非饱和土的蒸发效应与影响因素分析

采用非饱和土等温流和考虑热湿耦合的非等温流方程,结合不同的蒸发计算模型,分析了大气蒸发效应对非饱和土吸力变化的影响,并对控制蒸发量计算的多个参数进行了影响程度分析。结果表明,考虑实际蒸发量和非饱和土热湿耦合的水分运动公式能较好地模拟大气影响下非饱和土吸力变化状态,通常所采用的等温流方程加以潜在蒸发量计算非饱和土吸力变化会高估表层蒸发和吸力的下降程度。控制地面蒸发的主导因素是外部气象条件,尤其以太阳净辐射量和风速为最,土体自身特性参数影响程度有限。     

蒸发蒸腾作用下非饱和土的吸力和变形影响因素分析

采用热湿耦合非等温流方程,结合实际蒸发和植物蒸腾的边界条件,考虑了水分迁移所引发的非饱和土应力变形行为,从而建立了大气-非饱和土相互作用模型。采用该模型对控制非饱和土吸力和变形的大气蒸发和植物蒸腾等多参数进行了影响程度分析。结果表明,控制地面吸力和变形的主导因素是外部气象条件,尤其太阳净辐射量和风速,土体水力和热力特性参数影响程度有限;在植被覆盖条件下,吸力变化主要取决于植物叶面积指数LAI,而总腾发量的大小决定了地表变形量。     

Integrating variations in the soil chloride profile and evaporativity for in-situ estimation of evaporation in arid zones: an application in south-eastern Morocco

In arid regions, knowledge of the evaporation rate from the water table is essential for appropriate management of scarce resources and to prevent land degradation. Soil chloride profiles in the unsaturated zone of a bare soil in an arid area of south-eastern Morocco were used to assess the evaporation flux, using chloride inventories in conjunction with evaporative demand. Moisture fluxes were calculated from measured chloride concentrations on the basis of a steady-state flow model. The chloride profiles displayed large variations in concentrations and had (1) low chloride concentrations near the soil surface, (2) maximum chloride concentrations at depths of 11–14 cm beneath the soil surface, respectively in July and February, and (3) gradually decreasing chloride concentrations while depth increased below these peaks. Evaporative demands were found to be inversely proportional to the depth of evaporation fronts and proportional to evaporation fluxes. In addition, the evaporation along the profiles seems to be controlled by the soil composition and texture. The investigation of chloride profiles in February and July enabled the determination of a value for annual evaporation (～30 mm), which is in good agreement with the value estimated by the Allison-Barnes type model (～32 mm).     

Predicting effective stress parameter of unsaturated soils using neural networks

In the effective stress equation for unsaturated soils proposed by Bishop, shear strength in these soils depends on the effective stress parameter, χ, a function of soil suction [1]. To estimate the shear strength of unsaturated soils, one must already know this parameter and its variation with soil suction. Though theories on the shear strength of unsaturated soils are consistent with experimental measurements, estimating the effective stress parameter directly from tedious laboratory tests is impractical. Thus, researchers have performed numerous intensive studies to effectively obtain the unsaturated shear strength using simplified empirical methods.This paper shows an adaptive learning neural network method for predicting this parameter, χ. The proposed network is a multilayer perceptron network with six neurons in the input layer representing the air entry value, the volumetric water content at residual and saturated conditions, the slope of soil water characteristic curve, the net confining stress and suction. The available literature uses a database prepared from triaxial shear test results to train and test the network. The results show the suitability of the proposed approach for estimating the effective stress parameter. Network analysis indicates that the χ-parameter depends strongly on the net mean stress.     

Effect of soil parameters on the threshold wind velocity and maximum eroded mass in a dry environment

The present study aims to investigate the relationships between several soil parameters (texture, organic matter and CaCO₃ content) and the threshold wind velocity and erodibility of different soil types. Our aim was to determine the role of these soil parameters play in soil loss due to wind erosion and also to statistically evaluate these correlations. The erodibility studies were carried out in wind tunnel experiments, and the resulting data were analysed with multiple regression analysis and the Kruskal-Wallis test. We found that both the threshold wind speed and the erodibility of soils were mostly determined by silt fraction (0.05–0.02 mm), while sand fractions had a lesser effect on it. Our experiences with organic matter and CaCO₃ similar, i.e. in spite of their correlation with the erosion, their contribution was not significant in the multivariate regression model. Consequently, based on mechanical composition of soils, one can predict threshold wind velocity and erodibility of soils.     

