Poroelastic behaviour of fine compacted soils in the unsaturated to saturated transition zone

The behaviour of quasi-saturated materials is important to consider when designing cuttings and embankments in which earthwork materials are compacted to the optimum proctor density. Under this condition, the in-pore gaseous phase takes the form of air pockets and bubbles embedded within the liquid phase, which significantly affects the overall behaviour of the soil. The assessment of highly saturated soils thus requires a precise understanding of hydro-chemo-mechanical couplings between the entrapped air, the in-pore liquid and the solid skeleton. This paper presents a fully coupled poromechanical model that separates the kinematics and the mechanical behaviours of the phases in their interactions with each other (e.g., liquid water, dissolved air, gaseous air and solid matrix). The assumptions about the entrapped air behaviour are defined from a bibliographic study, and linear elastic behaviour is used for both the liquid phase and the solid skeleton. The model is implemented in the FEM code COMSOL and is subsequently used to simulate oedometric tests under different loading paths: undrained compression or imposed liquid pressure variation at constant stress. The behaviour, which shows a continuous transition from unsaturated to saturated, is logical and consistent with available experimental data.     

盐度对多孔介质中DNAPL运移和分布的影响

选用四氯乙烯（PCE）作为典型DNAPL污染物,以NaCl作为地下水中溶解盐代表,研究盐度对DNAPL在饱和多孔介质中运移和分布的影响。通过批次实验测定NaCl水溶液/石英砂/PCE三相体系下的接触角和界面张力,结果表明,PCE在石英砂表面的接触角随着水中NaCl浓度的增大而减小,而PCE和NaCl水溶液的界面张力随着NaCl浓度的增大而增大,尤其当氯化钠浓度较高时（>0.1 mol/L）,影响程度更为显著。在此基础上,采用透射光法监测不同介质情景下DNAPL在二维砂箱中的运移和分布,定量测定DNAPL在介质中的饱和度。实验结果表明,地下水盐度的增加将促进DNAPL的垂向入渗,减少被截留在运移路径上的DNAPL量,使得DNAPL运移路径及累积形成的池状DNAPL（pool）向水流方向偏移。在均质多孔介质和含有透镜体的非均质多孔介质中,随着盐度的增加,DNAPL在横向和垂向上的展布均呈现出增加趋势,导致污染源区变大,且介质中以离散状存在的DNAPL量明显增加。     

Effects of restricted flow at the surface of saturated clay

The deflection of, and the pore pressure at, the surface of a saturated clay mass are computed by modelling the mass mathematically as a consolidationg half-space. Specifically, attention is focused on the effect of a surface cover which impedes, though does not entirely prevent, the flow of the pore water. A number of results of interest are reported, generalizing work reported earlier by McNamee and Gibson⁸.     

河岸带表土层氮素非饱和入渗运移规律模拟

自制土槽装置,室内模拟氮素在河岸带表土层中非饱和入渗的过程.通过观测土槽中不同点位渗流水体出流时间的先后顺序以及各点水体中TN、NH₄+-N浓度随入渗时间的变化,探讨氮素在河岸带表土层中非饱和入渗的运移规律.试验结果表明:在河岸带表土层非饱和入渗过程中,水分水平运移速度小于垂直运移速度,NH₄+-N的运移滞后于水分的运移;在土壤"干"-"湿"-"干"过程中,各点TN、NH₄+-N浓度值随时间先急剧增加再缓慢减少然后趋于稳定.另外,河岸带表土层对非饱和入渗氮素的截留效果较好,试验11 h时,在1.08m的坡长距离上,距土表面12 cm深度TN、NH₄+-N浓度比进水分别下降74.23%和68.02%.     

