Unsaturated elasto-plastic constitutive equations for compacted kaolin under consolidated drained and shearing-infiltration conditions

Transient process of water flow changes the equilibrium conditions of an unsaturated soil, resulting in volume change of a soil. The volume change alters the hydraulic properties of the soil and thus influences the transient process of water flow through the soil. Therefore, the interactive processes between stress-strain behavior and pore-water pressure are the primary processes affecting the mechanical behavior of unsaturated soils. This paper presents coupled elasto-plastic constitutive equations for unsaturated compacted kaolin under consolidated drained and shearing-infiltration conditions. The study focused on the development of the suction increase (SI) yield curve that incorporates changes in matric suction during transient processes. In addition, the relationship of change in specific water volume with respect to net mean stress and matric suction was also proposed by incorporating the hysteresis of soil-water characteristic curve. The simulated results by the proposed constitutive model were compared with those obtained from isotropically consolidated drained tests and shearing infiltration tests of compacted kaolin to verify the proposed model. The simulated results are in close agreement with the experimental results.     

考虑变形效应的非饱和土相对渗透系数模型

 相对渗透系数是土体非饱和渗流分析的重要参数,Mualem统计模型在土体相对渗透系数预测中得到广泛应用。然而,Mualem统计模型假定土体具有刚性的孔隙结构,因而无法反映变形对非饱和渗透系数的影响。笔者最近提出的变形土土水特征曲线模型为基础,建立考虑变形效应的非饱和土相对渗透系数的修正Mualem统计模型。修正模型引入某一饱和度条件下被水充满的平均孔隙半径这一参量,来反映有效饱和度及土体变形过程中孔隙形态变化对其非饱和渗透特性的影响。采用4种不同类型土体的非饱和渗透试验数据对修正模型进行验证。结果表明,修正模型给出的相对渗透系数预测值与实测值的均根误差相比Mualem统计模型减小27%,说明修正模型对于变形条件下土体的相对渗透系数具有更强的预测能力。研究成果为非饱和土渗流规律以及水–力耦合数值模拟等研究提供了更为可靠的理论模型。     

非饱和土的性状及膨胀土边坡稳定问题

主要叙述非饱和膨胀土及其边坡稳定研究方面的新进展。首先讨论了非饱和土研究中与土坡稳定性有关的若干重要特性,指出非饱和土的气 水形态问题是非饱和土研究的一个基本问题。并提出随含水率由小变大,非饱和土存在四种气 水形态,不同气 水形态的土具有不尽相同的性状。然后,对非饱和土的吸力和土水特征曲线以及不同气 水形态下的强度作了讨论,并对非饱和土的本构关系作了简要的介绍。接着以南水北调中线膨胀土渠道工程为背景,以吸力问题为中心,对非饱和膨胀土边坡滑动的各种内在的和外界的因素进行了分析,尤其对新近研究的降雨入渗和裂隙影响的研究进行了定量的分析,改变了以往对这方面只进行定性研究的情况。在此基础上对边坡失稳的机理和考虑裂隙及雨水入渗的稳定分析方法进行了研究。     

高饱和度土的压缩和固结特性及其应用

高饱和度土往往存在于湖海地区的含气泡沉积物以及工程中高含水率的压实土。试验表明高饱和度土中气相主要以封闭气泡形式存于孔隙水中,并随水一起流动。假设高饱和度土为具有可压缩流体的两相土,考虑水气混合物的压缩性,推导出不排水条件下土体压缩公式,并建立高饱和度土的排水固结方程,获得一维简化解析解。在此基础上,深入分析了荷载作用下高饱和度土的孔压系数、三轴试验的反压饱和过程与围压饱和过程、以及一维情况下高饱和度土的排水固结特性,重点讨论它与饱和土的区别。最后还对影响高饱和度土压缩和固结特性的主要参数进行了分析。     

湿度和密度双变化条件下的非饱和黄土渗气渗水函数

饱和度和密度都会对非饱和土的渗透系数产生影响,而准确确定非饱和土的渗透系数在饱和/非饱和土体渗流分析中非常重要。利用改进的非饱和土水气运动联合测定仪,对重塑非饱和黄土试样进行不同增湿级数和初始干密度条件下的渗透试验,研究饱和度、干密度、渗气系数及渗水系数间的关系,建立湿度和密度双变化条件下的渗气渗水函数,并通过试验验证该函数的适用性和准确性。研究表明,该函数既可用于确定渗水系数,又可用于定量描述非饱和土孔隙中水气运动之间的相互影响,其在非饱和土固结分析、稳定分析及饱和非饱和渗流分析中具有较强的应用前景。     

