The combined effect of clay and moisture content on the behavior of remolded unsaturated soils

The behavior of unsaturated clayey soil is highly influenced by the coupled interaction between water and clay content. Various aspects of the behavior of artificial clay–sand mixtures with variable water content were experimentally studied. Laboratory tests were utilized for the determination of consistency limits, the stress–strain relationship, strength parameters, hydraulic conductivity, and volume change characteristics for various combinations of water and clay content in soil mixtures.

Results presented for various clay–sand mixtures include: new normalized consistency limits; the combined effect of clay content and water content on the stress–strain relationship and on the strength parameters (c and φ); and the effect of clay content on hydraulic conductivity and swelling potential. The cohesion of clayey sand is found to increase with increasing water content to a certain limit, above which it decreases. The angle of internal friction for clayey sand is found generally to decrease with increasing water content. The degree of saturation is found to be better than the water content in explaining the strength behavior. The hydraulic conductivity sharply decreases with increasing clay content up to 40% beyond which the reduction becomes less significant. Simple empirical equations are proposed for predicting the swelling potential of clayey soils as a function of either the clay content or plasticity index.     

Modelling the effect of temperature on unsaturated soil behaviour

A simple thermohydromechanical (THM) constitutive model for unsaturated soils is described. The effective stress concept is extended to unsaturated soils with the introduction of a capillary stress. This capillary stress is based on a microstructural model and calculated from attraction forces due to water menisci. The effect of desaturation and the thermal softening phenomenon are modelled with a minimal number of material parameters and based on existing models. THM process is qualitatively and quantitatively modelled by using experimental data and previous work to show the application of the model, including a drying path under mechanical stress with transition between saturated and unsaturated states, a heating path under constant suction and a deviatoric path with imposed suction and temperature. The results show that the present model can simulate the THM behaviour in unsaturated soils in a satisfactory way.     

Lattice Boltzmann modelling of liquid distribution in unsaturated granular media

We use capillary condensation simulated by a multiphase Lattice Boltzmann model as a means to generate homogeneous distributions of liquid clusters in 2D granular media. Liquid droplets condense from the vapour phase between and on the grains, and they transform into capillary bonds and liquid clusters as thermodynamic equilibrium is approached. As the amount of condensed liquid is increased, liquid clusters of increasing connectivity are formed and the distribution of liquid undergoes topological transitions until the whole pore space is filled by the liquid. We investigate the cluster statistics and local grain environments. From extensive simulations, we also obtain the mean Laplace pressure as a function of the amount of liquid, which is found to be quite similar to the well-known experimental retention curve in soil mechanics. The tensile stress carried by the grains increases as a function of the amount of condensed liquid up to a peak in the funicular state beyond which the stress falls off as a result of pressure drop inside the merging clusters.     

复杂水力路径下非饱和流动的数值分析

多孔介质中的流动问题,与孔隙介质的特性,含水量状态以及含水量的变化历史密切相关。基于毛细循环滞回理论模型,考虑含水量变化历史对土水特征关系的影响,在开发的U-DYSAC2有限元程序中进行了相应的数值实施。在试验给定的初边值条件下进行了非饱和渗流模拟分析,并将模拟结果与实测数据比较,表明在压力边界条件反复变化下,考虑滞回效应能获得更接近实测的结果,证实该模型在模拟各种循环变化条件下非饱和土渗流初边值问题的适用性与必要性。对入渗重分布反复变化条件下非饱和土柱流动的数值模拟表明,考虑滞回与不考虑滞回条件下,含水量、孔隙水压力和湿峰的迁移的预测在入渗后的重分布过程差异较大。考虑滞回效应时,土柱上部的脱湿速率、下部的吸湿速率比不考虑滞回时要低。从而证实了非饱和多孔介质中的土水状态依赖于含水量变化,而且强烈依赖于土体的水力路径变化。因此,循环边界条件变化下,毛细滞回效应在非饱和渗流模拟中的影响显著,必须加以考虑。     