考虑地表径流与地下渗流耦合的斜坡降雨入渗研究

为简化分析，在模拟斜坡降雨入渗暂态渗流时，通常没有考虑入渗和产流的耦合过程。笔者提出了一种新的考虑地表径流与地下渗流耦合的斜坡降雨入渗分析方法，它较好地模拟了入渗和产流的过程，并通过算例，研究了考虑与不考虑该耦合作用对斜坡孔隙水压力分布的影响。     

层状非均质结构包气带入渗过程单相流与两相流数值模拟对比研究

包气带水分入渗过程受多种因素的影响。定量研究层状非均质岩性结构和入渗速率对其影响,有助于解决根据不同条件选择单相流模型或水气二相流模型模拟包气带水分入渗过程的问题。结合填埋场等场地地层条件及污废水入渗特征,分别建立了“上细下粗”和“上粗下细”包气带层状非均质岩性结构水分入渗单相流和水气二相流模型,探讨不同层状非均质岩性结构条件下模型的适用性。在“上粗下细”岩性结构模型基础上,进一步探究入渗速率对水气两相运移结果的影响。基于论文模型研究表明：（1）在包气带岩性结构为“上细下粗”的条件下,气相的影响基本可以忽略,可直接采用单相流模型对包气带水分运移进行模拟;在“上粗下细”岩性结构和本次模型设定的底部压力保持不变及污废水泄漏前场地未接受降水入渗补给等条件下,当包气带上下层介质渗透率比值大于16倍时,气相会对水相运移产生明显影响,且下层介质渗透率越小、上下层介质渗透率比值越大,单相流与两相流的运移结果差别越大,需要采用水气二相流模型模拟包气带水分运移。（2）在包气带“上粗下细”岩性结构条件下,入渗速率越大,气相对水流入渗的阻滞作用越明显,此时包气带水分运移模拟应采用水气二相流模型。     

Tracing of residual multiple DNAPL sources in the subsurface using 222Rn as a natural tracer at an industrial complex in Wonju, Korea

Depth-discrete tracing of residual dense non-aqueous phase liquid (DNAPL) sources in the subsurface is of great importance in making decisions related to contaminated groundwater remediation. Temporal variations in the natural tracer ²²²Rn and contaminant concentrations in groundwater contaminated with multiple chlorinated contaminants, such as trichloroethene, carbon tetrachloride, and chloroform, were examined to trace residual multiple DNAPL contaminants at an industrial complex in Wonju, Korea. The ²²²Rn activities and multiple DNAPL concentrations in the groundwater fluctuated irregularly according to the groundwater recharge. The natural tracer ²²²Rn in groundwater present in the soil layer, originating from the underlying crystalline biotite granite, had a wide range from 29,000 to 179,000 Bq/m³, and total concentrations of chlorinated solvents ranged from 0.06 to 17.77 mg/l, indicating the ambiguous results of ²²²Rn for tracing the residual DNAPL sources. In this paper, a method is presented to locate zones with a high probability of containing depth-discrete residual multiple DNAPL sources using ²²²Rn and considering relative contaminant concentrations. The results demonstrate that the combination of the ²²²Rn activities as a natural tracer and the relative contaminant concentrations is able to be used as a useful tool for tracing residual DNAPLs.     

Numerical simulation of NAPL flow in the subsurface

A three-dimensional, three-phase numerical model is presented for simulating the movement of immiscible fluids, including nonaqueous-phase liquids (NAPLs), through porous media. The model is designed to simulate soil flume experiments and for practical application to a wide variety of contamination scenarios involving light or dense NAPLs in heterogeneous subsurface systems. The model is derived for the three-phase flow of water, NAPL, and air in porous media. The basic governing equations are based upon the mass conservation of the constitutents within the phases. The descretization chosen to transform the governing equations into the approximating equations, although logically regular, is very general. The approximating equations are a set of simultaneous coupled nonlinear equations which are solved by the Newton-Raphson method. The linear system solutions needed for the Newton-Raphson method are obtained using a matrix of preconditioner/accelerator iterative methods. Because of the special way the governing equations are implemented, the model is capable of simulating many of the phenomena considered necessary for the sucessful simulation of field problems including entry pressure phenomena, entrapment, and preferential flow paths. The model is verified by comparing it with several exact analytic test solutions and three soil flume experiments involving the introduction and movement of light nonaqueous-phase liquid (LNAPL) or dense nonaqueous-phase liquid (DNAPL) in heterogeneous sand containing a watertable. This revised version was published online in August 2006 with corrections to the Cover Date.     