基于动态多步流动法的非饱和土水力特性测试研究

非饱和土水力特性在非饱和土的研究中起重要作用。鉴于传统的测试方法耗时较长,提出能够快速测定非饱和土水力特性的动态多步流动方法。该方法基于WeiDewoolkar提出的多孔介质热动力学混合物理论模型,推导出能够描述非平衡态的非饱和土饱和度的演化方程,通过对粉土和粉质黏土两种试样开展动态多步流动试验,并根据动态试验结果求解饱和度演化方程,得出试样处于平衡状态时的土水特征曲线和渗透函数。该方法不需要求解初边值问题,仅仅需要动态多步流动的试验数据求解饱和度的演化方程,进而得出试样的水力特性参数,与其它方法相比,该方法更加简单有效。     

不排水条件下非饱和土水力–力学耦合特性数值模拟

非饱和土即便在不排水、不排气条件下也将发生体变,这为预测非饱和土不排水、不排气力学特性带来了困难。基于三相孔隙介质理论,结合非饱和土的弹塑性本构模型推导了不排水、不排气条件下非饱和土三轴剪切三相耦合控制方程,对非饱和土的不排水、不排气三轴剪切特性进行预测。数值模拟结果表明,此方法能够描述非饱和土在不排水、不排气条件下的水力–力学特性,如体缩特性、孔隙水压力和孔隙气压力增长以及饱和度增大等特性。数值计算结果与室内试验结果一致,证实了所提数值分析方法的合理性。     

非饱和带水–气二相流数值模拟研究

为了真实反映非饱和带中流体(水和气)的运移规律和渗透特性,将非饱和带中的孔隙水和孔隙气作为研究对象,基于水-气二相流理论,建立了水-气二相流数学模型。该模型采用积分形式的有限差分法对控制方程进行离散,在应用Newton-Raphson迭代方法得到相应的线性方程组的基础上,编制了三维计算程序SEEP2P。通过对土坝稳定渗流和降雨入渗情况的计算分析,得出稳定状态的渗流可以近似采用单相流模型来模拟,降雨入渗下的非稳定渗流问题采用水-气二相流模型来模拟更加符合实际。采用水-气二相流模型模拟得到水相、气相的流动状态,为定量研究孔隙气压力对入渗水流的影响提供了有效途径。     

考虑微观孔隙结构的非饱和土水–力耦合本构模型

非饱和土在应力和水力路径的作用下均会产生微观孔隙结构的变化,同时,不同的孔隙类型和结构也会对非饱和土宏观水力、力学特性产生不同的影响,尤其是在膨胀土、压实黏土等双孔结构土中,这种影响尤为显著。以Wheeler建立的非饱和土水–力全耦合模型(GCM)为理论框架,引入有效饱和度来描述土体内部宏观和微观孔隙对水–力特性的不同影响,提出考虑孔隙结构影响的Bishop有效应力表达式,建立了各向等压状态下考虑微观结构的非饱和土水–力耦合本构模型,并实现了模型的预测功能。通过与非饱和土等向压缩试验结果的对比,初步验证了所建立模型的合理性和有效性。     

Laboratory measurement of hydraulic conductivity functions of two unsaturated sandy soils during drying and wetting processes

The importance of applying unsaturated soil mechanics to geotechnical engineering design has been well understood. However, the consumption of time and the necessity for a specific laboratory testing apparatus when measuring unsaturated soil properties have limited the application of unsaturated soil mechanics theories in practice. Although methods for predicting unsaturated soil properties have been developed, the verification of these methods for a wide range of soil types is required in order to increase the confidence of practicing engineers in using these methods. In this study, a new permeameter was developed to measure the hydraulic conductivity of unsaturated soils using the steady-state method and directly measured suction (negative pore-water pressure) values. The apparatus is instrumented with two tensiometers for the direct measurement of suction during the tests. The apparatus can be used to obtain the hydraulic conductivity function of sandy soil over a low suction range (0–10 kPa). Firstly, the repeatability of the unsaturated hydraulic conductivity measurement, using the new permeameter, was verified by conducting tests on two identical sandy soil specimens and obtaining similar results. The hydraulic conductivity functions of the two sandy soils were then measured during the drying and wetting processes of the soils. A significant hysteresis was observed when the hydraulic conductivity was plotted against the suction. However, the hysteresis effects were not apparent when the conductivity was plotted against the volumetric water content. Furthermore, the measured unsaturated hydraulic conductivity functions were compared with predictions using three different predictive methods that are widely incorporated into numerical software. The results suggest that these predictive methods are capable of capturing the measured behavior with reasonable agreement.     

