism is clearly understood or searched by using evolutionary algorithm. The coefficients to be determined are then searched in global optional space. With the new evolutionary algorithm, the non‐linear stress–strain–time constitutive law to describe strain softening behaviours of diatomaceous soil under consolidated and undrained state was recognized by learning stress–strain–time behaviour of an intact sample under consolidated pressure of σ_c=0.1 MPa and strain velocity of_a=0.175%/min. This model gave reasonable prediction for diatomaceous soils under varying consolidated pressures (0.1–3.5 MPa) and strain velocities (0.0044–1.75%/min). It indicates that the methodology proposed in this paper is robust enough and strongly attractive for recognition of non‐linear constitutive model of soil and rock materials. Copyright © 2002 John Wiley & Sons, Ltd.     

Mechanized tunneling induced ground movement and its dependency on the tunnel volume loss and soil properties

This study investigates the ground movements due to mechanized tunnel excavation by applying two-dimensional finite element analyses. To assess the contribution of the compressibility and plasticity of the soil on the ground movements, different constitutive models are employed to describe the soil behavior. The influence of volume loss around the tunnel on the surface volume loss is investigated, and a quadratic correlation between them is proposed. Consequently, the empirical Gaussian distribution curve, which is generally used to determine the tunneling induced settlement trough, is improved by applying the proposed quadratic correlation between surface volume loss and tunnel volume loss. Furthermore, the settlement trough width parameter has been derived by a linear function of tunnel volume loss as well. The proposed equations are validated via a case study of centrifuge tests from the literature. The results show that the proposed modification enhances the empirical solution by having better knowledge on the model parameters. Additionally, tunnel overburden and coefficient of lateral earth pressure at rest (K₀) are taken into account to study their influence on the tunneling induced surface settlements. Finally, global sensitivity analysis is applied to evaluate the relative importance of corresponding model parameters in terms of their influence on the ground movements.     

超固结土热黏弹塑性本构关系

热黏弹塑性本构模型是描述土在温度（热）和时间（黏）耦合作用下的应力-应变关系的本构模型。在一些新型岩土工程诸如高放核废料地质处置、地热资源开发与贮存的建设中,需要同时考虑温度和时间对土的影响,所以建立一个热黏弹塑性本构模型具有理论和实际意义。将温度变化对黏土体积和强度参数的影响引入笔者之前提出的超固结土等向应力-应变-时间关系,建立了一个等向应力条件下的应力-应变-时间-温度关系。随后,基于该关系推导了屈服面硬化定律,并将其与超固结土统一硬化模型的屈服方程和流动法则结合,建立了超固结土的热黏弹塑性本构模型。最后,使用新模型预测室内试验,证明新模型能够反映时间和温度对土体积、一维压缩曲线和前期固结压力的耦合影响。     

Study on the Electrical Conductivity of 0.025 mol NaCl Solution at 0.25–3.75 GPa and 20–370°C

The electrical conductance of 0.025 mol NaCl solution was measured at 0.25–3.75 GPa and 20–370°C As shown by the results, the conductance increases with temperature, and there is a liner relation between the reciprocal of temperature and the logarithm of the conductance but their slopes are different at different pressures. The relations between the conductance and pressure is rather complex and there are some discontinuities: in the range of 2.25–3.75 GPa, the conductance increases with the pressure; in the range of 1.25–2.0 GPa, the conductance is not related to the pressure; and at a pressure of 0.75 GPa, the conductance is higher than that at the pressures nearby. This reflects that the NaCl solution has rather different properties of electronic chemistry at various pressures, and probably is an important cause for the existence of the layers with high electrical conductance and low velocity in the Earth's crust and mantle.     

Implementation of a hydromechanical elastoplastic constitutive model for fully coupled dynamic analysis of unsaturated soils and its validation using centrifuge test results

Prediction of unsaturated soil behavior during earthquake loading has received increasing attention in geotechnical engineering research and practice in recent years. Development of a fully coupled analysis procedure incorporating a coupled hydromechanical elastoplastic constitutive model for dynamic analysis of unsaturated soils has, however, been limited. This paper presents the implementation of a coupled hydromechanical elastoplastic constitutive model into a fully coupled dynamic analysis procedure and its validation using a centrifuge test. First, the fully coupled finite element equations governing the dynamic behavior of unsaturated soils with the solid skeleton displacement, pore water pressure, and pore air pressure as nodal unknowns are briefly presented. The closest point projection method is then utilized to implement the coupled hydromechanical elastoplastic constitutive model into the finite element equations. The constitutive model includes hysteresis in soil–water characteristic curves, cyclic elastoplasticity of the solid skeleton, and the coupling mechanisms between the SWCCs and the solid skeleton. Finally, the analysis procedure is validated using the results from a dynamic centrifuge test on an embankment constructed of compacted unsaturated silt subjected to base shaking. Reasonable comparisons between the predicted and measured accelerations, settlements, and deformed shapes are obtained.

