土应力应变关系的粘-弹-塑模型

土的动力非线性实验一般给出无量纲动力剪切模量G/G_(max)与剪应变γ的关系曲线以及滞回阻尼比λ与剪应变γ的关系曲线。同时,在地震工程主要关心的周期范围(例如0.1—10秒)内的地震荷载频率对以上两类经验曲线的影响不明显。为了获得一个与以上实验结果相一致的应力-应变关系,本文提出了一种具有粘性、弹性及塑件三种效应的应力-应变关系物理模型——粘-弹-塑模型(Visco-Elasto-Plastic Model),简称VEPM。这一模型能同时拟台G/G_(max)-γ和λ-γ两条实验曲线,且能很好地满足它们与加载频率无关的要求。本文还将VEPM与以往模型进行了对比,讨论了VEPM的物理意义。最后,给出了将VEPM应用于土层地震反应计算的两个实例。

基于ADINA的软土粘弹塑性模型二次开发研究

由于软土具有蠕变特性,而商业化软件提供的岩土本构模型往往不能满足软土地区实际工程数值分析的需求。将一维广义西原模型的应力-应变关系推广为三维应力状态的弹-粘弹-粘塑本构方程,采用D-P屈服准则推导出用于数值计算的粘弹性柔度矩阵和粘塑性柔度矩阵,并成功添加到ADINA有限元程序中。然后利用曲线拟合法确定流变模型参数,并利用二次开发的粘弹塑模型,对软土的三轴蠕变进行数值模拟,通过数值模拟和三轴蠕变试验的结果比较,验证了模型的正确性和可靠性。文中二次开发采用的方法具有普遍性,可以为其他粘弹塑模型的开发提供借鉴和参考。

沥青砂粘弹塑本构模型及其验证

将Burgers模型与粘塑性模型串联,得到能描述沥青砂3个蠕变阶段的粘弹塑本构模型。开展了40℃,不同应力水平下沥青砂单轴蠕变实验,通过非线性拟合确定了模型中材料参数。利用模型预测了沥青砂蠕变变形和应变率变化,并与实验结果和Burgers模型预测结果进行了比较。结果表明,该模型不仅能描述沥青砂蠕变过程的前2个阶段变形,而且能反映第3阶段的加速蠕变特征,且预测结果与实验相当吻合。

Influence of features of natural soft clay on behaviour of embankment

The influence of different features of natural soft clays,namely anisotropy,destructuration and viscosity,on modelling the time-dependent behaviour of Murro embankment was investigated.The newly developed elasto-viscoplastic models were enhanced for determining viscosity parameters in a straightforward way and adopted for the finite element analysis.The same set of common parameters determined from conventional triaxial and oedometer tests was employed for all models,with additional parameters required for representing different soil features.The finite element predictions by using models coupled with BIOT's consolidation theory were compared with each other and with field data for settlement,horizontal displacement and excess pore pressures.In addition,the stress paths under the embankment loading were also compared with each other to improve the understanding of the effect of different soil features.All simulations demonstrate that all three features significantly influence the predictions.As a consequence,accounting for soil features needs to be carefully considered when they are applied to a construction site.

Model Test and Numerical Analysis on Long-Term Mechanical Properties of Stepped Reinforced Retaining Wall

Model tests and numerical analyses of stepped reinforced retaining wall were performed to investigate the effects of rheology of backfill and creep of geogrids on the long-term performance of the structure.The geogrid tensions,soil pressures,wall deformations and foundation pressure were measured during model construction and loading.A visco-elasto-plastic model and an empirical nonlinear visco-elastic model were utilized to simulate the stresses and deformations of geogrid-reinforced earth-retaining wall under long-term loads.By comparing test data with numerical results,it is shown that the foundation pressure distribution is nonlinear,and the lateral constraint of geogrids for backfill can cause a redistribution of foundation pressure.The curve of soil pressure is outside convex at each step initially,and it is close to the distribution for the case of vertical wall subsequently.The variation trend of geogrid tensions at different heights is obtained.Moreover,the failure mechanism and development mode of potential slip surface in retaining wall are proposed.