A numerical model for nonlinear large deformation dynamic analysis of unsaturated porous media including hydraulic hysteresis

A numerical model based on the theory of mixtures is proposed for the nonlinear dynamic analysis of flow and deformation in unsaturated porous media. Starting from the conservation laws, the governing differential equations and the finite element incremental approximations suitable for nonlinear large deformation static and dynamic analyses are derived within the updated Lagrangian framework. The coupling between solid and fluid phases is enforced according to the effective stress principle taking suction dependency of the effective stress parameter into account. The effect of hydraulic hysteresis on the effective stress parameter and soil water characteristic curve is also taken into account. The application of the approach is demonstrated through numerical analyses of several fundamental nonlinear problems and the results are compared to the relevant analytical solutions. The effects of suction, large deformations and hydraulic hysteresis on static and dynamic response of unsaturated soils are particularly emphasized.     

Head-based isogeometric analysis of transient flow in unsaturated soils

An isogeometric analysis (IGA) is introduced to obtain a head-based solution to Richards equation for unsaturated flow in porous media. IGA uses Non-Uniform Rational B-Spline (NURBS) as shape functions, which provide a higher level of inter-element continuity in comparison with Lagrange shape functions. The semi-discrete nonlinear algebraic equations are solved using a combination of implicit backward-Euler time-integration and Newton-Raphson scheme. The time-step size is adaptively controlled based on the rate of changes in the pore pressure. The results from the proposed formulation are compared and verified against an analytical solution for one-dimensional transient unsaturated flow in a homogenous soil column. The proposed method is then applied to four more complex problems including two-dimensional unsaturated flow in a two-layered soil and a semi-circular furrow. The test cases in two-layered soil system involve sharp variations in the pressure gradient at the intersection of the two media, where the pore water pressure abruptly changes. It is shown that the proposed head-based IGA is able to properly simulate changes in pore pressure at the soils interface using fewer degrees of freedom and higher orders of approximation in comparison with the conventional finite element method.     

A semi-analytical solution for the transient response of one-dimensional unsaturated single-layer poroviscoelastic media

This paper develops a semi-analytical solution for the transient response of an unsaturated single-layer poroviscoelastic medium with two immiscible fluids by using the Laplace transformation and the state-space method. Using the elastic–viscoelastic correspondence principle, we first introduce the Kelvin–Voigt model into Zienkiewicz’s unsaturated poroelastic model. The vibrational response for unsaturated porous material can be obtained by combining these two models and assuming that the wetting and non-wetting fluids are compressible, the solid skeleton and solid particles are viscoelastic, and the inertial and mechanical couplings are taken into account. The Laplace transformation and state-space method are used to solve the basic equations with the associated initial and boundary conditions, and the analytical solution in the Laplace domain is developed. To evaluate the responses in the time domain, Durbin’s numerical inverse Laplace transform method is used to obtain the semi-analytical solution. There are three compressional waves in porous media with two immiscible fluids. Moreover, to observe the three compressional waves clearly, we assume the two immiscible fluids are water and oil. Finally, several examples are provided to show the validity of the semi-analytical solution and to assess the influences of the viscosity coefficients and dynamic permeability coefficients on the behavior of the three compressional waves.     

压力下无纺织物部分堵塞的水力特性研究

对压力作用下的部分堵塞无纺织物进行了室内试验,表明有土颗粒进入的织物压缩性和渗透性都降低,提出织物滤层的等效孔径折减系数RF,并建议用于贴坡排水时RF 取1.5～2.5。织物无堵塞压力条件下,Giroud（1996年）提出的公式对织物渗透系数有满意的预测值。在压力作用下对比分析了部分堵塞织物Giroud法和等效孔隙率法的渗透系数预测值,也与实测渗透系数值进行了比较,表明提出的等效孔隙率法能较好地预测渗透系数。     

Numerical simulation of vertical ground heat exchangers: Intermittent operation in unsaturated soil conditions

The effect of varying the thermal properties of inhomogeneous unsaturated soil on the intermittent operation of a vertical ground heat exchanger (GHE) was simulated by a conjugate heat transfer simulation using a transient conductive heat transfer model. A three-phase soil model was used to introduce soil properties that vary with depth. The performance during the first few hours was significantly different from that of an analytical infinite line source model that assumes steady-state borehole conditions, although relatively good agreement was obtained thereafter. Unsaturated soil conditions afforded a 40% lower mean heat exchange rate than saturated conditions. This demonstrates the importance of considering unsaturated conditions in the design and performance evaluation of GHEs.     

