Modelling of unsaturated ground behaviour influenced by vegetation transpiration

Vegetation contributes to weak soil stabilisation through reinforcement of the soil, dissipation of excess pore pressure and increasing the shear strength by induced matric suction. This paper describes the way vegetation influences soil matric suction, shrinkage and ground settlement in the vadose zone through transpiration. A mathematical model for the rate of root water uptake, including the root growth rate considering ground conditions, type of vegetation and climatic parameters, has been developed. A finite element approach is employed to solve the transient coupled flow-deformation equations. The finite element mesh is built using partially saturated soil elements capable of representing the salient aspects of unsaturated permeability and the soil water characteristic curve. The model formulation is based on the effective stress theory of unsaturated soils. Based on this proposed model, the distribution of the ground matric suction profile adjacent to the tree is numerically analysed. Current field measurements of soil matric suction and moisture content collected from Miram site located in Victoria State, Australia by the authors are compared with the numerical predictions. The results indicate that the proposed root water uptake model incorporated in the numerical analysis can be used for prediction of ground properties influenced by tree roots.     

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.     

场地类别分类方案研究

基于性能的抗震设计,要求工程师设计出具有预期抗震性能的结构,一个关键因素是地震作用的确定,这在很大程度上取决于局部场地条件。通过收集和分析北京、苏州和唐山城区956个钻孔资料,建立地表20 m和30 m深土层走时平均剪切波速VS20和VS30的关系式;现场钻探获取北京城区深105 m的典型钻孔原状土样,试验给出各类土体动剪切模量和阻尼比曲线;建立北京城区170个钻孔的场地反应计算模型,采用Nakamura提出的HVSR法和陈国兴等提出的弱震法估算场地基本周期TS值,结合国内外现行抗震规范的场地分类及一些学者对场地分类的研究成果,提出两种新的场地分类建议方案：基于等效剪切波速VSE和覆盖土层厚度H（地表至剪切波速VS ≥ 500 m/s的基岩深度）的双指标场地分类方案及基于VSE、H和TS的三指标场地分类方案。提出的场地分类方案对我国现行抗震规范场地分类方法的改进有参考价值。     

黄土高原关键带全剖面土壤水分空间变异性

土壤水分是黄土高原关键带水循环、地下水补给和植被恢复的关键因素。为揭示黄土高原关键带黄土整个剖面的土壤水分空间变化特征,通过土芯钻探的方式获取了黄土高原关键带5个典型样点（杨凌、长武、富县、安塞和神木）从地表到基岩的土壤水分样品,采用经典统计学和地统计学相结合的方法分析了剖面土壤水分的分布规律、变异特征及空间结构。结果表明:黄土高原关键带剖面土壤水分从南往北,土壤平均含水量由高变低;5个样点的土壤水分均为中等变异,随着深度由40 m增加到200 m,土壤水分变异性变弱,且样点之间的土壤含水量差异降低;地统计学分析表明样点的半方差函数能被理论模型较好地拟合（杨凌除外）,指数模型能够描述大部分样点深剖面的空间变异结构。相关结果有助于了解黄土高原深层土壤水分状况及分布规律,对于黄土高原土壤水资源估算和区域植被恢复具有重要价值。     

Drained bearing response of shallow foundations on structured soils

This paper examines the drained bearing response of circular footings resting on structured soil deposits. Numerical simulations have been carried out using a finite element formulation of the Structured Cam Clay model. A parametric study was conducted by varying the parameters that govern the behaviour of structured soils and guidelines are given for designers to identify when effects of the soil structure are important. Under fully drained conditions, deformation within the structured soil supporting the footing usually occurs as a local or punching shear failure due to high compressibility of the structured soil and the mobilised bearing pressure increases with the footing movement, without reaching an ultimate value. A novel approximate method is presented to obtain the load–displacement response of a rigid circular footing resting on the surface of a structured soil deposit. This requires the properties of the soil in the reconstituted state and two additional parameters, which govern the natural structure of the soil. The proposed method has been applied to a published case study, where plate load test results are given for rigid circular steel plates resting on structured soil deposits. Fair agreement is observed between the computed and experimental results, suggesting the approximate method may be useful in design studies of foundations on structured soil deposits.     

Local versus regional processes: can soil characteristics overcome climate and fire regimes by modifying vegetation trajectories?