一种非饱和土表层蒸发过程的预测方法

探索了一种由土壤基本物性指标、气象数据预测非饱和土表层蒸发过程的方法。通过土壤基本物理性质（颗粒分布、土粒相对密度、干密度）预测出土-水特征曲线（SWCC）,进而得到土壤气相对湿度与含水率的关系,采用Penman-Wilson模型预测出了非饱和土表面的蒸发曲线。通过该方法,只需实地采取土样,获取其基本物理性质,由任意时刻土壤的含水率及气象数据就能预测出该时刻土表的蒸发速率。采用自制的自动蒸发测量系统进行浅层土蒸发试验,得到了蒸发曲线,与预测结果进行比较发现预测蒸发过程与实测结果一样皆由临界含水率和风干含水率将其分为3个阶段：稳定阶段、减速阶段和残余阶段,且蒸发量吻合,证明了所提出方法的准确性和实用性,对工程界估计土表蒸发量,确定水流量边界有重要意义。     

Limitations of suction‐controlled triaxial tests in the characterization of unsaturated soils

Behavior of unsaturated soils is influenced by many factors, and the influences of these factors are usually coupled together. Suction‐controlled triaxial (SCTX) tests are considered to allow researchers to investigate influences of individual variables on unsaturated soils under specified stress path with controls of stresses, pore water, and air pressures. In the past 50 years, SCTX testing method has been established as a standard approach to characterize constitutive behavior of unsaturated soils. Most important concepts for modern unsaturated soil mechanics were developed upon results from the SCTX tests. Among these, one of the most important contributions in the constitutive modeling of elasto‐plastic behavior for unsaturated soils is the Barcelona basic model (BBM) proposed by Alonso et al. in 1990. The BBM successfully explained many features of unsaturated soils and received extensive acceptance. However, the SCTX tests are designed based upon the divide‐and‐conquer approach in which an implicit assumption is used: soil behavior is stress‐path independent. However, it is well‐established that unsaturated soil behavior is elasto‐plastic and stress‐path dependent. It is found that the SCTX tests in fact cannot control the stress path of an unsaturated soil during loading. This incapability, in combination with complicated loading/collapse behavior of unsaturated soils, makes the SCTX tests for characterizing unsaturated soil questionable. This paper discusses the limitations of the SCTX tests in the characterization of unsaturated soils. A possible solution to the problem was proposed based on a newly developed modified state surface approach. The discussions are limited for isotropic conditions. Copyright © 2015 John Wiley & Sons, Ltd.     

Evaporation,shrinkage and intrinsic permeability of unsaturated clayey soil: analytical modelling versus experimental data

This paper presents the experimental study conducted on a clayey soil originating from the region of Béja, north-west of Tunisia. The evaporation, shrinkage and permeability behaviours were studied. The Soil Water Retention Curve (SWRC) was determined from the slurry state to dry state, under the desiccation path (called initial drying curve). The Crack Intensity Factor (CIF), settlement and void ratio were also studied to characterise the shrinkage phenomenon during desiccation. Moisture content (ω), saturation degree (S_r) and evaporation rate (R_e) evolutions during desiccation path were also presented. This type of slurry clay presents three stages during the desiccation process (pendular, funicular and capillary regimes). During desiccation process, the evaporation rate presents a linear relationship as a saturation degree function. Furthermore, the evaporation rate versus suction presents two phases: quasi-saturated and unsaturated states. This paper introduces a study of the hygroscopic and mechanical parameters naturally modified during a desiccation process and proposes some analytical models to describe clay behaviour. Using these parameters, we can determine the intrinsic permeability during the desiccation process.     

Determining the active region model parameter from dye staining experiments for characterizing the preferential flow heterogeneity in unsaturated soils

The active region model (ARM) has been developed as a practical and effective approach for characterizing and representing preferential flow patterns in unsaturated soils. However, studies on methods to determine the ARM parameter (γ) are very limited. The major objective of this work was to refine the methods for determining the ARM parameter (γ) using the data from field-scale dye staining experiments. For this purpose, 13 field-scale dye staining experiments were conducted in silty clay, loam and sand with various initial and boundary conditions. The distributions of soil water content and fraction of stained region were measured to determine the ARM parameter (γ). Three determination methods of ARM parameter (γ) were presented according to the different distribution patterns of the soil water content of stained region. The efficiency of these approaches was demonstrated with the results from 13 field-scale dye staining tests that were directly related to preferential flow patterns. Impacts of soil texture on soil water redistribution, and effects of initial soil water content and infiltration amount on the preferential flow heterogeneity were also discussed in this research.     