Garnet‐controlled very low velocities in the lower mantle transition zone at sites of mantle upwelling

Deep mantle plumes and associated increased geotherms are expected to cause an upward deflection of the lower–upper mantle boundary and an overall thinning of the mantle transition zone between about 410 and 660 km depth. We use subsequent forward modelling of mineral assemblages, seismic velocities, and receiver functions to explain the common paucity of such observations in receiver function data. In the lower mantle transition zone, large horizontal differences in seismic velocities may result from temperature‐dependent assemblage variations. At this depth, primitive mantle compositions are dominated by majoritic garnet at high temperatures. Associated seismic velocities are expected to be much lower than for ringwoodite‐rich assemblages at undisturbed thermal conditions. Neglecting this ultralow‐velocity zone at upwelling sites can cause a miscalculation of the lower–upper mantle boundary on the order of 20 km.     

Use of geostatistical models in DNAPL source zone architecture and dissolution profiles assessment in spatially variable aquifer

Following the accidental subsurface release of dense nonaqueous phase liquids (DNAPLs), spatial variability of physical and chemical soil/contaminant properties can exert a controlling influence on infiltration pathways and organic entrapment. DNAPL spreading, fingering, and pooling typically result in source zones characterized by irregular contaminated regions with complex boundaries. Spatial variability in aquifer properties also influences subsequent DNAPL dissolution and aqueous transport dynamics. An increasing number of studies have investigated the effects of subsurface heterogeneity on the fate of DNAPL; however, previous work was limited to the examination of the behavior of single-component DNAPL in systems with simple and well-defined aqueous and solid surface chemistry. From a DNAPL remediation point of view, such an idealized assumption will bring a large discrepancy between the designs based on the model simulation and the reality. The research undertaken in this study seeks to stochastically explore the influence of spatially variable porous media on DNAPL entrapment and dissolution profiles in the saturated groundwater aquifer. A 3D, multicomponent, multiphase, compositional model, UTCHEM, was used to simulate natural gradient water flooding processes in spatially variable soils. Porosity was assumed to be uniform or simulated using sequential Gaussian simulation (SGS) and sequential indicator simulation (SIS). Soil permeability was treated as a spatially random variable and modeled independently of porosity, and a geostatistical method was used to generate random distributions of soil permeability using SGS and SIS (derived from measured grain size distribution curves). Equally possible 3D ensembles of aquifer realizations with spatially variable permeability accounting of physical heterogeneity could be generated. Tetrachloroethene (PCE) was selected as a DNAPL representative as it was frequently discovered at many contaminated groundwater sites worldwide, including Thailand. The randomly generated permeability fields were incorporated into UTCHEM to simulate DNAPL source zone architecture under 96-L hypothetical PCE spill in heterogeneous media and stochastic analysis was conducted based on the simulated results. Simulations revealed considerable variations in the predicted PCE source zone architecture with a similar degree of heterogeneity, and complex initial PCE source zone distribution profoundly affected PCE recovery time in heterogeneous media when subject to natural gradient water flush. The necessary time to lower PCE concentrations below Thai groundwater quality standard ranged from 39 years to more than 55 years, suggesting that spatial variability of subsurface formation significantly affected the dissolution behavior of entrapped PCE. The temporal distributions of PCE saturation were significantly altered owing to natural gradient water flush. Therefore, soil heterogeneity is a critical factor to design strategies for characterization and remediation of DNAPL contaminated sites. The systematic and comprehensive design algorithm developed and described herein perhaps serves as a template for application at other DNAPL sites in Thailand.     

