非饱和土水-力本构模型及其隐式积分算法

在已有工作基础上建立了水力-力学耦合的非饱和土本构模型,在硬化方程中考虑饱和度的影响,同时在土水特征曲线中考虑了塑性体变的影响,从而使模型可以反映非饱和土中的毛细现象与土中弹塑性变形现象的耦合行为。采用隐式积分方法,建立了非饱和土耦合模型的数值模型,并推导了得到了水力-力学耦合的非饱和土的一致切线模量。利用该算法编制了本构模型计算的子程序,使其能向外输出切线刚度矩阵,用于有限元计算。为了验证该算法和程序的正确性,用所编制程序对不同路径下的土体行为进行了预测。通过预测结果与试验结果相对比,表明程序预测结果与试验数据相吻合,模型可以较好地模拟土体的水力-力学耦合行为特性。     

Implicit integration under mixed controls of a breakage model for unsaturated crushable soils

This paper discusses a series of stress point algorithms for a breakage model for unsaturated granular soils. Such model is characterized by highly nonlinear coupling terms introduced by breakage‐dependent hydro‐mechanical energy potentials. To integrate accurately and efficiently its constitutive equations, specific algorithms have been formulated using a backward Euler scheme. In particular, because implementation and verification of unsaturated soil models often require the use of mixed controls, the incorporation of various hydro‐mechanical conditions has been tackled. First, it is shown that the degree of saturation can be replaced with suction in the constitutive equations through a partial Legendre transformation of the energy potentials, thus changing the thermomechanical state variables and enabling a straightforward implementation of a different control mode. Then, to accommodate more complex control scenarios without redefining the energy potentials, a hybrid strategy has been used, combining the return mapping scheme with linearized constraints. It is shown that this linearization strategy guarantees similar levels of accuracy compared with a conventional strain–suction‐controlled implicit integration. In addition, it is shown that the use of linearized constraints offers the possibility to use the same framework to integrate a variety of control conditions (e.g., net stress and/or water‐content control). The convergence profiles indicate that both schemes preserve the advantages of implicit integration, that is, asymptotic quadratic convergence and unconditional stability. Finally, the performance of the two implicit schemes has been compared with that of an explicit algorithm with automatic sub‐stepping and error control, showing that for the selected breakage model, implicit integration leads to a significant reduction of the computational cost. Such features support the use of the proposed hybrid scheme also in other modeling contexts, especially when strongly nonlinear models have to be implemented and/or validated by using non‐standard hydro‐mechanical control conditions. Copyright © 2015 John Wiley & Sons, Ltd.     

非饱和土的清华弹塑性模型

为研究土在非饱和增湿情况下的应力、应变和强度关系,提出将含水率引入清华模型的硬化参数中建立非饱和土的清华弹塑性模型。通过在干土中掺加冰屑的方法进行增湿试验,表明该模型可以预测不同含水率的非饱和土的应力-变形和强度关系,从天然风干状态增湿到其他含水率的应力-应变全过程的计算结果也与试验结果相符合。将含水率直接引入弹塑性模型而不去研究复杂的基质吸力,可能是非饱和土本构模型工程应用的一条简便途径。     

上海软土各向异性弹塑性模型

根据同济大学真三轴仪所做的上海典型软土的排水试验、Kirkgard和Lade真三轴仪所做的旧金山湾淤泥的不排水试验资料，以及日本太田—关口(0hta—Sikiguchi)模型，提出上海各向异性弹塑性模型(简称上海软土模型)，该模型包括形状函数、屈服函数、应力—应变关系。为了证明模型的合理性，从理论和试验中对形状函数和屈服函数进行了检验。最后，提供了上海软土模型的有关参数。     

软土的弹塑性损伤模型在BiOt固结有限元中应用

在Biot固结理论基础上，引入弹塑性损伤本构关系，建立了固结分析有限元方程。对某试验路堤进行了损伤模型计算与弹塑性模型计算的结果比较。结果表明：损伤模型的计算结果大于弹塑性模型的计算结果；损伤随着加荷时间的增长而增大；路基中心线和路基坡脚下的土损伤程度较大。     

土的基本力学特性及其弹塑性描述

Cam-clay模型是在等向加载试验基础上建立起来的且能够描述正常固结土在常规三轴试验条件下应力-应变关系的最简单弹塑性模型,是建立各种土的弹塑性本构模型的基础。文中分别按加载方式的不同和土组构的不同,对描述与应力状态、应力历史、应力路径、持荷时间等因素有关的加载方式下土应力应变特性的弹塑性本构模型进行了归纳。对反映不同组构所形成的结构性、各向异性和颗粒破碎等特性的弹塑性本构模型进行了综述,并着重介绍了描述应力状态的三维化方法、反映应力历史影响的超固结土模型和模拟渐近状态路径的渐近状态模型等典型研究成果。     