     

On finite volume method implementation of poro‐elasto‐plasticity soil model

Accurate prediction of the interactions between the nonlinear soil skeleton and the pore fluid under loading plays a vital role in many geotechnical applications. It is therefore important to develop a numerical method that can effectively capture this nonlinear soil‐pore fluid coupling effect. This paper presents the implementation of a new finite volume method code of poro‐elasto‐plasticity soil model. The model is formulated on the basis of Biot's consolidation theory and combined with a perfect plasticity Mohr‐Coulomb constitutive relation. The governing equation system is discretized in a segregated manner, namely, those conventional linear and uncoupled terms are treated implicitly, while those nonlinear and coupled terms are treated explicitly by using any available values from previous time or iteration step. The implicit–explicit discretization leads to a linearized and decoupled algebraic system, which is solved using the fixed‐point iteration method. Upon the convergence of the iterative method, fully nonlinear coupled solutions are obtained. Also explored in this paper is the special way of treating traction boundary in finite volume method compared with FEM. Finally, three numerical test cases are simulated to verify the implementation procedure. It is shown in the simulation results that the implemented solver is capable of and efficient at predicting reasonable soil responses with pore pressure coupling under different loading situations. Copyright © 2015 John Wiley & Sons, Ltd.     

Implicit integration of a mixed isotropic–kinematic hardening plasticity model for structured clays

In recent years, a number of constitutive models have been proposed to describe mathematically the mechanical response of natural clays. Some of these models are characterized by complex formulations, often leading to non‐trivial problems in their numerical integration in finite elements codes. The paper describes a fully implicit stress‐point algorithm for the numerical integration of a single‐surface mixed isotropic–kinematic hardening plasticity model for structured clays. The formulation of the model stems from a compromise between its capability of reproducing the larger number of features characterizing the behaviour of structured clays and the possibility of developing a robust integration algorithm for its implementation in a finite elements code. The model is characterized by an ellipsoid‐shaped yield function, inside which a stress‐dependent reversible stiffness is accounted for by a non‐linear hyperelastic formulation. The isotropic part of the hardening law extends the standard Cam‐Clay one to include plastic strain‐driven softening due to bond degradation, while the kinematic hardening part controls the evolution of the position of the yield surface in the stress space. The proposed algorithm allows the consistent linearization of the constitutive equations guaranteeing the quadratic rate of asymptotic convergence in the global‐level Newton–Raphson iterative procedure. The accuracy and the convergence properties of the proposed algorithm are evaluated with reference to the numerical simulations of single element tests and the analysis of a typical geotechnical boundary value problem. Copyright © 2007 John Wiley & Sons, Ltd.     

一种新的本构理论及其在基桩分析中的应用

首次将岩土体荷载与位移特征曲线划分为3种类型,并定义高地应力的判别标准。提出一种新的本构理论,可以描述在不同正压力（或不同围压）下应力（或荷载）与应变（或位移）的全过程特征,只有3个描述材料结构和状态的力学参数。该理论不仅可以描述单个材料,也可以描述多种材料组成的复合材料和两种不同材料界面等力学特性,变形可达任意值。对于长沙湾特大桥基桩,在不同静载荷作用下不同土体与桩之间的力学特性采用新本构理论进行数值模拟,结果表明该本构理论可以描述不同岩土体和岩土体与基桩之间的力学特性。     

Numerical investigations of shear localization in a micro‐polar hypoplastic material

In this paper a micro‐polar continuum approach is proposed to model the essential properties of cohesionless granular materials like sand. The model takes into account the influence of particle rotations, the mean grain size, the void ratio, the stresses and couple stresses. The constitutive equations for the stresses and couple stresses are incrementally non‐linear and based on the concept of hypoplasticity. For plane strain problems the implementation of the model in a finite element program is described. Numerical studies of the evolution of micro‐polar effects within a granular strip under plane shearing are presented. It is shown that the location and evolution of shear localization is strongly influenced by the initial state and the micro‐polar boundary conditions. For large shearing the state quantities tend towards a stationary state for which a certain coupling between the norm of the stress deviator and the norm of the couple stress tensor can be derived. Copyright © 2003 John Wiley & Sons, Ltd.     

基于不同弹性本构方程的塑性区宽度及封孔长度研究

煤层注水是煤矿开采过程中重要的增透卸压手段,而封孔长度是影响注水压裂效果的一个关键因素。为了确定出合理的封孔长度,根据Lippmann煤层失稳基本理论,选取了6种不同的弹性区本构方程,引入非线性动态瓦斯压力分布的表达式,构建了含瓦斯煤层力学理论模型,得到煤层垂直、水平应力和塑性区宽度的解析式,之后结合高家堡的现场情况,对比分析6种弹性本构方程下的煤层应力分布曲线及塑性区宽度,并进行了现场封孔试验。结果表明：封孔长度在9.5 m时孔洞漏水量小、未出现异常现象,封孔效果良好,最终确定出了合理的封孔长度为9.5 m;相对于其他3种弹性本构方程,选取第i、v、vi种弹性本构方程对封孔长度进行理论计算分析,更加符合现场实际情况。     