Study of fate and transport of chromium in unsaturated zone- saturated zone

<正>Effluent irrigation can ease the water shortage area agricultural water shortage,and waste-water recycling,as well as bring a series of scientific and socio-economic issues,such as environmentalissues,food safety,sustainable development and so on,which has attracted the attention of people.So,paper to Kaifeng city several important dirt irrigation district for     

Comparison of the anisotropic behaviour of undeformed sandstones under dry and saturated conditions

This article presents a systematic analysis of the anisotropic behaviours of the Bentheim and Rothbach sandstones using ultrasonic P-wave velocity, electrical conductivity and magnetic susceptibility measurements. For each sandstone, the data were obtained from three core samples drilled perpendicularly to each other and tested in dry- and water-saturated conditions. For acoustic and magnetic investigations, the same statistical analysis was applied in order to present the data on comparable stereoplots. Surprisingly, the Bentheim sandstone which appeared homogeneous at macroscopic scale showed a stronger elastic and electrical anisotropy than the Rothbach sandstone in which cross-laminations were clearly identified, as confirmed by a sedimentary magnetic fabric. A discussion on the velocity contrasts between dry and saturated samples led us to consider two different origins for the observed anisotropies. First, by comparing electrical and acoustic properties in the Bentheim sandstone, we conclude that the nature of the anisotropic behaviour is linked to the anisotropy of pore shape: the inclusion model developed by Kachanov (Kachanov, M., 1993. Elastic solids with many cracks and related problems. Advances in Applied Mechanics, vol. 30. Academic Press, Boston, MA, pp. 259–445) accounts for our observations if one considers that the pore space is made of parallel flat pores with moderate pore aspect ratio. Second, acoustic, electrical and magnetic properties indicate that the observed anisotropy in the Rothbach sandstone can be attributed to the matrix, and more specifically to cementation: we modified the Dvorkin and Nur (Geophysics 61 (5) (1996) 1363) model of cemented granular media by introducing a spatially variable contact length, and the model suggests that a very small variability of cemented contact length is enough to account for the observed P-wave velocity anisotropy. We emphasise the fact that combining several kinds of measurements is of great help in capturing the nature of the anisotropic behaviour of porous rocks.     

基于VSF的非饱和-饱和带耦合过程的三维水流数值模拟

包气带作为"四水转化"的关键带,在地下水流与溶质运移模拟中起着重要作用。VSF是一款基于MODFLOW-2000开发的用于模拟非饱和-饱和带水分变化和运移过程的数值模拟程序。本文首先介绍了VSF程序的设计原理与结构,并重点介绍了VSF区别于MODFLOW-2000的用以概化非饱和带水分运移和边界条件设置的子程序包模块,包括Richards方程渗流(REF)、浸润面(SPF)、地表滞水(PND)、地表蒸发(SEV)和植物根区蒸散发(RZE)等子程序包。本文以REF、SPF、PND和SEV等子程序包为例,将VSF与MODFLOW-2000耦合应用于一个非饱和-饱和带水流运移过程的模拟算例。结果表明,VSF可与MODFLOW耦合实现饱和带水位与非饱和带饱和度的同步计算,并能刻画模型中单元格饱和状态的变化过程,其多种实用的边界条件可提供较为全面的模拟条件,是模拟实际包气带-饱和带耦合问题的有效工具。     

浅层包气带水汽昼夜运移规律及其数值模拟研究

西北干旱、半干旱地区,浅层包气带水分通量主要由水汽组成,而水汽在运移过程中产生的能量转换和质量迁移是地表质能平衡计算不可缺少的重要源汇项。在野外进行一个沙坑实验,发现土壤水在中午(12:00—15:00)达到最大值(10cm深度,5.9~6.1cm3/cm3;30cm深度,11.9~13.1cm3/cm3),而在凌晨(02:00—05:00)出现最小值(10cm深度,4.4~4.5cm3/cm3;30cm深度,10.4~10.8cm3/cm3)。为进一步验证该实验条件下的土壤水运移及分布规律,考虑了土壤水、汽、热耦合运移的HYDRUS-1D模型被用来对实验过程进行模拟,模拟结果与实测结果吻合较好。为描述土壤水分昼夜运移模式,笔者将土壤水耦合运移的时间信息和空间信息进行同步分析;并根据土壤水运移的不同驱动力,分别对温度梯度、基质势梯度作用下的液态水及汽态水通量进行了分析。     

