A linear constitutive model for unsaturated poroelasticity by micromechanical analysis

This paper extends the Biot theory of poroelasticity from the saturated to unsaturated case. The Biot phenomenological model uses parameters that are easily observable, such as the deformation of porous frame, total stress, pore pressure, and fluid specific discharge. Such model is preferred for engineering applications. At this macroscopic level, the extension of Biot theory from saturated to unsaturated is straightforward. The constitutive constants, however, are combined properties of solid, pore space, and fluids. In the unsaturated case, the constants are functions of the degree of saturation. Their measurements and tabulation over a range of saturation is generally not feasible for practical applications. In this work, a Biot-Willis type analysis is performed for the unsaturated case to provide a theory that the bulk material constants can be evaluated using laboratory measurable micromechanical constants under saturated condition, plus a capillary pressure curve (saturation versus suction pressure) typically available for unsaturated porous medium, without the need of measurement at each state of saturation. In particular, it is demonstrated that the surface energy contained in the meniscus interface manifests as a “capillary modulus,” given by the negative inverse slope of the capillary pressure curve. A rigorous analysis based on the thermodynamic variational energy approach is also conducted to lend theoretical support to the phenomenological approach. The presented model can bring a closure to the practical engineering modeling of the deformation of partially saturated porous medium that lacks the information of material constants over the range of saturation.     

Evaluation of probabilistic flow in two unsaturated soils

A variably saturated flow model is coupled to a first-order reliability algorithm to simulate unsaturated flow in two soils. The unsaturated soil properties are considered as uncertain variables with means, standard deviations, and marginal probability distributions. Thus, each simulation constitutes an unsaturated probability flow event. Sensitivities of the uncertain variables are estimated for each event. The unsaturated hydraulic properties of a fine-textured soil and a coarse-textured soil are used. The properties are based on the van Genuchten model. The flow domain has a recharge surface, a seepage boundary along the bottom, and a no-flow boundary along the sides. The uncertain variables are saturated water content, residual water content, van Genuchten model parameters alpha (α) and n, and saturated hydraulic conductivity. The objective is to evaluate the significance of each uncertain variable to the probabilistic flow. Under wet conditions, saturated water content and residual water content are the most significant uncertain variables in the sand. For dry conditions in the sand, however, the van Genuchten model parameters α and n are the most significant. Model parameter n and saturated hydraulic conductivity are the most significant for the wet clay loam. Saturated water content is most significant for the dry clay loam. Electronic Publication     

干密度与含水率对重塑红黏土抗剪强度参数影响研究

为探究了解桂林重塑红黏土抗剪强度特性,利用三轴试验研究饱和及未饱和重塑红黏土的干密度、含水率对土体抗剪强度的影响。试验结果表明:相同最优含水率下的饱和重塑红黏土的黏聚力与干密度的二次多项式拟合曲线呈凹状,未饱和土的黏聚力与干密度的二次多项式拟合曲线呈凸状,两者黏聚力曲线在1.41 g ·cm-3附近两者差值最大;饱和重塑红黏土的内摩擦角与干密度的二次多项式拟合曲线呈凹状,未饱和土的内摩擦角与干密度的二次多项式拟合曲线呈凸状,两者在干密度为1.35 g ·cm-3附近差值最大;未饱和重塑红黏土的含水率对抗剪强度参数影响显著。     

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.     

温度对粘性土介质力学特性的影响

分析了温度效应对粘性土介质若干基本力学特性的影响。对粘性土的热固结问题、温度对粘性土介质渗透特性的影响、粘性土中的热传导规律及热阻抗特性、土-水体系在温度效应下的作用机理、温度作用下粘性土的本构规律等进行了深入研究。特别就温度诱致的孔隙水压力的变化机理及不同性质土类粘性土体积变化的可逆性问题进行了探讨。     

Coupled thermo-hydro-mechanical modelling of bentonite buffer material

Mechanistic model development and numerical analyses were carried out on coupled thermo-hydraulic-mechanical processes in bentonite-based buffer material for the geological disposal of high-level radioactive waste with small-scale laboratory experiments and a full-scale test. The mechanism of water movement in compacted bentonite was identified by applying theoretical equations to the experimental results. The application clearly explained the observed results of the temperature dependence of the hydraulic conductivity in the saturated state and the water diffusivity in the unsaturated state for the compacted bentonite and the dry density dependence of the diffusivity. The full-scale coupled test, BIG-BEN, was conducted at PNC (Power Reactor and Nuclear Fuel Development Corporation) Tokai Works. The results of the numerical analyses for the full-scale test which are based on the present knowledge of coupled processes and our small-scale experiments were in good agreement with the measured results except for mechanical phenomena. Copyright © 1999 John Wiley & Sons, Ltd.     