非饱和土的水力和力学特性及其弹塑性描述

简单回顾了非饱和土本构模型研究的发展历程,总结了近几年非饱和土弹塑性本构模型最新研究成果,重点介绍了能统一模拟非饱和土水力性状和力学性状耦合的弹塑性本构模型。通过对建立模型过程中的几个核心问题讨论,较详细地说明该类模型的结构、性能以及相关问题。非饱和土水力性状的滞回性用假定存在饱和度弹性区间的弹塑性过程来模拟;该类耦合模型不仅考虑了吸力对非饱和土水力性状和力学性状的影响,还考虑了饱和度对应力-应变关系和强度的影响以及土体变形对土-水特征曲线的影响。用同一套模型参数,耦合模型可统一预测在吸力控制或含水率控制下沿各种应力路径下非饱和土的水力-力学特性,并简单介绍了膨胀性非饱和土的弹塑性本构模型以及耦合模型在有限元数值计算中的应用。     

非饱和土毛细滞回与变形耦合弹塑性本构模型

在修正剑桥模型的基础上,提出了一个非饱和土毛细滞回与骨架变形耦合的弹塑性本构模型。该模型考虑了基质吸力与饱和度对屈服应力的影响,可以同时描述非饱和土的弹塑性变形特性与毛细循环滞回效应。根据塑性体变的产生使非饱和土进气值增大的特点,建立了变形对土-水特征曲线影响的数学描述。该模型有效地考虑了饱和度对前期屈服应力的作用,准确地反映了土体在不同土-水状态条件下（脱湿和吸湿过程）强度特性的变化,而且还可以有效地描述水力循环历史对土体变形的影响。通过与试验数据对比,证明了该模型能够模拟非饱和土的主要力学特性。     

Implicit integration algorithm for Hoek-Brown elastic-plastic model

A realistic strength criterion often used to describe the yielding behaviour of a jointed rock mass at a continuum level is the well-known Hoek and Brown criterion. This paper is concerned with a 3-D stress generalization of the Hoek-Brown failure criterion by means of an elliptical functional which leads to a smooth deviatoric trace in the stress space. For its incorporation into a finite element analysis involving plasticity calculations, the formulation of an implicit stress integration algorithm is presented. The key computational methodology alludes to the notion of consistent tangent modulus and implicit return mapping schemes (radial and closest point return) for stress integration in a finite element analysis. Within the context of non-linear elastoplastic analysis, it is found that formulation of such consistent modulus and success into achieving numerical efficiency are closely intertwined. Indeed, the procedure results into accurate and rapid convergence of the displacement finite element scheme during the search for equilibrium. This means that considerable savings in computational time can be achieved for large scale problems. Numerical examples which focus on the Hoek-Brown plasticity model are presented in order to fully appreciate the robustness of the algorithm, and hence the viability of such method to practical problems.     

An elastoplastic mechanical constitutive model for microbially mediated cemented soils

Microbially induced calcite precipitation (MICP) is an innovative bio-mediated soil improvement technique that develops cementation within originally loose and potentially collapsible soils. This method utilizes biogeochemical processes with microbes. It has the advantage of being friendly to the environment and sustainable. In spite of the current interest in the MICP technique, the mechanical modeling of MICP-treated soils is still limited. In this paper, a constitutive model for MICP-treated sands is presented. The core components of the proposed approach include: a critical state yield surface, sub-loading concepts, a mechanism to account for the MICP-induced cementation enhancement, and an evolution law to consider bonding degradation effects during shearing. The mathematical framework is presented in detail. The model is then applied to analyze recently published experiments involving MICP-treated samples, with different calcite contents, and tested under different conditions (i.e., various confining pressure and loading paths). The model was able to properly capture the main features of MICP-treated sands behavior observed in the tests. It also assisted to interpret the response of this type of soil under different loading conditions.

     

三维弹塑性模型在路堤软基固结分析中应用

针对基于SMP准则改进的剑桥模型(MCC-SMP),采用弹性预测和塑性修正两阶段的回映算法,给出了应力更新算法,根据其算法编制了非线性有限元子程序,进行了三轴压缩、三轴伸长及平面应变等单元试验问题的数值模拟,通过数值分析和试验结果的对比,验证了子程序的精度;采用该本构模型,对澳大利亚某试验段路堤软土地基固结进行了三维有限元分析,重点分析和对比了孔压、沉降及水平位移的变化规律。     

Pollution effects of wastewater sludge application to sandy soils with particular reference to the behaviour of mercury

Hammarsdale, an industrial centre about 40 km inland from the coastal city of Durban, South Africa experiences effluent treatment problems typical of highly industrialised centres. The majority of the factories in the area manufacture textiles, with the remainder comprising chemical manufacturers and a large chicken processing plant. Effluent produced by these industries constitutes approximately 97% of the flow to the local wastewater treatment works. This is operated by Umgeni Water, a regional water supply authority which has an interest in pollution control as part of its integrated catchment management strategy. The activated sludge treatment process employed at the works produces a sludge which, following dewatering, is transported to a nearby land disposal site which has been engineered to ensure environmentally acceptable use is made of the site. The heavy metal concentrations of the sludges produced by the Hammarsdale Wastewater Works have long been a matter of concern, in particular the elevated concentrations of Hg. Investigations of the land disposal site showed that the Hg is bound in the upper topsoil (0–10 cm), and only very low concentrations are found in the subsoil. As the soils at the disposal site possess low cation exchange capacities, the difference between Hg concentrations in the top and subsoils may be attributed to binding of the Hg by organic matter introduced by sludge addition. Current research on the disposal area is focused on the movement of Hg, N and other elements through the soil profile into plants, surface and groundwater resources, with the ultimate aim of identifying suitable crops for cultivation at the site following its use as a sludge disposal facility.     