The effectiveness of two capillary barriers on a 10% slope

Summary Capillary barriers, consisting of fine-over-coarse soil layers, are being considered as an alternative cover component for waste-disposal facilities, especially in dry climates. Infiltrating water is removed from the fine layer by evaporation or transpiration, or percolation into the coarse layer (failure). If the fine-coarse interface is sloped, water in the fine layer can also drain laterally under unsaturated conditions. The effectiveness of two capillary barriers in laterally diverting water was tested. The barriers were 7 m long and 1.2 m thick, built on a 10% slope. One had a homogeneous fine layer, while the fine layer of the other was layered to increase its ability to divert water laterally. The barriers were first subjected to constant infiltration (10 mm/day) followed by exposure to ambient climatic conditions. The layered capillary barrier was successful in laterally diverting water near the interface and did not permit any water to enter the coarse layer. In contrast, the homogeneous capillary barrier failed over its entire length. These results indicate that a significant lateral diversion capacity can be designed into capillary barriers, greatly increasing their effectiveness.     

The role of uncertainty in bedrock depth and hydraulic properties on the stability of a variably-saturated slope

We investigate the uncertainty in bedrock depth and soil hydraulic parameters on the stability of a variably-saturated slope in Rio de Janeiro, Brazil. We couple Monte Carlo simulation of a three-dimensional flow model with numerical limit analysis to calculate confidence intervals of the safety factor using a 22-day rainfall record. We evaluate the marginal and joint impact of bedrock depth and soil hydraulic uncertainty. The mean safety factor and its 95% confidence interval evolve rapidly in response to the storm events. Explicit recognition of uncertainty in the hydraulic properties and depth to bedrock increases significantly the probability of failure.     

Temperature effects on the hydraulic behaviour of an unsaturated clay

The influence of temperature on the hydraulic properties of unsaturated clays is of major concern in the design of engineered barriers in underground repositories for high-level radioactive waste disposal. This paper presents an experimental study centred on the investigation of the influence of temperature on soil hydraulic properties related to water retention and permeability. Laboratory tests were conducted on artificially prepared unsaturated fabrics obtained from a natural kaolinitic-illitic clay. Special attention is given to the testing procedures involving controlled suction and temperature oedometer cells and the application of the vapour equilibrium technique at high temperatures. Retention curves at different temperatures show that total suction tends to reduce with increasing temperatures at constant water content. Temperature influence on water permeability is more relevant at low matric suctions corresponding to bulk water preponderance (inter-aggregate zone). Below a degree of saturation of 75% no clear effect is detected. Experimental data show that temperature dependence on permeability at constant degree of saturation and constant void ratio is smaller than what could be expected from the thermal change in water viscosity. This behaviour suggests that phenomena such as porosity redistribution and thermo-chemical interactions, which alter clay fabric and pore fluid, can be relevant.     

测定非饱和土水力参数的一步流动方法研究

详细介绍了改进的常流速联合测试系统,该系统增加了气泡冲刷和量测装置,能够冲刷试验过程中溢出陶土板的气泡并测量气泡的体积,进而对溢出水量进行修正,使测量值更加接近于真值。使用该系统对试样进行一步脱湿、吸湿流动试验,得出溢出水量随时间变化的关系曲线,利用HYDRUS-1D水分运移模型对该曲线进行拟合并得出相关参数。根据这些参数,反算得出试样脱湿段和吸湿段的土-水特征曲线和渗透函数并和实测数据进行对比。结果表明,该方法能很好的拟合实测溢出水量随时间变化的关系曲线,通过模型得出的土-水特征曲线和实测值比较接近,证实了该方法的可靠性。并且,与传统的测试方法相比,该方法能够节约大量的时间。因此,通过一步流动试验测定非饱和土-水力学参数的方法是可行的。     