基于氢氧同位素的华北平原降水入渗过程

华北平原地下水浅埋区水循环主要以垂直方向上的入渗、蒸发和蒸腾的方式存在,同位素可以作为一种有效"示踪剂"揭示降水入渗补给地下水的过程。选择华北平原中、东部地下水浅埋区的衡水和沧州为典型实验点,研究不同降水特征、土壤质地和植被条件下入渗过程的差异性。结果表明,土壤非均质条件下(沧州),降水入渗补给过程中伴随着蒸发、植被蒸腾作用以及与土壤前期水分的强烈混合作用,活塞流入渗的同时土壤100 cm深度可能还存在大孔隙优先流;土壤均质条件下(衡水),降水向下均匀入渗,入渗速度较快,土壤水运动以新水基本代替老水的活塞流为主要形式,并经过强烈的蒸发浓缩作用补给地下水。     

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.     

Transfer of moisture through the unsaturated zone in the tropical forest using the neutron probe

Knowledge of moisture content is crucial in assessing spatial and temporal movement of water through the unsaturated zone. Moisture storage is also important for monitoring the soil water balance and for validation of water balance models. The purpose of this work was to determine and analyse moisture content profiles at point locations in the unsaturated zone of a lateritic soil around Nsimi, south of Cameroon. Neutron probe has been connected to a set of tensiometers in an area of 60 ha. A comparative study between a site covered with vegetation and a site uncovered was conducted to assess the influence of vegetation in the process of moisture transfers. The results showed that the spatial distribution of moisture profiles varied according to the site and the texture of the soil, with in general increasing of moisture from the surface horizon toward the deeper layers. The mean values of moisture varied fromθ_m = 0,397 cm³/cm³ on barren site, against 0 = 0,429 cm³/cm³ in vegetation. Values of suction were generally strong in surface and at depth, but weak in the intermediate layers.     

Influence of Unsaturated Soil Properties Uncertainty on Moisture Flow Modeling

The quality of a numerical modeling solution of moisture flow through unsaturated soil, in part, depends on properly described unsaturated soil properties. The variability of the Soil Water Characteristic Curve, SWCC, is attributed to hysteresis and reproducibility of measurement. Because the unsaturated conductivity function is rarely directly measured, the variability of the unsaturated soil hydraulic conductivity function is attributed to the uncertainty associated with the estimation of this parameter with currently available fitting functions, and hence a range of reasonable variation was considered. One-dimensional modeling of expansive soil under dry initial conditions (suction of 1,500 kPa) was performed; both potential evaporation and infiltration boundary conditions were considered. It was found that small variations in the unsaturated soil hydraulic conductivity function result in significantly different modeling outputs, as expected, while substential variation in SWCC alone (assuming the same unsaturated soil hydraulic conductivity for all SWCCs) produced almost identical soil response in terms of soil suction when the slope of the SWCC is similiar. Thus, proper characterization of the slope of the SWCC is important to proper suction profile determination.     

非饱和渗流基质吸力对边坡稳定性的影响

降雨入渗是致使边坡岩土体稳定性下降并最终导致崩滑地质灾害发生最为常见的环境因素。通常对边坡中地下水的影响分析采用的是经典的静水压力假定 ,并考虑适当的折减系数。本文研究了非饱和土强度随基质吸力变化的规律 ,对基质吸力影响边坡稳定性的机制进行探讨 ,并提出了相应的分析方法。运用上述方法分析了某露天矿边坡实例。结果表明 ,该方法十分有效。     

The relation of seasonal pattern in stable carbon compositions to meteorological variables in the leaves of <Emphasis Type="Italic">Sabina</Emphasis><Emphasis Type="Italic">przewalskii</Emphasis> Kom. and <Emphasis Type="Italic">Sabina chinensis</Emphasis> (Lin.) Ant.

The seasonal variation of foliar δ¹³C values in Sabina przewalskii Kom. and Sabina chinensis (Lin.) Ant. was measured. The relationships between foliar δ¹³C values and branch δ¹³C values as well as environmental factors (monthly total precipitation, monthly average air temperature, monthly average soil temperature, monthly total solar duration, relative humidity, atmospheric pressure, vapor pressure, wind speed and potential evaporation) were investigated. The results showed that the foliar δ¹³C values were negatively correlated with air pressure, and positively correlated with air temperature, precipitation, vapor pressure, potential evaporation, solar duration, wind speed and soil temperature. No significant relationship between δ¹³C values and relative humidity was detected. This demonstrates that the foliar δ¹³C of Sabina is a successful empirical indictor of these meteorological factors within the usual range of C₃ whole-leaf δ¹³C values. Furthermore, the δ¹³C signature of leaf tissue is similar to that of wood tissue and the responses of δ¹³C values in S. przewalskii Kom. to environmental factors are also relatively stronger than that of S. chinensis (Lin.) Ant. These results provided strong evidence that it is feasible to extract climatic information from tree-ring δ¹³C series and S. przewalskii Kom. is a dendroclimatologically promising tree species.     