We analysed charcoal and pollen from sediments obtained from two lakes in the northwestern mixed‐wood Canadian boreal forest in order to reconstruct fire‐return intervals and vegetation dynamics over the last 8000 years. Sites were selected with contrasting soil properties (mesic versus dry‐sandy soils), allowing an estimation of the potential influence of soils on long‐term vegetation and fire dynamics. The sites likely experienced fewer fires during the period extending from 8000 to 4000 cal. a BP than over the last 4000 years. At both sites, eastern white pine (Pinus strobus) populations were most extensive shortly after deglaciation, with vegetation later shifting towards mixed woodlands with less P. strobus and more extensive Picea and Pinus banksiana populations. This gradual vegetation shift was probably induced by the establishment of colder and moister conditions along with a fire‐regime change. In spite of the parallel long‐term vegetation trajectories, vegetation composition differed between the two sites in both the past and present. Whereas Picea was more abundant at the mesic site, the fire‐adapted P. banksiana populations were more extensive at the sandy‐soil site. These differences in vegetation composition indicate that, in addition to climate changes and fire occurrence, soil properties also influenced vegetation dynamics. A likely increase in fire frequency in the Canadian boreal forest during the 21st century might therefore favour the expansion of these two disturbance‐adapted trees with spatial heterogeneity in the populations due to varying soil types. Copyright © 2012 John Wiley & Sons, Ltd.     

土-水特征曲线滞后现象的微观机制与计算分析

对吸湿与脱湿过程中引起非饱和土土-水特征曲线滞后性质进行机制分析,认为在微观角度看,接触角的差异是主要原因。基于热力学原理,假定粒间弯液面为一圆弧,综合考虑不等径颗粒半径比、接触角等因素,应用几何关系导得了两接触土颗粒填充角之间的迭代关系,并进行编程计算。在此基础上,引入热力学弯液面压强计算公式和水量体积计算公式,得到了不等径土颗粒间基质吸力－水量计算方法。应用该方法对不同接触角条件下的单个弯液面和不同堆积形式土粒的土-水特征曲线进行了解算,结果表明,由于接触角不同,土-水特征曲线具有明显的滞后现象;孔隙填充将引起低吸力状态土体含水率增加,其对松散颗粒影响较大,含水率可增大达90%;颗粒堆积形式及级配对接触角不同引起的滞后现象影响不大。此外,仅考虑接触角因素无法解释滞回圈的形成,其计算理论有待进一步深入研究。     

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。     

干湿过程中花岗岩残积土抗拉强度变化研究

利用自制的土体直拉强度测试仪,研究了重塑花岗岩残积土在不同含水率下、增湿过程和干燥过程中抗拉强度的变化规律,并从胶结力和湿吸力的角度讨论了抗拉强度变化规律的微观机制。结果表明：不同含水率条件下抗拉强度随含水率增加呈现先增后减,峰值两侧呈较好的指数规律;增湿过程中抗拉强度随含水率增加呈先增后减趋势,峰值两侧为一次函数规律;干燥过程中抗拉强度随含水率降低出现指数增加阶段、平稳阶段和小幅降低阶段3个阶段,其抗拉强度峰值为不同含水率条件下抗拉强度的4倍。花岗岩残积土在不同含水率和增湿过程中的抗拉强度的变化主要受湿吸力控制,而干燥过程中抗拉强度变化的同时受湿吸力和胶结力的控制,且胶结力对抗拉强度的贡献超过70%。土体的干燥开裂过程对应干燥过程中抗拉强度变化的几个阶段,干燥时湿吸力是土体内部拉应力的来源,这说明湿吸力既是抗拉强度的贡献者,同时也是抗拉强度的破坏者。该研究结果从另一种角度解释了土体抗拉强度的形成来源及其变化的主控因素。     

