Numerical modelling for clays with internal discontinuity surfaces

A numerical model capable of performing deformation analysis of a medium containing discontinuity surfaces is presented. The discontinuity can be either a crack, which can be open or closed, or a shear band. The model consists of two separate numerical algorithms, which are coupled together by means of the principle of superposition. In particular, an integral equation scheme based on the theory of dislocations is adopted for modelling the discontinuity, while a finite element discretization is used for the continuous medium. In this paper the discontinuity modelling is illustrated in detail, together with the specific formulation of the principle of superposition adopted, and some simple examples of application are presented. The well-known modelling approach based on Fracture Mechanics theory is also briefly discussed. The two models are compared and some advantages and drawbacks of each are pointed out, comments are made regarding their applicability in the specific case of soil mechanics, and conclusions are drawn as regards the conditions under which one or the other is appropriate. Finally, a full-scale example of deformation analysis using the proposed model is presented. Copyright © 1999 John Wiley & Sons, Ltd.     

Modelling microcrack kinetics in rocks

Mathematical concepts of statistical mechanics are use here to formulate the problem of microcrack kinetics. A stochastic distribution function is defined to characterize some modelling parameters of microcracks. This formalism results in an integral–differential equation of the Boltzmann type. A functional form for the microcrack production rate is imposed to model dilatancy and failure under shear-loading histories. This simple model illustrates some concepts of this theoretical approach.     

应用极化雷达估算农作物覆盖地区土壤水分相对变化

通过应用一阶离散植被模型,结合前人研究成果及雷达极化特性提出了多时相多极化雷达后向散射消除农作物覆盖层影响的算法:①应用已知的假设关系将植被层的体散射用交叉极化的总散射代替;②分析并将垂直极化的总后向散射中贡献很小的植被-土壤多次散射忽略;③将直接地表的后向散射分解成土壤水分与地表粗糙度的函数,使用重轨数据消除了地表粗糙度和农作物覆盖层的影响,并使用多时相全极化L波段(频率为1.2GHz)机载雷达测量数据进行验证,成功的估算了地表土壤水分的相对变化。     

填埋场中气体运移的非稳定耦合渗流数学模型

基于多孔介质渗流力学和流固耦合理论，建立了填埋场内挥发性气体在土壤中迁移气－固耦合数学模型，并根据数学模型的非线性特征，采用摄动法求出模型的拟解析解。通过解析求解研究了挥发性气体在土壤中耦合与非耦合渗流情况下孔隙压力分布动态特征，为预测和预报挥发性气体在土壤中的扩散状况，以及对环境污染的治理，控制污染的扩散理论了理论依据。     

Adequacy of field pullout testing of soil nails

In practice, field pullout testing of soil nails is a widely accepted method for the construction quality control and the performance assessment of soil nail walls. At the design stage, field pullout tests provide useful information about the design bond strength of the nail–soil interface, which is critical in the determination of minimum soil nail length. To obtain a reliable and representative estimate of the design bond strength, it is necessary to conduct a sufficient number of field pullout tests. This study demonstrates the use of the concepts of simple random sampling and stratified random sampling to assess the adequacy of the ultimate bond strength adopted for design based on the minimum recommended number of field pullout tests.     

A simple critical state interface model and its application in prediction of shaft resistance of non-displacement piles in sand

A simple semi-hyperbolic state-dependent constitutive model for sand-structure interfaces is proposed. The model formulation is consistent with critical state soil mechanics since void ratio evolves continuously with shear strain from initial state towards asymptotic critical state at extremely large shear strains. The model takes into account influence of normal stiffness on volume change and stress path. The proposed interface model is implemented in a pile segment analysis scheme for simulation of shaft resistance mobilization in non-displacement piles. Results reveal that the proposed pile segment analysis can well predict shaft resistance of model piles embedded in different sands.     