Numerical studies on lateral load response of fin piles

In the recent past, in order to cater lateral loads, provision of fins to monopiles is evolving as an option for enhancing their lateral load capacity. Numerical and experimental studies pertaining to fin piles subjected to lateral loading are very limited. It is very difficult to understand the behaviour of fin piles through conventional soil pile theories, due to their different fin configurations. In the present study, three-dimensional finite element analyses were performed on regular piles (pile without fins) as well as fin piles. For fin piles, different fin lengths, orientation and position were considered during the analyses. The results have shown that fins placed at the top portion of the piles are more effective than fins placed at bottom of the pile. In all cases, fin piles exhibited more lateral load-carrying capacity compared to regular piles. When fin’s length is less than half of the pile length, lateral load-carrying capacity of fin piles is same irrespective of their orientation. However, fin’s orientation is significant, when fin length is more than 0.5 times length of piles, star fin piles exhibit more lateral load-carrying capacity compared to diagonal and straight fin piles. Fin piles experienced less bending moment compared to regular piles for a given lateral load. Further, it was also noticed that lateral load-carrying capacity of the fin piles depend upon fins’ length and their orientation.     

均质非饱和土一维稳态流水头垂直分布的解析解

渗流场水头分布计算是进行渗流量和渗流水力坡降计算的基础,准确、有效地求取渗流场水头分布是渗流计算的关键环节。对均质非饱和土体一维稳态流的流动方程进行分析,考虑到渗透系数是与基质吸力相关的函数,通过数学变换,给出了稳定渗流场的解析通式,并基于渗透性函数中的Gardner模型,给出了非饱和土一维稳态流水头垂直分布的解析解。该解析通式表明,均质非饱和土一维稳态流水头垂直分布主要受地表水头、深度和流动率3个因素控制。分别计算了一维稳态蒸发条件下粉土和黏土两种典型土类水头沿垂直方向的分布。计算结果表明：稳态蒸发条件下粉土层和黏土层内的水头分布表现出相似的变化规律,即自地表至地下水位处随着土层深度的增加,水头分布呈现出加速递减的趋势;在相同的蒸发条件下,对于相同深度处的黏土和粉土而言,黏土层内水头更高些;对同一种土类而言,在较大的蒸发状态下同一深度处土层内水头更高。反之,则较低。     

Modelling hydraulic and capillary-driven two-phase fluid flow in unsaturated concretes at the meso-scale with a unique coupled DEM-CFD technique

The goal of the research was to demonstrate the impact of thin porous interfacial transition zones (ITZs) between aggregates and cement matrix on fluid flow in unsaturated concrete caused by hydraulic/capillary pressure. To demonstrate this impact, a novel coupled approach to simulate the two-phase (water and moist air) flow of hydraulically and capillary-driven fluid in unsaturated concrete was developed. By merging the discrete element method (DEM) with computational fluid dynamics (CFD) under isothermal settings, the process was numerically studied at the meso-scale in two-dimensional conditions. A flow network was used to describe fluid behaviour in a continuous domain between particles. Small concrete specimens of a simplified particle mesostructure were subjected to fully coupled hydro-mechanical simulation tests. A simple uniaxial compression test was used to calibrate the pure DEM represented by bonded spheres, while a permeability and sorptivity test for an assembly of spheres was used to calibrate the pure CFD. For simplified specimens of the pure cement matrix, cement matrix with aggregate, and cement matrix with aggregate and ITZ of a given thickness, DEM/CFD simulations were performed sequentially. The numerical results of permeability and sorptivity were directly compared to the data found in the literature. A satisfactory agreement was achieved. Porous ITZs in concrete were found to reduce sorption by slowing the capillary-driven fluid flow, and to speed the full saturation of pores when sufficiently high hydraulic water pressures were dominant.     