Experiment and validation of numerical simulation of coupled thermal,hydraulic and mechanical behaviour in the engineered buffer materials

Evaluation of the coupled heat transfer, water flow and stress changes in the engineered clay barrier is an important issue in the performance assessment of the high‐level radioactive waste disposal. To demonstrate the function of the engineered barrier system, the large‐scale experiment is conducted, which is called Big Bentonite facility (BIG‐BEN). The facility consists of an electric heater surrounded by glass beads, carbon steel overpack, buffer material and man‐made rock. The buffer is a mixture of bentonite and sand. The heater is operated at 0·8 kW. Water is injected from the interface between the buffer and the man‐made rocks at the pressure of 0·05 MPa. The duration of the experiment is 20 months. The change in temperature and swelling pressure are continuously monitored and gravimetric water content is measured by sampling. The coupled thermal, hydraulic and mechanical processes are simulated with a finite element code THAMES, which can simulate the fully coupled phenomena in the saturated and unsaturated clay under anisothermal condition. To examine the validity of the code, all the parameters used in the model are evaluated from the other laboratory tests. The simulated results are compared with the measured ones without calibration of the parameter values using the results from the BIG‐BEN experiment. It can be concluded that the changes in temperature and gravimetric water content within the buffer can be simulated reasonably well and that the mechanical effect such as swelling pressure is difficult to realize. Copyright © 2000 John Wiley & Sons, Ltd.     

非饱和土降雨诱发塌陷成因探讨

有些土体塌陷由降雨诱发，特别是久旱无雨后突降暴雨，有时会导致岩溶塌陷成群出现。同样，在干旱季节农田灌溉也会导致塌陷大量出现，可见塌陷的产生与土壤的干湿状态密切相关。自然土层接近地表部位为非饱和的包气带，以下才是饱水带，大气降雨及农田灌溉水入渗地下要有一个由非饱和带向饱水带运动的过程。由于土体为固气液和收缩膜四相体系，在地表水入渗地下的过程中，入渗的水峰会对土体结构中的气体产生驱动作用，对固体骨架也会产生动力作用。本文采用非饱和土力学的基本原理，对于非饱和土中吸力和水压传递机理以及降雨诱发塌陷进行了分析，并按提出的成因机制对暴雨诱发塌陷实例进行了分析。     

Determination of clay-water contact angle via molecular dynamics and deep-learning enhanced methods

Acta Geotechnica - Molecular dynamics modeling is a useful tool to study the interface physics of the clay-water system at the nanoscale. A key parameter to characterize the interface physics of...     

Impacts of saturation-dependent anisotropy on the shrinkage behavior of clay rocks

Geomaterials such as soils and rocks can exhibit inherent anisotropy due to the preferred orientation of mineral grains and/or cracks. They can also be partially saturated with multiple types of fluids occupying the pore space. The anisotropic and unsaturated behaviors of geomaterials can be highly interdependent. Experimental studies have shown that the elastic parameters of rocks evolve with saturation. The effect of saturation has also been shown to differ between directions in transversely isotropic clay rock. This gives rise to saturation-dependent stiffness anisotropy. Similarly, permeability anisotropy can also be saturation-dependent. In this study, constitutive equations accommodating saturation-dependent stiffness and hydraulic anisotropy are presented. A linear function is used to describe the relationship between the elastic parameters and saturation, while the relative permeability–saturation relationship is characterized with a log-linear function. These equations are implemented into a hydromechanical framework to investigate the effects of saturation-dependent properties on the shrinkage behavior of clay rocks. Numerical simulations are presented to demonstrate the role of saturation-dependent stiffness and hydraulic anisotropy in shrinkage behavior. The results highlight that strain anisotropy and time evolution of pore pressures are substantially influenced by saturation-dependent stiffness and hydraulic anisotropy.

     

A coupled poroplastic damage model accounting for cracking effects on both hydraulic and mechanical properties of unsaturated media

Damage induced by microcracking affects not only the mechanical behaviour of geomaterials but also their hydraulic properties. Evaluating these impacts is important for many engineering applications, such as the safety assessment of radioactive waste disposal facilities. This paper presents a new constitutive model accounting simultaneously for the impact of damage on hydraulic and mechanical properties of unsaturated poroplastic geomaterials. The hydro‐mechanical coupling is formulated by means of the thermodynamic framework for partially saturated media, extended by taking into account isotropic damage and plasticity. State and complementary laws are governed by the so‐called plastic effective stress and equivalent pore pressure. Assuming a bimodal pore size distribution for cracked porous media, the hydraulic part (water retention curve and hydraulic conductivity) is modelled using phenomenological functions of damage variable. The participation of damage on both mechanical and hydraulic part enables this model to describe bilateral couplings between them. This coupled model is then validated against a number of experimental data obtained from Callovo‐Oxfordian argillite, which is the possible host rock for a radioactive waste disposal in France. Parametric studies are also carried out to check the consistency and to better demonstrate the bilateral couplings in the model. Copyright © 2015 John Wiley & Sons, Ltd.     