非饱和土与特殊土力学的基本理论研究

对非饱和土与特殊土开展了持久深入的研究,取得了系统的创造性成果：在国内率先研制成非饱和土固结仪、直剪仪、渗气仪、标准三轴仪、温控三轴仪、多功能三轴仪和土工CT-三轴仪等一系列仪器设备,揭示了非饱和土与特殊土的水气运动规律及变形、强度、屈服、水量变化、湿陷、湿胀、细观结构演化、温度效应等许多重要力学特性规律;构建了岩土力学的公理化理论体系与多种组合形式的非饱和土的应力状态变量;提出了各向异性多孔介质的有效应力理论公式与非饱和土的有效应力理论公式;建立了非饱和土、湿陷性黄土和膨胀土的本构模型谱系（包括非线性、弹塑性、结构性损伤与热力耦合模型）与分别考虑密度、净平均应力和偏应力影响的广义土-水特征曲线模型谱系;创立了非饱和土三维固结理论及其固结模型谱系;自主研发了分析固结问题的系列软件,求得一维固结问题的解析解和二维固结问题的数值解,形成了完整的理论体系。应用研究成果解决了多项工程的疑难问题,表明所建理论能够指导实践,为工程决策提供理论支持和科学依据。     

荆门膨胀土土–水特征曲线特征参数分析与非饱和抗剪强度预测

 为探讨荆门原状、压实、石灰改良膨胀土的非饱和抗剪强度与土–水特征曲线(SWCC)的相关关系,利用Fredlund-Xing模型对3种膨胀土的SWCC试验数据进行非线性拟合,获得相应模型参数;利用Fredlund公式(1996年)对3种膨胀土控制吸力下的固结排水剪三轴压缩试验抗剪强度成果进行相关分析。分析结果表明：(1) 使用Fredlund-Xing模型拟合3种膨胀土SWCC试验数据的效果均较好。(2) 荆门原状、压实、石灰改良膨胀土的进气值分别为210,68和18 kPa;石灰改良膨胀土的持水能力(水稳定性)最强,压实膨胀土次之,原状膨胀土最弱;3种膨胀土的残余吸力可均取为3 000 kPa。(3) 3种膨胀土的有效内摩擦角(j´)和与基质吸力相应的内摩擦角(jb)均非常数;j´与基质吸力相关,jb与基质吸力和应力状态均相关,这意味着Mohr-Coulomb破坏包面是双向弯曲的。(4) 使用Fredlund公式,能够在较大的吸力范围内较好地预测荆门膨胀土的非饱和抗剪强度,原状、压实、石灰改良膨胀土的土性参数κ分别为2.4,2.7及3.4。(5) 预测值与实测值间存在差别的主要原因是Fredlund公式本身未考虑到jb与应力状态相关,若要考虑该因素,可将应力状态对SWCC的影响引入到Fredlund公式中。     

A macro-structural constitutive model for partially saturated expansive soils

The mechanical behaviour of unsaturated expansive soils is an important factor in the design of barriers for nuclear waste disposal. Double structural constitutive models for expansive soil have been developed based on the distinction between micro-structure and macro-structure of the expansive soil. Nevertheless, the micro-parameters and the coupling function of these models are hard to determine. To eliminate the limitations of these models, the concept of a macro-structural neutral loading line was introduced by adopting a simple assumption concerning the plastic swelling potential of unsaturated expansive soil. A macro-structural volume change equation of unsaturated expansive soil was derived using a macro-structural neutral loading line. The model was based on an existing model for non-expansive unsaturated soils. Only one new macro-parameter was introduced into the constitutive model compared to the original model. Finally, a comparison between predicted results and experimental data indicates that the proposed model has a remarkable ability to reproduce the mechanical and hydraulic responses of unsaturated expansive soils.     