CO_2注入下岩层变形和流体运移分析(I):两相流-岩层耦合模型

目前国内关于CO2咸水层封存尚处于先导性和试验性研究阶段,对超临界CO2注入过程中岩层力学响应和流体运移的理论与技术方面的认识还不完善。为研究CO2注入下岩层变形和流体运移,基于两相流动数学模型,给出了超临界CO2和咸水质量守恒方程;采用毛细压力和有效饱和度的关系式,将质量守恒方程变换成以毛细压力为变量的表达式,以便于考虑流体压力对岩层的影响。提出了无流体压力影响下的岩层力学本构模型,该模型能够同时考虑岩层的塑性变形和损伤。分析了两相流体-岩层相互作用机制:一方面,采用有效应力原理,考虑流体压力对岩层的力学影响;另一方面,通过岩层固有渗透率变化考虑岩层变形对流体运移的影响。     

Strength and Deformation Performances of Siltstone

The fundamental mechanical properties, time and crisscross effects of a kind of siltstone under different stress states and along different stress paths are studied with laboratory testing. A failure criterion, constitutive equations with one dimensional rate dependence and three dimensional volume deformation are proposed in this paper.     

粉砂岩的强度特性和变形特性

本文通过实验,研究了一种介于岩石与土之间的软岩--粉砂岩在不同应力状态和应力路径下的基本力学性质、时间效应和交叉效应,提出了一维率相关型本构方程、三维体积应变本构方程以及破裂准则。     

A FORMULATION OF FULLY COUPLED THERMAL–HYDRAULIC–MECHANICAL BEHAVIOUR OF SATURATED POROUS MEDIA—NUMERICAL APPROACH

The theoretical aspects of fully coupled thermohydromechanical behaviour of saturated porous media are presented. The non-linear behaviour of soil skeleton is assumed. A new concept called ‘thermal void ratio state surface’ is introduced to include thermal effects, and the stress state level influence on volume changes. The fluid phase flows according to Darcy's law and energy transport is assumed to follow Fourier's law classically. Variation of water permeability, water and solid unit weight due to thermal effects and pore pressure changes are included. A finite element package is developed based on final matrix form obtained from discretization of integral form of field equations by finite element method and integration in time. A very good agreement between the theoretical predictions and the experimental results was obtained for the several simple problems proposed by other authors. © 1997 by John Wiley & Sons, Ltd.     

Numerical modelling of in situ behaviour of the Callovo–Oxfordian argillite subjected to the thermal loading

To enhance the understanding of thermal impact on the in situ behaviour of the Callovo–Oxfordian argillite, this paper presents an interpretation of an in situ heating experiment carried out in the Meuse/Haute Marne Underground Research Laboratory (M/HM URL). The argillite was heated successively by two constant heating powers: 277 W and 925 W. When subjected to thermal loading, the argillite exhibits an important volume change and a strong pore pressure response that significantly affect its hydraulic and mechanical behaviour. Numerical analysis has been performed by using a coupled theoretical formulation that incorporates a constitutive model especially developed for this material. Based on Biot's theory, this model includes the influence of interstitial pressure on the mechanical behaviour. The simulation obtained reproduces satisfactorily the results of the in situ experiment and the main observed patterns of behaviour. The interpretation and discussion of numerical results provide additional data that can help us to understand the thermo-hydromechanical behaviour concepts of saturated argillite formation.     

紫坪铺面板坝堆石料颗粒破碎试验研究

采用大型三轴仪对紫坪铺面板坝堆石料进行了单调和循环荷载下的固结排水剪切试验,研究了不同孔隙比情况下颗粒破碎及剪胀的变化规律。试验表明：（1）单调和循环荷载条件下,堆石料颗粒破碎率与塑性功之间存在一致的双曲线关系;（2）峰值应力处剪胀率与颗粒破碎率在半对数坐标中呈近似线性关系;（3）峰值应力处主应力比与相应的剪胀率呈近似线性关系,且上述结果受初始孔隙比的影响不大。研究成果有助于进一步了解堆石料的颗粒破碎特点,对建立复杂应力条件下考虑颗粒破碎和状态相关性的弹塑性本构模型,分析紫坪铺面板堆石坝汶川地震破损机制是十分有益的。     

A visco‐elasto‐plastic model for granular materials under simple shear conditions

The numerical simulation of rapid landslides is quite complex mainly because constitutive models capable of simulating the mechanical behaviour of granular materials in the pre‐collapse and post‐collapse regimes are still missing. The goal of this paper is to introduce a constitutive model capable of capturing the response of dry granular flows from quasi‐static to dynamic conditions, in particular when the material experiences a sort of solid‐to‐fluid phase transition. An ideal assembly of identical spheres under simple shear conditions is considered. In the constitutive model, void ratio and granular temperature have been chosen as state variables, and both shear and normal stresses are computed as the sum of two contributions: the quasi‐static one and the collisional one. The former is determined by using a perfect elasto‐plastic model including the critical state concept, while the latter is derived from the kinetic theory of granular gases. The evolution of the granular temperature, fundamentally governing the material phase transition, is obtained by imposing the kinetic fluctuating energy balance. The constitutive relationship has been integrated, under both constant pressure and constant volume conditions, and the influence of shear strain rate, initial void ratio and normal pressure on the mechanical response has been investigated. Copyright © 2015 John Wiley & Sons, Ltd.