包气带黄土环境研究与放射性废物处置

黄土中的粘粒含量较高、比表面积较大、吸附性较强、阳离子交换容量也较高,这些因素对迟滞放射性核素的迁移有利。而且黄土的非饱和渗透系数K(θ)远远低于它的饱水渗透系数K_(?)。使得放射性核素在黄土中的迁移很慢。因此,包气带黄土作为中、低放射性废物处置库的环境屏障是可行的。     

Experimental study on height simulation of capillary fringe

In this paper, the plexiglass experimental column was used to analyze the capillary fringe thickness of three kinds of lithologies-silty sand, silt and silty clay-providing a basis for defining the interface in the study of hydrodynamics at the water table between vandose water and groundwater. The capillary fringe generally refers to the subsurface layer in which the groundwater seeps up to the air-entry suction value due to capillary action, and is nearly saturated with water. The thickness of the capillary fringe varies with different lithologies. In this experiment, self-made stable water supply devices were used to study the height of capillary rise, capillary water volume and capillary fringe thickness of the three lithologies through capillary experiment and numerical simulation. Experimental results show as follows:(1) Rising height of capillary water is related to time, particle radius, volume, etc., and the relationship between height and time is in line with the Hill model.(2) The smaller the particle radius, the more water the pores contain, and the ratio of the unsaturated portion of capillary water to the total water content gradually rises. Experimental results obtained by numerical simulation, segmentation and actual measurement are consistent.(3) The thickness of the capillary zone is related to the lithology. The larger the particle size, the smaller the thickness of the capillary fringe, and vice versa. In silty sand, the thickness measures about 13 cm. The figure rises to 16 cm in silt, and 37 cm in silty clay. This work studies the law of soil water transport at saturated-unsaturated interface. Experimental results are of great significance to the study of soil water and salt transport and soil salinization control in unsaturated zone.     

An elastoplastic strainhardening model for soil allowing for hydraulic bonding–debonding effects

The paper presents a strainhardening constitutive model for unsaturated soil behaviour based on energy conjugated stress variables in the framework of superposed continua. The proposed constitutive law deals with hydro‐mechanical coupling phenomena. The main purpose is to develop within a consistent framework a model that can deal with possible mechanical instabilities occurring in partially saturated materials. The loss of capillary effects during wetting processes can, in fact, play a central role in unstable processes. Therefore, it will be shown that the bonding effects due to surface tensions can be described in a mathematical framework similar to that employed for bonded geomaterials to model weathering or diagenesis effects, either mechanically or chemically induced. The results of several simulations of common laboratory tests on partially saturated soil specimens are shown. The calculated behaviour appears to be in good qualitative agreement with that observed in the laboratory. In particular it is shown that volumetric collapse phenomena due to hydraulic debonding effects can be successfully described by the model. Finally, it will be highlighted the ability of the model to naturally capture the transition to a fully saturated condition and to deal with possible mechanical instabilities in the unsaturated regime. Copyright © 2008 John Wiley & Sons, Ltd.     

On the capillary stress tensor in wet granular materials

This paper presents a micromechanical study of unsaturated granular media in the pendular regime, based on numerical experiments using the discrete element method, compared with a microstructural elastoplastic model. Water effects are taken into account by adding capillary menisci at contacts and their consequences in terms of force and water volume are studied. Simulations of triaxial compression tests are used to investigate both macro and micro‐effects of a partial saturation. The results provided by the two methods appear to be in good agreement, reproducing the major trends of a partially saturated granular assembly, such as the increase in the shear strength and the hardening with suction. Moreover, a capillary stress tensor is exhibited from capillary forces by using homogenization techniques. Both macroscopic and microscopic considerations emphasize an induced anisotropy of the capillary stress tensor in relation with the pore fluid distribution inside the material. Insofar as the tensorial nature of this fluid fabric implies shear effects on the solid phase associated with suction, a comparison has been made with the standard equivalent pore pressure assumption. It is shown that water effects induce microstructural phenomena that cannot be considered at the macro level, particularly when dealing with material history. Thus, the study points out that unsaturated soil stress definitions should include, besides the macroscopic stresses such as the total stress, the microscopic interparticle stresses such as the ones resulting from capillary forces, in order to interpret more precisely the implications of the pore fluid on the mechanical behaviour of granular materials. Copyright © 2009 John Wiley & Sons, Ltd.     