邓肯-张模型和沈珠江双屈服面模型的改进

邓肯-张非线性弹性模型和沈珠江双屈服面弹塑性模型已经被广泛地应用于高土石坝等土工建筑物应力位移分析中，但是邓肯-张模型存在不能反映土剪胀性的缺陷，沈珠江模型在高围压或较大剪应变条件下也存在模拟土剪胀性偏大的不足。笔者提出了一个能够统一模拟低围压到高围压条件下土剪胀特性的数学表达式，对上述两个模型进行了改进。采用粗粒土的大型三轴试验结果的初步验证分析表明，改进模型能较好地拟合粗粒土在低围压条件下剪胀以及高围压条件下剪缩的应-应变特性。     

Implicit numerical integration for a kinematic hardening soil plasticity model

Soil models based on kinematic hardening together with elements of bounding surface plasticity, provide a means of introducing some memory of recent history and stiffness variation in the predicted response of soils. Such models provide an improvement on simple elasto‐plastic models in describing soil behaviour under non‐monotonic loading. Routine use of such models requires robust numerical integration schemes. Explicit integration of highly non‐linear models requires extremely small steps in order to guarantee convergence. Here, a fully implicit scheme is presented for a simple kinematic hardening extension of the Cam clay soil model. The algorithm is based on the operator split methodology and the implicit Euler backward integration scheme is proposed to integrate the rate form of the constitutive relations. This algorithm maintains a quadratic rate of asymptotic convergence when used with a Newton–Raphson iterative procedure. Various strain‐driven axisymmetric triaxial paths are simulated in order to demonstrate the efficiency and good performance of the proposed algorithm. Copyright © 2001 John Wiley & Sons, Ltd.     

A nonlinear Hvorslev surface for highly overconsolidated soils: elastoplastic and hypoplastic implementations

The clayfills are being produced in open-pit mining. The stress state in the stiff lumps of the clayfills is significantly lower than in situ level. As a result, their current states lie on the dry side of the critical state. The linear Hvorslev surface is widely used due to its simplicity and capability to model the limit stress condition of soils on the dry side. However, it may overestimate the strength at very low stress level. For this purpose, a series of drained triaxial tests were performed on a silty clay at very small stress levels. The failure points of the tested soil confirm a nonlinear relationship in \(p^{\prime }\)–q plane on the dry side of the critical state. The degree of nonlinearity increases after being normalized by the Hvorslev equivalent pressure, which can be well modeled by a nonlinear power law criterion proposed by Atkinson (Géotechnique 57(2):127–135, 2007). Based on the test data and the critical state concept, a new failure line is proposed with help of the equivalent Hvorslev pressure. The nonlinear Hvorslev surface is then incorporated into an elastoplastic model and a hypoplastic model. Comparisons between the experimental data and simulations reveal that the proposed models can well represent the behavior observed in the laboratory.     

Regionally representative selection of soil samples for an environmental specimen bank with particular reference to loess soils

Digital evalutation of a series of correlative geo-specific maps in different scales and subsequent multivariate data processing in the form of entropy and neighbourhood analyses are the basis of a comparative selection of soil samples for the environmental specimen bank of the Federal Republic of Germany. By means of regionalisation algorithms, partly developed for, or specifically adapted to the present purpose, the optimum location of sampling points was determined and the result corroborated on large-scale maps or by visual inspection, respectively. In addition an index of representativity was defined, grouping soil taxa in terms of acreage and spatial autocorrelation.Once the spatial structure of the soil associations in an area to be sampled is thus determined variogram analysis, in the second step, contributes to definitely select the most appropriate specimens on the basis of a representative sampling grid and with respect to relevant properties, for instance cation or anion exchange capacities and biodegradability potential.     

Effect of degree of saturation on mechanical behaviour of unsaturated soils and its elastoplastic simulation

The two stress-state variable approach has been widely used in interpreting unsaturated soil behaviour. However this approach cannot take into account the effect of degree of saturation or water contents on the stress–strain behaviour and strength of unsaturated soils. The triaxial test results presented in this paper show that even if the same path of net stress and suction is followed, the stress–strain relation and strength are different due to different degrees of saturation. When other conditions are the same, the higher the degree of saturation for the soil sample is, the higher the stress ratio corresponding to a given axial strain will be. This effect can be modeled by using an elasto-plastic constitutive model coupling hydraulic and mechanical behaviour of unsaturated soils. Comparisons between the predicted and measured results are presented, which demonstrate that the model can quantitatively simulate the influence of the degree of saturation on stress–strain behaviour and strength of unsaturated soils.