含节理黄土渗流数值模型研究

本文建立了在考虑黄土中有节理存在时,水分场数值计算的有限元方法。采用质量守恒的观点推导了质点元中饱和度的变化与流速的关系。进而利用达西定律得到以水头为变量的渗流基本方程。针对黄土垂直节理的渗流特点,确定了节理渗流基本方程中的参数。采用四边形等参元,利用Galerkin加权余量法建立考虑节理影响的黄土非饱和渗流的有限元形式。对局部水头边界条件下的黄土节理二维水分入渗问题进行了数值分析。结果表明,节理对黄土场地湿润峰的迁移有很大影响。     

Slip line theory applied to a retaining wall–unsaturated soil interaction problem

An extension of slip line theory to unsaturated soils is presented and applied to the problem of a rigid retaining wall rotating about its toe into unsaturated soils. Suction is introduced using the effective stress concept. Soil–wall interface friction is defined carefully. The influence of suction on limiting passive earth pressures is analysed for two soils under steady state evaporation and infiltration. Suction increases the limiting passive stress at the soil–wall interface, with a dependence on the steady state flow type. The displacement of the retained soil is studied assuming the wall undergoes a rotation increment. The results show a clear difference in the displacement for evaporation and infiltration.     

A constitutive model for unsaturated soils with consideration of inter-particle bonding

The paper presents a physically-based constitutive model for unsaturated soils that considers the bonding effect of water menisci at inter-particle contacts. A bonding factor has been used to represent the magnitude of the equivalent bonding stress, defined as the bonding force per unit cross-sectional area. The average skeleton stress is employed to represent the effect of average fluid pressures within soil pores. Based on an empirical relationship between the bonding factor ζ and the ratio e/e_s (where e and e_s are void ratios at unsaturated and saturated states, respectively, at the same average skeleton stress), we propose an elasto-plastic constitutive model for isotropic stress states, and then extend this model to triaxial stress states within the framework of critical state soil mechanics. Because only one yield surface is needed in the proposed model, a relatively small number of parameters are required. Comparisons between experimental data and model results show that, in most cases, the proposed model can reasonably capture the important features of unsaturated soil behavior.     

Numerical studies on the effect of temperature on the unsaturated hydraulic response of geotextiles

A series of coupled thermo-hydraulic simulations were performed on a soil–geotextile column to understand the effect of temperature on suction distribution throughout the soil column and on the hydraulic performance of the geotextile as a drainage/capillary barrier layer. Two different constant temperatures of 0 °C and 38 °C and a temperature gradient of 4 °C along the column were modeled. Changing the temperature from 0 °C to 38 °C did not have a significant effect on the suction head distribution in the soil–geotextile column. The temperature gradient resulted in appreciable thermal vapor flow and changes in suction head and hydraulic conductivity of the geotextile. During drainage, the temperature gradient and lower temperature at the top of the column increased suction in the geotextile and its ability to function as a capillary barrier. During capillary rise, the temperature gradient and lower temperature at the top of the column decreased the suction in the geotextile and its ability to function as a capillary barrier. Changing the direction of the thermal gradient reversed the water vapor flow direction and its effect on the suction in the geotextile. A temperature gradient did not have a noticeable effect on the suction head of the geotextile when positive pore pressure was developed in the geotextile and adjacent soil during drainage.     

Semi-analytical solution to one-dimensional consolidation for viscoelastic unsaturated soils

This paper presents a semi-analytical solution to one-dimensional consolidation of viscoelastic unsaturated soils with a finite thickness under oedometric conditions and subjected to a sudden loading. The solution is obtained by using Lee’s correspondence principle based on the semi-analytical solution to one-dimensional consolidation of elastic unsaturated soils. The boundary contains the top surface permeable to water and air and the bottom impermeable to water and air. A typical example is given to show the evolution of excess pore-air and pore-water pressures as well as the total degree of consolidation of the soil layer with time for different ratios of air–water permeability coefficient, elastic modulus and viscoelastic coefficient. The one-dimensional consolidation behavior of viscoelastic unsaturated soil is discussed according to the semi-analytical solution. These results contribute to a better understanding of the consolidation behavior of viscoelastic unsaturated soils.     