Consolidation in spatially random unsaturated soils based on coupled flow‐deformation simulation

This paper integrates random field simulation of soil spatial variability with numerical modeling of coupled flow and deformation to investigate consolidation in spatially random unsaturated soil. The spatial variability of soil properties is simulated using the covariance matrix decomposition method. The random soil properties are imported into an interactive multiphysics software COMSOL to solve the governing partial differential equations. The effects of the spatial variability of Young's modulus and saturated permeability together with unsaturated hydraulic parameters on the dissipation of excess pore water pressure and settlement are investigated using an example of consolidation in a saturated‐unsaturated soil column because of loading. It is found that the surface settlement and the pore water pressure profile during the process of consolidation are significantly affected by the spatially varying Young's modulus. The mean value of the settlement of the spatially random soil is more than 100% greater than that of the deterministic case, and the surface settlement is subject to large uncertainty, which implies that consolidation settlement is difficult to predict accurately based on the conventional deterministic approach. The uncertainty of the settlement increases with the scale of fluctuation because of the averaging effect of spatial variability. The effects of spatial variability of saturated permeability k_sat and air entry parameters are much less significant than that of elastic modulus. The spatial variability of air entry value parameters affects the uncertainties of settlement and excess pore pressure mostly in the unsaturated zone. Copyright © 2016 John Wiley & Sons, Ltd.     

非饱和成都黏土瞬时剖面法渗透性试验研究

非饱和渗透系数是土体渗流分析的基础,成都黏土作为一种典型的非饱和膨胀土,具有吸水膨胀、失水收缩的特性,在受侧限的浸水过程中,土颗粒的膨胀致使孔隙体积减小,渗透性降低,使得直接对其进行非饱和渗透试验十分困难。根据瞬时剖面法的原理,利用EC-5土壤水分传感器测含水率、MPS-2电介质水势传感器直接同步测量同一位置的基质吸力,通过水平渗透试验研究了非饱和成都黏土在侧限条件下的渗透性。含水率和基质吸力的同步测量,保证了其测试条件的一致性,避免了采用其他土水特征曲线的影响。试验表明,试样的非饱和渗透系数为（1.33×10-11~3.14×10-9）m·s-1,非饱和渗透系数与基质吸力并非单调线性关系。基质吸力较高时,受膨胀土颗粒吸水膨胀的影响,渗透系数未出现明显变化,基质吸力降低到一定程度后,渗透系数快速增大。试验结束时土体已接近饱和,土中气体排出较慢,过水断面增加缓慢,促使渗透系数仍然持续增大。采用VG模型拟合k-s曲线,拟合参数α=0.048 kPa-1,n=1.79,m=0.48,试验结果可以用于成都黏土地区的渗流分析。     

黄土高原北部坡面尺度土壤饱和导水率分布与模拟

为探明黄土高原北部坡面土壤饱和导水率(K_s)空间分布特征,为土壤水文过程模拟与预测提供理论依据,采用经典统计学和地统计学的空间变异分析方法,分析了坡面尺度土壤K_s的空间变异特征,并用一阶自回归状态空间模型对K_s的空间分布进行了模拟.研究区坡面尺度K_s的变异为中等程度变异,具有中等程度空间依赖性,变程为42 m.K_s与容重、砂粒、粉粒和黏粒含量在不同滞后距离下均具有自相关关系和交互相关关系.容重和土壤颗粒是影响坡面K_s空间分布的主要因素.状态空间模拟结果表明,基于容重和土壤颗粒的状态空间方程可以很好地解释坡面K_s的变异状况(R2>0.9).一阶自回归状态空间模型可用于田间条件下坡面尺度K_s分布特征的预测.     

Micromechanical approach to swelling behavior of capillary-porous media with coupled physics

A detailed multiscale analysis is presented of the swelling phenomenon in unsaturated clay-rich materials in the linear regime through homogenization. Herein, the structural complexity of the material is formulated as a three-scale, triple porosity medium within which microstructural information is transmitted across the various scales, leading ultimately to an enriched stress-deformation relation at the macroscopic scale. As a side note, such derived relationship leads to a tensorial stress partitioning that is reminiscent of a Terzaghi-like effective stress measure. Otherwise, a major result that stands out from previous works is the explicit expression of swelling stress and capillary stress in terms of micromechanical interactions at the very fine scale down to the clay platelet level, along with capillary stress emerging due to interactions between fluid phases at the different scales, including surface tension, pore size, and morphology. More importantly, the swelling stress is correlated with the disjoining forces due to electrochemical effects of charged ions on clay minerals and van der Waals forces at the nanoscale. The resulting analytical expressions also elucidate the role of the various physics in the deformational behavior of clayey material. Finally, the capability of the proposed formulation in capturing salient behaviors of unsaturated expansive clays is illustrated through some numerical examples.