A triaxial creep model for coal containing gas and its stability analysis

Triaxial creep tests on CCG specimens were systematically performed using aself-made creep seepage experimental apparatus for determining the creep law of CCG.An improved triaxial creep model of CCG was established on the basis of a Nishiharamodel and another visco-elasto-plastic model,parameters of which were fitted on test data.Furthermore,the creep model is validated according to the result of triaxial creep experiments,and the outcome shows that the proposed triaxial creep model can properly characterizethe properties of various creep deformation phases of CCG,especially the acceleratingcreep phase.At the same time,the instability conditions of CCG were presentedbased on the discussion of the improved model's stability in terms of stability theories ofdifferential equation solution.

Visco-elastoplastic damage constitutive model for compressed asphalt mastic

In order to describe the three-stage creep behavior of compressed asphalt mastic, a visco-elastoplastic damage constitutive model is proposed in this work. The model parameters are treated as quadratic polynomial functions with respect to stress and temperature. A series of uniaxial compressive creep experiments are performed at various stress and temperature conditions in order to determine these parameter functions, and then the proposed model is validated by comparison between the predictions and experiments at the other loading conditions. It is shown that very small permanent deformation at low stress and temperature increases rapidly with elevated stress or temperature and the damage may initiate in the stationary stage but mainly develops in the accelerated stage. Compared with the visco-elastoplastic models without damage, the predictions from the proposed model is in better agreement with the experiments, and can better capture the rate-dependency in creep responses of asphalt mastic especially below its softening point of 47 ℃

Uniqueness of rate-dependency, creep and stress relaxation behaviors for soft clays

This work focuses on the uniqueness of rate-dependency, creep and stress relaxation behaviors for soft clays under one-dimensional condition. An elasto-viscoplastic model is briefly introduced based on the rate-dependency of preconsolidation pressure. By comparing the rate-dependency formulation with the creep based formulation, the relationship between rate-dependency and creep behaviors is firstly described. The rate-dependency based formulation is then extended to derive an analytical solution for the stress relaxation behavior with defining a stress relaxation coefficient. Based on this, the relationship between the rate-dependency coefficient and the stress relaxation coefficient is derived. Therefore, the uniqueness between behaviors of rate-dependency, creep and stress relaxation with their key parameters is obtained. The uniqueness is finally validated by comparing the simulated rate-dependency of preconsolidation pressure, the estimated values of secondary compression coefficient and simulations of stress relaxation tests with test results on both reconstituted Illite and Berthierville clay.

软土流变性对高速公路路基变形的影响分析

在对软粘土变形特性进行分析和在已有的粘弹塑性本构模型的基础上,结合高速公路工程进行了较多的室内三轴排水剪、三轴流变试验,通过试验确定了软土粘弹塑本结构模型的参数,将为本构模型引入BOIT固结理论有限元分析程序,对影响路基稳定的沉降、侧向位移、工后沉降及土体稳定进行了分析,并对实测的资料进行分析比较,从而阐明了软粘土地基进行变形分析时考虑土体流变性的必要性。

An elastic-viscous-plastic model for overconsolidated clays

The influences of time on clays are discussed first,and the concept of the instant normal compression line is proposed by analyzing the existing theories and experimental results.Based on the creep law,the relationship between the aging time and the overconsolidation parameter is built.With the reloading equation of the UH model(unified hardening model for overconsolidated clays) used to calculate the instant compression deformation,a one-dimensional stress-strain-time relationship is proposed.Furthermore,the evolution of this relationship is analyzed,and the characteristic rate that is a function of the overconsolidation parameter is defined.Then a three-dimensional elastic-viscous-plastic constitutive model is suggested by incorporating equivalent time into the current yield function of the UH model.The new model can describe not only creep,rate effect and other viscous phenomena,but also shear dilatancy,strain softening and other behaviors of overconsolidated clays.Besides,compared with the modified Cam-clay model it requires only one additional parameter(the coefficient of secondary compression) to consider the creep law.Finally,because the proposed model can be changed into the UH model under instantaneous loading,the elastic-plastic and elastic-viscous-plastic frameworks are unified.