离心实验在污染物迁移研究中的应用

离心实验模拟以其可以获得与原型一致的应力水平并且能够大大缩短原型历时而被应用于污染物迁移实验研究。在离心相似理论及离心实验模拟的相似基础上,重点回顾了离心机在饱和带水分、非饱和带水分、保守性溶质、NAPLs、重金属、核素迁移以及污染场地修复方面的实验应用。最后讨论了离心模拟中土壤预制和加速度选择的问题,并简单介绍了离心监测方法。可以得出结论:离心机能够成功用于各类物质迁移的实验研究中,离心实验模拟能够为理论和数值等分析方法快速提供真实可靠的参数依据,但离心实验的理论基础和监测方法需要进一步完善,此外也应积极开展更接近实际情况下的离心实验模拟研究。     

基质吸力变化引起的体积应变研究

与降雨有关的边坡表层变形位移,其根源是基质吸力降低引起的非饱和土体积应变。利用某滑坡体的原状样及其重塑样,分别在100kPa、200kPa围压作用下,采用单纯增加基质吸力的应力路径,对基质吸力引起的体积应变,进行试验测试。结果显示: (1)试样的体应变,随基质吸力的增大,单调增大,服从对数函数关系;(2)围压由100kPa增加到200kPa的试样体积模量,随基质吸力的增大,单调增大,服从幂函数关系;(3)与基质吸力有关的体积变化系数,随基质吸力的增大,单调减小,服从幂函数关系;并且当基质吸力大于200kPa以后,随基质吸力增大,体积变化系数接近于常数,约为0.0059左右。在此基础上,本文针对这种由单纯基质吸力变化引起的非饱和土体积应变规律及机理,进行了探讨。     

A constitutive-relationship model for film flow on rough fracture surfaces

Film flow on fracture surfaces may be an important mechanism for fast flow in unsaturated fractured rocks. Incorporating this mechanism into a numerical model requires knowledge of constitutive relationships for film flow. Based on fractal concepts and a conceptual argument of Tokunaga et al. that water films could be treated as analogues to water in unsaturated porous media, a simple constitutive-relationship model has been developed. The validity of the model is supported by excellent agreements between calculation results and experimental observations for two different fracture surfaces.

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Resumen El flujo pelicular en la superficie de las fracturas puede ser un mecanismo importante para el flujo rápido en rocas fracturadas no saturadas. Incorporarlo en un modelo numérico requiere conocer las relaciones constitutivas del flujo pelicular. Se ha desarrollado un modelo de una relación constitutiva sencilla partiendo de conceptos fractales y de un argumento conceptual de Tokunaga et al., según el cual las películas de agua pueden ser tratadas como análogos del agua en medios porosos no saturados. La validez del modelo es corroborada por los excelentes ajustes entre los resultados numéricos y las observaciones experimentales en dos superficies de fracturas diferentes.

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Résumé Lécoulement en film sur les surfaces de fracture peut être un mécanisme important pour lécoulement rapide dans les roches fracturées non saturées. Lincorporation de ce mécanisme dans un modèle numérique nécessite la connaissance des relations fondamentales pour lécoulement en film. Basé sur des concepts fractals et sur un argument conceptuel de Tokunaga et al. selon lequel des films deau peuvent être considérés comme des analogues de leau en milieu poreux non saturé, un modèle simple des relations fondamentales a été développé. La validité de ce modèle est confirmée grâce à une excellente concordance entre les résultats du calcul et les observations expérimentales pour deux surfaces différentes de fractures.

     

Solution of dynamic Green׳s function for unsaturated soil under internal excitation

The closed form three-dimensional Green׳s function of a semi-infinite unsaturated poroelastic medium subjected to an arbitrary internal harmonic loading is derived, with consideration of capillary pressure and dynamic shear modulus varying with saturation. By applying the Fourier expansion techniques and Hankel integral transforms to the circumferential and radial coordinates, respectively, the general solution for the governing partial differential equations is obtained in the transformed domain. A corresponding boundary value problem is formulated. The integral solutions for the induced displacements, pore pressure and net stress are then determined considering the continuity conditions. The formulas are compared with the degenerated solution of saturated soils and confirmed. Numerical results reveal that the response of the unsaturated half-space depends significantly on the saturation by altering dynamic shear modulus to account for the effects of matric suction on soil stiffness. Slight differences between the results occur if only the saturation is taken into account. Moreover, a large source-depth results in a pronounced contribution to the reduction of surface displacement amplitudes. The analytical solutions concluded in the study offer a broader application to dynamic response associated with axi-symmetric and asymmetric conditions.     

Free-Swell and Swelling Pressure of Unsaturated Compacted Clays; Experiments and Neural Networks Modeling

Expansive soils have received attentions of several investigators in the past half of century in the problematic soils context. Volume change behavior of unsaturated compacted soils in presence of water and change of degree of saturation was observed in two form of heave or collapse. Low water content and low density compacted soils in presence of enough surcharge pressure lose stability and collapse, because of their metastable and susceptible structure to change of degree of saturation. Free-swell and swelling pressure of five compacted clays, covering low to high plastic clays have been investigated in respect to compaction states and swelling pressure was compared with collapse pressure threshold. The results of experiments were utilized in two Artificial Neural Networks to predict free-swell percent and swelling pressure of a soil sample based on index properties and compaction state.