非饱和土广义有效应力原理

综述了非饱和土有效应力的沿革,并就非饱和土有效应力的研究和发展中存在的问题进行了讨论。基于多相孔隙介质理论推导得到的变形功的表达式,提出了非饱和土广义有效应力原理。该原理认为,由非饱和土中的单应力变量的有效应力或双应力变量理论很难唯一地确定非饱和土的变形和强度。广义有效应力原理实质上就是要综合考虑影响非饱和土变形和强度的三种广义应力以及与其对偶的广义变形,给出考虑因素更为全面、理论基础更为坚实的广义有效应力原理。它为非饱和土基本性质的研究和本构方程的建立奠定了坚实而科学的理论基础。     

内蒙古中西部地区土壤水分对沙尘暴的贡献

以2001年4月至2002年6月内蒙中部地区逐时观测的土壤水分资料为基础，论证了该区日土壤水分和日平均风速的变化规律，统计分析了日平均风速与土壤水分对沙尘暴的成生综合贡献。得出的主要结论是：①该地区沙尘暴发生时日平均风速的最小值是3．5m／s。如果日平均风速大于8．0m／s，预示着有沙尘暴的发生。②在平均风速大于3.5m／s的条件下，观测样本中18．4％日数发生沙尘暴，而81．6％日数没有发生。说明大风的天气条件不应当被视其为沙尘暴的唯一重要的因子。③在同一地点、同样的风力条件下，在沙尘暴发生时，日平均风速与日平均土壤水分呈明显的反相关变化；而它们的反相关关系不明显时，沙尘暴就很少发生。     

内蒙古中西部地区土壤水分对沙尘暴的贡献

以2001年4月至2002年 6月内蒙中部地区逐时观测的土壤水分资料为基础，论证了该区日土壤水分和日平均风速的变化规律，统计分析了日平均风速与土壤水分对沙尘暴的成生综合贡献。得出的主要结论是：①该地区沙尘暴发生时日平均风速的最小值是 3．5 m／s。如果日平均风速大于 8．0 m／s，预示着有沙尘暴的发生。②在平均风速大于 3．5 m／s的条件下，观测样本中 18．4％日数发生沙尘暴，而81．6％日数没有发生。说明大风的天气条件不应当被视其为沙尘暴的唯一重要的因子。③在同一地点、同样的风力条件下，在沙尘暴发生时，日平均风速与日平均土壤水分呈明显的反相关变化；而它们的反相关关系不明显时，沙尘暴就很少发生。     

Numerical analysis of seepage–deformation in unsaturated soils

A coupled elastic–plastic finite element analysis based on simplified consolidation theory for unsaturated soils is used to investigate the coupling processes of water infiltration and deformation. By introducing a reduced suction and an elastic–plastic constitutive equation for the soil skeleton, the simplified consolidation theory for unsaturated soils is incorporated into an in-house finite element code. Using the proposed numerical method, the generation of pore water pressure and development of deformation can be simulated under evaporation or rainfall infiltration conditions. Through a parametric study and comparison with the test results, the proposed method is found to describe well the characteristics during water evaporation/infiltration into unsaturated soils. Finally, an unsaturated soil slope with water infiltration is analyzed in detail to investigate the development of the displacement and generation of pore water pressure.     

Application of HYDRUS-1D in understanding soil water movement at two typical sites in the North China Plain

Recharge and discharge, such as rainfall infiltration and evapotranspiration in vertical direction, are major processes of water cycle in the shallow groundwater area of the North China Plain. During these processes, soil water movement in the unsaturated zone plays an important role in the transformation from rainfall infiltration to groundwater. The soil water movement models were developed by using HYDRUS-1D software at two typical experimental sites in Cangzhou (CZ) and Hengshui (HS) with different soil, vegetation and similar climate conditions. As shown in the results, the comparison in precipitation infiltration features between the two sites is distinct. The soil water experiences strong evaporation after precipitation infiltration, which accounts for 63% of the total infiltration at the HS site where the soil is homogenous. It is this strong evaporation effect that leads to slight increase of soil water storage. At the CZ site, where the soil is heterogeneous, the evaporation effect exists from July to October of the simulation period. The total evaporation accounts for 33% of the total infiltration, and the evaporation rate is slow. At the end of the simulation period, the soil water storage increases and the water table decreases, indicating a strong storage capacity at this site.