Evaluation of allowable withdrawn volume of groundwater based on observed data

To control land subsidence due to groundwater withdrawal, it is important to estimate allowable withdrawn volume of groundwater in a soft deposit. This technical note presents a simple approach for estimating the allowable withdrawn volume of a deposit. A regression analysis method was used based on measured land subsidence and recorded net withdrawn volume. This approach was proposed based on the principle of soil compression at different effective stresses, i.e. the soil compression is small when the consolidation stress is lower than the yield stress of the deposit, but large when the consolidation stress is higher than the yield stress. Two case studies are presented in this technical paper to demonstrate how to use the simple approach to estimate the allowable withdrawn volume.     

非饱和土力学中几个基本问题的探讨

近些年非饱和土力学的研究非常活跃,但对一些基本问题的认识并不一致,有时甚至概念混淆。针对非饱和土力学的几个基本问题：非饱和土状态变量的选择、非饱和土有效应力变量的选择、吸力概念的界定和轴平移技术的局限性、非饱和土的结构的表征方法等问题进行了分析和探讨。其中非饱和土状态变量和有效应力的选择对于非饱和土力学的理论和相应本构模型的建立具有重要影响,因此,首先深入讨论了这一问题,概括论述了非饱和土有效应力的演变并深入探讨了目前各种形式有效应力的优缺点。其次,指出由于受负压孔隙水气化（液-气相变化）的影响,在实际场地中大于某一界限值的基质吸力是不存在的;目前被广泛使用的轴平移试验技术却掩盖了这一情况,而基于此所建立的非饱和土强度和变形理论的适用性需要进一步的研究和论证。再次,指出非饱和土的结构除了包括组构和颗粒之间作用力的综合效应外,还建议增加孔隙水和孔隙气的分布以及各相之间的相互作用和物理-化学作用。最后对一些容易混淆的概念进行了梳理。其目的是希望国内同行在今后的研究中对这些问题加以关注,并建立正确的认识,促进非饱和土力学沿着正确的方向发展。     

基于多源信息融合法的岩土力学参数概率分布推断

岩土力学参数的概率分布类型直接影响岩土工程可靠性分析的计算结果和精度。在现场有限个小样本和多个先验分布的条件下,探讨如何综合利用已有的经验资料确定岩土参数的概率分布,提出了多源信息融合方法。通过分析各先验分布和现场试验信息的差异程度,确定各先验分布的权重,进而进行整体融合,实现统计意义上概率分布类型的优化。     

水压致裂法三维地应力测量的理论探讨

本文对现有的水压致裂三维地应力测量方法进行了深入的力学分析,在此基础上提出了新的理论模型。新的理论模型采用了最小主应力破坏准则,与现有方法采用的最小切向应力破坏准则相比,更客观的表述了水压致裂过程中的力学机制。为说明该方法的工作步骤及其可行性,文中利用遗传算法反演技术,对假设的地应力状态进行了试算。试算结果表明该方法是可行的。      

Probabilistic slope stability analysis by a copula-based sampling method

In probabilistic slope stability analysis, the influence of cross correlation of the soil strength parameters, cohesion and internal friction angle, on the reliability index has not been investigated fully. In this paper, an expedient technique is presented for probabilistic slope stability analysis that involves sampling a series of combinations of soil strength parameters through a copula as input to an existing conventional deterministic slope stability program. The approach organises the individual marginal probability density distributions of componential shear strength as a bivariate joint distribution by the copula function to characterise the dependence between shear strengths. The technique can be used to generate an arbitrarily large sample of soil strength parameters. Examples are provided to illustrate the use of the copula-based sampling method to estimate the reliability index of given slopes, and the computed results are compared with the first-order reliability method, considering the correlated random variables. A sensitivity study was conducted to assess the influence of correlational measurements on the reliability index. The approach is simple and can be applied in practice with little effort beyond what is necessary in a conventional analysis.     