吸力历史对非饱和粉土动力变形特性的影响

以非饱和粉土为研究对象,利用吸力可控的动三轴仪对其进行动力变形试验,得到了在不同净围压和不同吸力路径下非饱和粉土试样的骨架曲线、动弹性模量和阻尼比,研究了先脱湿后吸湿的吸力历史对其动力变形特性的影响。结果表明,在同一净围压和吸力下,非饱和粉土试样经历过先脱湿后吸湿的骨架曲线和动弹性模量比仅经过脱湿的要高,而试样经历过先脱湿后吸湿的阻尼比比仅经过脱湿的要小;此外,随着非饱和粉土试样经历过最大吸力的增大,其骨架曲线和动弹性模量增大,而阻尼比减小。试验结果可用非饱和土的平均骨架应力和弯液面水的表面张力作用来解释,并用两项联合预测非饱和土试样的最大剪切模量。     

Reliability bearing capacity analysis of footings on cohesive soil slopes using RFEM

Reliability analysis of bearing capacity of a strip footing at the crest of a simple slope with cohesive soil was carried out using the random finite element method (RFEM). Analyses showed that the coefficient of variation and the spatial correlation length of soil cohesion can have a large influence on footing bearing capacity, particularly for slopes with large height to footing width ratios. The paper demonstrates cases where a footing satisfies a deterministic design factor of safety of 3 but the probability of design failure is unacceptably high. Isotropic and anisotropic spatial variability of the soil strength was also considered.     

Research of soil–water characteristics and shear strength features of Nanyang expansive soil

Nanyang expansive soil is investigated in its unsaturated state in this paper. The wetting–drying cycle tests of soil–water characteristics of Nanyang expansive soil have been performed in the laboratory. The test results show that the soil–water characteristic curve of the pre-load specimen can well reflect the soil property function of expansive soil. The strength features of the different suction states of the unsaturated expansive soil are also investigated. The hyperbolic model of the suction strength is presented and the parameters of this model are easily determined by tri-axial tests of unsaturated soils. The hyperbolic model is conveniently applied to predict suction strength of an unsaturated soil.     

松嫩平原盐碱土的形成与新构造运动关系的研究

应用地质历史与气候-构造旋回相结合的分析方法,并采用卫星遥感技术,研究、揭示新构造运动对盐碱土形成的控制作用.研究表明,新构造运动的作用与过程为松嫩平原提供了苏打盐份的来源和搬运途径,并营造了盐份的聚集环境.苏打盐份主要富集于全新世盆地的沉降区,沉降中心为现代土壤盐碱化的重灾区.自全新世以来,由于气候的干暖化,积盐作用广泛进行,在地表1.5m深的范围内,发育多层碱土.中全新世以来,在原始的碱性草甸草原上,发育了植被和土被,形成了良好的覆盖层,它们对草原生态环境起了保护作用.自20世纪以来,随着人类对草原的过度开发和气候干暖化,导致土地迅速盐碱化.     

青藏高原坡面冻土土壤水分空间变异特性

为深入揭示坡面冻土水分运移规律及其主要影响因子,以青藏高原巴颜喀拉山北坡为例,结合冻融变化过程,研究不同地形条件冻土土壤水分空间变异特征,利用分类回归树模型（CART）和典范对应分析（CCA）识别影响坡面冻土土壤水分空间异质性的主控因子及其相互作用关系。研究结果表明:①受坡面地形与冻融过程影响,冻结期坡面冻土土壤水分侧向流动减弱,以垂直迁移为主,上坡位含量高于下坡位,反之,融化期上坡位含量则低于下坡位。②影响坡面冻土土壤水分的主要环境因子为高程、土壤质地、土壤温度和植被覆盖度,但在不同冻融阶段下其影响因子存在差异,在冻结状态下主要因子为高程、土壤质地和土壤温度,其相对贡献率分别达到19.97%、19.45%和9.56%;在融化阶段下主要因子为高程、植被覆盖度和土壤质地,其相对贡献率分别为37.4%、14.9%和10.7%。③ 0~20 cm浅层深度上影响坡面冻土土壤水分的主要因素为坡度、高程和植被覆盖度,其相关系数分别高达0.941 2、0.903 9和0.563 1;中下层深度上其主要影响因素较为复杂。     

Bearing capacity of unsaturated expansive soils

It is difficult to determine the bearing capacity of a foundation in unsaturated expansive soil, although this is most important. The bearing capacity of unsaturated expansive soil is related to the drying and wetting environment. Swelling pressure occurs when the soil volume change is constrained as an expansive soil is inundated. The expansive lateral pressure, induced by the swelling pressure is similar to the passive earth pressure. By considering the effect of the expansive lateral pressure in Terzaghi's bearing capacity formula, the bearing capacity of unsaturated expansive soil is derived. Because it is very difficult to measure suction in situ, the bearing capacity is expressed using the expansive lateral pressure offers a feasible approach to calculate the bearing capacity of a foundation in unsaturated expansive soil, when suction is not measured. Plate load tests to measure the bearing capacity in situ were performed for the foundation in natural soil and saturated soil immersed by water. The verification of the bearing capacity formulae presented in this paper is conducted by comparing the predicted results with the results of the plate load tests on unsaturated expansive soils in Handan and Bingxia, China.     