非饱和土水气迁移与相变：两类“锅盖效应”的发生机理及数值再现

结合非饱和土水气迁移的物理过程和内在机理,将“锅盖效应”分为两种情形。第一类“锅盖效应”定义为非饱和土水气的冷凝过程,而第二类“锅盖效应”定义为水气迁移成冰过程。相较于前者,第二类“锅盖效应”会造成覆盖层下土体含水率大幅度提高,且现有非饱和土水热气迁移理论无法给出合理解释。综合考虑水分的蒸发、冷凝和冻结3个相变过程,建立了非饱和冻土水热气耦合迁移的数学模型,并通过数值模拟求解,再现了第二类“锅盖效应”的形成过程。另外计算结果表明：温度梯度下的气态水迁移并成冰会造成覆盖层下土体接近饱和含水率;一定表层深度范围内,土体含水率增加存在两个陡升段。由于现行工程设计很少考虑防气隔气,在寒旱地区进行工程建设需对第二类“锅盖效应”引起足够重视。     

Détermination expérimentale de la conductivité hydraulique D'un Sol non saturé au laboratoire

When using Richards's equation to predict the spatio-temporal distribution of moisture or matric potential in an unsaturated porous medium, the knowledge of the relations between the soil hydrodynamics parameters, constituted an essential prerequisite. We propose an experimental procedure to determine the hydraulic conductivity and/or diffusivity, as a function of the volumetric water content and of the effective pressure head in soil expressed in pF term of the unsaturated soil. The calculation method is based on the analysis of transient volumetric water content and potential profile of a compacted soil column, during a vertical infiltration at constant water head.     

Numerical simulations of triaxial shearing-infiltration tests

The failure of steep slopes during rainfall is commonly associated with a decrease in matric suction in the unsaturated soil zone above the water table. The shear strength characteristics of residual soil under water infiltration have been studied in the laboratory using unsaturated triaxial tests. This paper presents a development of a numerical model for simulating a triaxial shearing-infiltration test to investigate the shear strength characteristics of a compacted kaolin under infiltration condition. Both the hydraulic and mechanical responses of the compacted kaolin are modeled using the commercial software SIGMA/W and in-house software YS-Slope. The numerical analyses result and their validation against laboratory test results are presented and discussed in this paper. The results from the numerical analyses show good agreements with those from the laboratory tests, indicating that the proposed numerical model can be used to simulate the triaxial shearing-infiltration tests in laboratory.     

非饱和土孔隙气、水、汽、热耦合运动之模拟(英文)

旨在介绍一个描述非饱和土孔隙气、水、汽、热耦合运动的理论模型。该模型假定孔隙气和孔隙水运动分别遵循达西定律 ,而影响水蒸汽运动的两种主要因素分别是分子扩散和孔隙气运动 ,其中受分子扩散影响的孔隙水蒸汽运动可用Fick定律描述。热转移则主要包括了三种形式 ,即传导、对流和汽化潜热。根据有限单元法 ,编制了一个三维的计算机程序用以模拟非饱和土孔隙气、水、汽、热的耦合运动。通过数值分析与干沙试验结果之比较 ,验证了文中之理论模型和计算机程序的可靠性     

关于非饱和土本构研究的几个基本规律的探讨

非饱和土是土不同状态的一种,所有的土都可能对于水处于部分饱和状态。因此,理想的土本构模型应该能够预测在所有可能的孔压和应力范围内土的性质;同时,也可以描述在该范围内任一的应力和水力路径。在过去大约 20 a 的时间中,非饱和土本构性质的研究经历了重大的发展,主要表现在以下几个基本理论方面：体积变化、剪切强度、屈服应力、土水滞回及水力学性质的耦合等。拟从对试验数据的预测效果、各理论间的一致性以及饱和与非饱和状态的连续性 3 个方面,对非饱和土本构基本理论的研究现况进行分析、归纳。     

非饱和土的水气运动规律及其工程性质研究

本文以压实黄土为对象,以各种改进的仪器设备为手段,在较大的湿度和密度范围内系统地研究了非饱和土的渗气性、渗水性、孔隙水压力和孔隙气压力在三轴不排水不排气剪切过程中的演化特性;用多种方法研究了非饱和土的吸力-密度-饱和度间的关系;在总结正反两方面经验的基础上,摸索出一套相应的测试方法.     

非饱和土性状及其与工程问题的联系

首先讨论非饱和土研究的现状,指出目前存在的几种观点及其特点,然后针对不同类型的工程问题分别讨论了非饱和土性状与这些工程问题的联系,最后对非饱和土理论实用化发展方向进行了思考和讨论。