Numerical modeling of cyclic mobility based on fuzzy-set concepts in plasticity theory

In this paper, the application of an efficient, transparent and accurate kinematic-cyclic constitutive model based on the fuzzy-set concepts and incremental plasticity theory is presented to show its capability in modeling cyclic mobility of saturated granular soil. The nature and kinematic mechanism of the membership functions in the fuzzy-set constitutive model are illustrated. The model’s capability of modeling soil dilatancy is investigated. Important features of volume change and pore water pressure build-up related to soil cyclic mobility are captured. The formulation of the proposed model is relatively simple and it can be readily implemented in finite element codes. The enhanced fuzzy-set model is capable of simulating ground motion problems particularly related to cyclic mobility, soil liquefaction, and spreading behavior.     

A Strain-Rate Dependent Clay Constitutive Model with Parametric Sensitivity and Uncertainty Quantification

This paper presents a strain-rate dependent plastic constitutive model for clays. Based on the concepts of critical-state soil mechanics and bounding surface plasticity theory, the model reproduces the mechanical response of clays under triaxial and simple shear loading conditions. The model parameters are determined for Boston Blue Clay, London Clay and Kaolin Clay, and the performance of the model in simulating the mechanical response of these clays is demonstrated for low to medium strain rates. The sensitivity of each model parameter is checked by perturbing the calibrated values by ±20 %. Subsequently, a probabilistic analysis using Monte Carlo simulations is performed by treating the model parameters as random variables and the impact of the statistics of the parameters on the undrained shear strength is investigated.     

An orthotropic interface damage model for simulating drying processes in soils

The study of drying process in soils has received an increased attention in the last few years. This is very complex phenomenon that generally leads to the formation and propagation of desiccation cracks in the soil mass. In recent engineering applications, high aspect ratio elements have proved to be well suited to tackle this type of problem using finite elements. However, the modeling of interfaces between materials with orthotropic properties that generally exist in this type of problem using standard (isotropic) constitutive model is very complex and challenging in terms of the mesh generation, leading to very fine meshes that are intensive CPU demanding. A novel orthotropic interface mechanical model based on damage mechanics and capable of dealing with interfaces between materials in which the strength depends on the direction of analysis is proposed in this paper. The complete mathematical formulation is presented together with the algorithm suggested for its numerical implementation. Some simple yet challenging synthetic benchmarks are analyzed to explore the model capabilities. Laboratory tests using different textures at the contact surface between materials were conducted to evaluate the strengths of the interface in different directions. These experiments were then used to validate the proposed model. Finally, the approach is applied to simulate an actual desiccation test involving an orthotropic contact surface. In all the application cases the performance of the model was very satisfactory.     

Limitations of suction‐controlled triaxial tests in the characterization of unsaturated soils

Behavior of unsaturated soils is influenced by many factors, and the influences of these factors are usually coupled together. Suction‐controlled triaxial (SCTX) tests are considered to allow researchers to investigate influences of individual variables on unsaturated soils under specified stress path with controls of stresses, pore water, and air pressures. In the past 50 years, SCTX testing method has been established as a standard approach to characterize constitutive behavior of unsaturated soils. Most important concepts for modern unsaturated soil mechanics were developed upon results from the SCTX tests. Among these, one of the most important contributions in the constitutive modeling of elasto‐plastic behavior for unsaturated soils is the Barcelona basic model (BBM) proposed by Alonso et al. in 1990. The BBM successfully explained many features of unsaturated soils and received extensive acceptance. However, the SCTX tests are designed based upon the divide‐and‐conquer approach in which an implicit assumption is used: soil behavior is stress‐path independent. However, it is well‐established that unsaturated soil behavior is elasto‐plastic and stress‐path dependent. It is found that the SCTX tests in fact cannot control the stress path of an unsaturated soil during loading. This incapability, in combination with complicated loading/collapse behavior of unsaturated soils, makes the SCTX tests for characterizing unsaturated soil questionable. This paper discusses the limitations of the SCTX tests in the characterization of unsaturated soils. A possible solution to the problem was proposed based on a newly developed modified state surface approach. The discussions are limited for isotropic conditions. Copyright © 2015 John Wiley & Sons, Ltd.     