土钉加固黏性土坡加载的离心模型试验研究

进行了不同坡度土钉加固边坡坡顶加载的离心模型试验,观测了土坡的破坏过程并测量土坡的位移场,研究了土钉加固黏性土坡的承载力、变形和破坏规律以及坡角对其破坏规律的影响。试验结果表明,坡顶荷载的增加引起土坡变形的增加,变形的集中产生和发展导致滑裂面的形成并使土坡发生破坏。土钉变形规律受加载阶段和加载底板的综合影响,坡顶荷载越大,接近坡顶的土钉弯曲挠度越大,钉土间的相互作用越强。土坡的坡角越大,承载力越低,土体呈现出更显著的向坡面位移的趋势。     

Distribution of CO_2 in Soil Air Affected by Vegetation in the Shilin National Park

This paper studies the CO2 distribution of soil atmosphere in the Shilin National Park. The measurement sites were chosen according to different topographic features and different vegetations. Seven measurement sites on 3 cross sections were chosen to pass through 3 karstic depressions or on the slopes of depressions. All measurement results show soils with pH values lower than 7.0 (from 5.4 to 6.6). There are 2 cases for the pH values of soil in different topographic features: the pH values of 2 profiles on the ridges or upper slopes of depressions are lower than those in the depressions; and the pH values of 2 soil profiles on the slopes of depressions are higher than those in the depressions. Most samples show relatively low humidity and CO2 contents on the ridges or slopes of depressions compared with soil profiles in the depressions. High CO2 contents occur at depths from -40 to -80 cm and high and dense grassland shows high CO2 contents in the soil atmosphere. Grass roots may grow and are distribu     

Seasonal effects on geophysical–geotechnical relationships and their implications for electrical resistivity tomography monitoring of slopes

Current assessments of slope stability rely on point sensors, the results of which are often difficult to interpret, have relatively high costs and do not provide large-area coverage. A new system is under development, based on integrated geophysical–geotechnical sensors to monitor groundwater conditions via electrical resistivity tomography. So that this system can provide end users with reliable information, it is essential that the relationships between resistivity, shear strength, suction and water content are fully resolved, particularly where soils undergo significant cycles of drying and wetting, with associated soil fabric changes. This paper presents a study to establish these relationships for a remoulded clay taken from a test site in Northumberland, UK. A rigorous testing programme has been undertaken, integrating the results of multi-scalar laboratory and field experiments, comparing two-point and four-point resistivity testing methods. Shear strength and water content were investigated using standard methods, whilst a soil water retention curve was derived using a WP4 dewpoint potentiometer. To simulate seasonal effects, drying and wetting cycles were imposed on prepared soil specimens. Results indicated an inverse power relationship between resistivity and water content with limited hysteresis between drying and wetting cycles. Soil resistivity at lower water contents was, however, observed to increase with ongoing seasonal cycling. Linear hysteretic relationships were established between undrained shear strength and water content, principally affected by two mechanisms: soil fabric deterioration and soil suction loss between drying and wetting events. These trends were supported by images obtained from scanning electron microscopy.     

Vapour Equilibrium and Osmotic Technique for Suction Control

The vapour equilibrium method and osmotic technique have gained widespread acceptance as reliable methods for controlling relative humidity and thereby suction in soil specimens. The ability to impose suction on soil specimens allows for drying and wetting stress paths to be imposed to evaluate resulting changes in strength, deformation and flow characteristics. The two methods presented and discussed in this paper have been adapted for use with a number of traditional laboratory tests including the oedometer, direct shear and triaxial tests. This report provides a summary of some recent developments and knowledge regarding the use of these techniques highlighting the limitations and drawbacks of the methods.