The influence of vegetation on turbulence and flow velocities in European salt-marshes

Flow hindrance by salt‐marsh vegetation is manifested in the structure of the tidal current; it has a significant impact on sediment transport and it has been related to increased sediment accretion. The flow characteristics in three different vegetation types (Spartina maritima, Sp. anglica and Salicornia sp./Suaeda maritima) were measured on three salt‐marshes in Portugal and England. These in situ measurements differ from laboratory flume experiments with ‘clean’ vegetation by the complexity of natural canopies. Skimming flow develops above the Spartina canopy when the vegetation is fully submerged. In this situation, a low turbulence zone with nearly constant velocity in the denser canopy is separated from the skimming flow above by an interface characterized by high Reynolds stresses. In the low turbulence zone, a positive relationship exists between turbulence intensity and shoot density, which is due to wake turbulence generated locally in the canopy. The rate of particle settling should be increased in that zone. The lower limit of skimming flow is best predicted by the height within the canopy that includes 85% of the biomass. For emergent Spartina canopies and the short Salicornia/Suaeda marsh, the maximal velocity‐gradient is shifted upwards compared to a standard boundary layer over bare sediment and the turbulence is attenuated near the bed, but to a lesser extent than for fully submerged Spartina canopies. A turbulence reduction near the bed was observed in all measured profiles; that should enhance sediment deposition and protects the bed against subsequent erosion.     

A two-phase model for the description of the influence of cracks on the P- and S-wave velocities in dry and saturated rock samples

The premonitory variations of seismic-wave velocities before earthquakes originate from various cracking processes before the fracture. It can be shown that these variations are comparable with wave-velocity variations in porous model samples with defined pore sizes. Furthermore, it is possible to describe analytically the variation of wave velocities as a function of the parameter K₀, which describes the fracturing process, and a material/depth parameter AP. On the basis of the wave velocity vs. pressure curves of rocks, it is possible to determine K₀ and A. Using the material/depth parameter AP sediments in covering strata and eruptive rocks in regions of earthquakes of shallow to medium depth can be descirbed. A relationship between Δυ_D and ΔK₀ can be established. For acid to ultrabasic rocks, a variation of up to 2 km/s indicates a variation of K₀ of 0.1–1.0.Moreover, it is possible to establish a relationship between K₀ and the number as well as the mean length of cracks in the rock. The solutions differ depending on the cracks being closed or open. For closed cracks a wave-velocity minimum of 6% results. For oper cracks the variations of the number and mean length of cracks are taken into account by means of a stochastic process; the resulting variations of K₀ and the wave velocity car adequately explain the variations in seismic-wave velocity. Variations of the pore pressure have an influence on K₀and the wave velocity only under most favourable geologica conditions; generally they are insignificant. Also for S-wavc velocities and for the ratio υ_p/υ_s the wave velocity vs. pressure equations are valid; it is possible to state K₀ and A-values.     

河水入渗过程中近岸带地下水中砷的形成与影响因素研究

沈阳黄家水源地是我国北方地区典型的傍河地下水水源地,近岸带地下水中铁(Fe)、锰(Mn)、砷(As)含量严重超标。为查明地下水中As的来源与影响因素,对研究区河水、地下水以及土壤样品进行采集与测试,分析了水样常规指标与碳硫稳定同位素、土样中典型矿物、砷的含量及赋存形态。结果表明,研究区河水中As含量很低,而地下水中As含量普遍超标。河水入渗初期,氧化性河水使部分含As矿物发生氧化而释放As;随着河水入渗,地下水向还原环境转变,含As的Fe/Mn矿物发生还原性溶解,地下水中As含量逐渐升高。研究区典型矿物有黄铁矿、菱铁矿、软锰矿、赤铁矿、针铁矿、菱锰矿等,通过可交换态砷解吸、有机质结合态砷氧化、铁锰氧化物结合态砷还原性溶解等,介质中的As释放至地下水中。地下水中As含量与酸碱度(pH)、氧化还原电位(Eh)呈一定负相关,与溶解有机碳(DOC)、 {\mathrm{HCO}}_3^{-}、Fe、Mn含量呈正相关。     