土体龟裂力学机理及理论模型研究进展

土体龟裂是一种常见的自然现象,它对土体的物理力学性质有重要影响,是许多工程地质及环境地质问题的重要诱因。开展土体龟裂研究,探讨其形成机理并构建相应的理论模型,一直是本课题的研究重点和难点。根据近年来国内外围绕土体龟裂所取得的研究成果,着重对土体龟裂的力学机理及相关理论模型研究进展进行了归纳和总结,并依据各模型的理论基础及其特点进行了分类和评价,得到如下主要认识:学界关于龟裂的形成机理尚未有统一认识,主流观点认为龟裂是土体张拉破坏的一种表现形式,张拉应力和抗拉强度是控制龟裂形成的两个关键力学指标,但上述力学机理不能解释所有的土体龟裂现象。与土体龟裂相关的理论模型总体上可以分为4大类:（1）以断裂力学为理论基础的模型:代表性的有线弹性断裂力学模型、弹塑性断裂力学模型和基于线弹性断裂力学的有限元模型;（2）以张拉破坏为理论基础的模型:代表性的有线弹性力学模型、剪切破坏模型、弹性力学模型和黏性开裂模型;（3）以土体干缩变形过程为基础的含水率-体积变化模型;（4）以土体固结理论为基础的应力路径分析模型。在此基础上,进一步对各模型的适用条件及不足之处进行了评价。最后,笔者对该课题今后的研究重点和方向提出了建议,包括:蒸发过程中土体微观结构变化及微观力学分析;蒸发过程中孔隙水的迁移过程、分布特征及赋存状态研究;土体收缩研究;土体抗拉强度研究;裂隙发育宽度及深度预测理论模型研究;龟裂发育几何形态特征预测理论模型研究。     

冻土路基动力分析模型及青藏铁路地震灾害评估

在大量试验研究成果的基础上,运用冻土物理学、冻土力学、数值传热学、高等土力学及土动力学等基本理论,建立了冻土区道路及铁道工程路基的水-热-动力耦合数学模型（有效应力法）,并编制了相应的数值分析程序。最后,以青藏铁路某典型断面为例,对冻土铁路路基在地震荷载作用下的动力响应问题进行了系统分析,并评价了路基的地震安全稳定性。结果表明,提出的冻土路基动力分析模型合理,能较好地模拟冻土铁路路基的地震反应问题。另外,当发生设计烈度以下的地震时,青藏铁路路基不会产生严重破坏,经运营管理部门的维修后即可投入正常生产。     

冻土静力学室内试验研究进展

冻土力学是冻土工程学的理论基础,以解决冻土工程问题为归宿。冻土力学分为冻土静力学和冻土动力学两个方面,而冻土静力学是冻土力学的重要部分。为此,对常规冻土静力学室内试验的研究进展进行了总结,系统地阐述了其在冻土的强度特性、变形特性及理论模型等方面所取得的重要研究成果,并指出研究中存在的一些不足之处。最后结合冻土静力学研究的特点及实际冻土工程建设的需要,对冻土静力学研究的未来发展进行了展望。     

复杂边界的FLAC二维地质模型构建

FLAC3D是一款优秀的岩土力学分析软件,但其建模效率较低,尤其面对复杂地质条件时,其应用受到一定程度的限制。本文分析了FLAC建模命令的特点,提出一种新的较简单的建模方法,并开发了相应的计算机辅助建模程序,该方法通过坐标变换、复杂边界点坐标均匀化处理、逐列定义子网格坐标,可生成具有复杂边界的地质模型,且便于赋材料参数及应力边界条件参数,有效提高工作效率。