Analytical solution for the 1D consolidation of unsaturated multi-layered soil

The 1D consolidation of unsaturated multi-layered soil is studied based on the theory proposed by Fredlund and Hasan, and an analytical solution for a typical boundary condition is obtained by assuming all material parameters remain constant during consolidation. In the derivation of the analytical solution, the eigenfunction and eigenvalue for the multi-layered problem are first derived through the transfer matrix method. Then, by using the method of undetermined coefficients and the orthogonal relation of the eigenfunction, the analytical solution is obtained. The present method is applicable to various types of boundary conditions. Finally, numerical examples are provided to investigate the consolidation behavior of unsaturated multi-layered soil.     

Investigating the scale of structural controls on chlorinated hydrocarbon distributions in the fractured-porous unsaturated zone of a sandstone aquifer in the UK

Contaminant migration behaviour in the unsaturated zone of a fractured porous aquifer is discussed in the context of a study site in Cheshire, UK. The site is situated on gently dipping sandstones, adjacent to a linear lagoon historically used to dispose of industrial wastes containing chlorinated solvents. Two cores of more than 100 m length were recovered and measurements of chlorinated hydrocarbons (CHCs), inorganic chemistry, lithology, fracturing and aquifer properties were made. The results show that selecting an appropriate vertical sampling density is crucial both to providing an understanding of contaminant pathways and distinguishing whether CHCs are present in the aqueous or non-aqueous phase. The spacing of such sampling should be on a similar scale to the heterogeneity that controls water and contaminant movement. For some sections of the Permo-Triassic aquifer, significant changes in lithology and permeability occur over vertical distances of less than 1 m and samples need to be collected at this interval, otherwise considerable resolution is lost, potentially leading to erroneous interpretation of data. At this site, although CHC concentrations were high, the consistent ratio of the two main components of the plume (tetrachloroethene and trichloroethene) provided evidence of movement in the aqueous phase rather than in dense non-aqueous phase liquid (DNAPL).     

Coupled solution of heat and moisture flow in unsaturated clay barriers in a repository geometry

An analytical solution in the Laplace transform domain is obtained for the transient heat and moisture transport in an unsaturated clay buffer with a geometry simulating repository conditions. A numerical inversion scheme based on Crump's method is used to obtain the time‐domain solution. The coupled effect of thermally driven moisture transport is especially investigated because of its importance to alter the flow field in low‐permeability buffers. The practical background is based on the case of an engineering bentonite barrier placed in a drift excavated in rock in the context of underground disposal of high‐level radioactive waste. Parametric study has been performed to assess the effects of dimensionless geometry and material parameters on flow field. Despite the simplified assumptions required in order to obtain analytical expressions, the results incorporate the main mechanisms involved in the coupled thermo‐hydraulic (T–H) problem, and they may be eventually used for validation purposes. Copyright © 2006 John Wiley & Sons, Ltd.     

根系和裂隙对土体水力和力学特性影响数值模拟

为探究根系和裂隙对土体水力及力学特性的影响,利用有限元软件计算降雨过程中裂隙和根土间隙对渗流场的影响,并以渗流计算结果为基础,分析降雨对根系固土作用的影响,采用分区强度折减法对降雨前后根?土复合体进行直剪试验模拟,同时考虑了侧根倾角的影响。结果表明,裂隙和根土间隙为雨水入渗提供优先通道,降雨影响深度随裂隙深度增加而增加;有根系时降雨影响深度由主根深度决定,侧根倾角对其影响较小,考虑根土间隙影响的降雨影响深度相较于无根系工况增加了93.3%;根系能显著提高土体的抗剪强度,相对于垂直主根方向的不同倾角,侧根增加土体抗剪强度由大至小依次为60o、45o、30o侧根和无根系;雨水入渗降低了土体强度,同时削弱了根系固土作用,使得降雨后根-土复合体抗剪强度大幅降低,是许多植被覆盖边坡仍发生浅层滑动原因之一。