Mathematical simulation on the oil slick spreading and dispersion in nonuniform flow fields

The viscous fluid boundary layer equations were adopted to characterize the velocity distribution across the vertical section of the oil slick on moving water. The velocity profile was found to be the combination of linear and parabolic distribution. A numerical model including spreading and dispersion was developed to describe the oil slick’s early movement in the open and ice-covered water. The flume test was conducted to determine the dispersion coefficients (K_x) and the effects of velocity and wave height on the slick’s dispersion were also investigated. In the open water, K_x increased from 4.34 cm²/s to 20.08 cm²/s as the velocity changed from 3 cm/s to 12 cm/s. Acoefficient β that characterized K_x fluctuated at 1.5 when wave heights were between 20 mm and 70 mm. Under ice, the slick didn’t move until the velocity exceeded 6 cm/s and K_x increased from 2.69 cm²/s to 5.64 cm²/s as the velocity changed from 6 cm/s to 12 cm/s. β remained 0.4 when wave heights were between 20 mm and 60 mm. The model performed very well in predicting the slick’s position and length during the gravitation-inertia phase for the two situations when relevant dispersion coefficients were input.     

Influence of structure on the compression and shear behaviour of a saturated lateritic clay

Lateritic clay is well recognized to contain significant amount of iron and aluminium oxides (sesquioxide). These oxides enhance the formation of soil aggregates which would greatly affect soil structure. So far, no study has been carried out purposely to investigate the influence of aggregate-dependent structure on the mechanical behaviour of the lateritic clay. In this study, structure effects on the compression and shear behaviour of a saturated lateritic clay were studied. Intact, recompacted and reconstituted specimens were studied through oedometer, isotropic compression and consolidated undrained shear tests. Microstructure of these specimens was determined using the mercury intrusion porosimetry and scanning electron microscopy (SEM) techniques and then used to explain the observed behaviour. It was found that the compressibility of recompacted/reconstituted specimens was about 90% larger than intact specimen. Different from soft clay, the influence of microstructure does not show an obvious reduction in compressibility after yielding. The peak shear strength of intact specimen was about 100% higher than those of reconstituted/recompacted specimens. The significant difference in compression and shear behaviour is mainly because the aggregates of intact specimen were about 90% larger than those of reconstituted/recompacted specimens, as revealed by the SEM results. As a result, particle contacts forming the force chain were therefore larger in the intact specimen. The intact specimen therefore became less compressible and more dilative.

     

Simulation of polychlorinated biphenyls transport in the vadose zone

Polychlorinated biphenyls (PCBs) are the main constituents of clophen (the liquid of the electric transformers and capacitors) and have been characterized as potential human carcinogens. PCBs can be a hazardous contaminant of soil and groundwater. We used the mathematical model variably saturated 2D flow and transport (VS2DT model) to simulate the transport of PCBs from the soil surface to groundwater for a time period of 30 years. We also used a mathematical model to simulate the colloid-facilitated PCB transport, under saturated flow conditions. The results showed that PCBs dissolved in water cannot be transported to large depths in unsaturated soils, because of their strong sorption onto soil and low solubility in water. For soils with very low or no organic matter content, PCB transport is much faster and the probability of groundwater contamination is much higher. PCBs can partition to colloids originating from dissolved organic matter in groundwater. Colloid-facilitated PCB transport is faster compared to PCB transport in aqueous solution with no colloids present.