土壤电阻率测量影响因素的试验研究

实验室测量的土壤电阻率准确性受到电极布置以及测试信号类型等影响。选取5种不同质地土壤,比较了四极法与二极法测量以及四极法测量中电压极探针渗入深度的影响,分析了直流信号与交流信号的测试差别以及电流强度及交流频率的影响。结果表明,二极法测量结果包含了土壤与电极的接触电阻,且随土壤含水率降低接触电阻不断增大;四极法可在电压极渗入深度较浅,即土壤扰动较小的情况下测量;土壤在直流电压下存在极化效应,土壤电阻率测量宜采用交流;土壤热容量有限,对电阻率在200 Ω•m以内的土壤测量的电流密度不宜超过1 mA/cm2;土壤电阻率随测量频率增加而逐渐下降。     

循环荷载作用下筋土界面抗剪特性的试验研究

利用自主研制的大型结构面剪切仪完成了一系列界面直剪试验,探讨在法向循环荷载与静荷载两种条件下土工格栅与石英砂界面的抗剪性能,比较其在两种法向荷载下的异同点。试验结果表明：循环荷载作用下剪应力随剪切位移的发展趋势与静荷载下的有相似之处,但曲线本身表现出了循环发展的特性;循环荷载作用下剪应力峰值和谷值的最大值与法向应力的关系近似符合摩尔-库仑定律;循环剪应力最大值随着法向循环荷载频率的增加而减小,但随着振幅的增加而逐渐增大;循环荷载下筋土界面剪应力随位移而波动,始终处于振幅上下限所对应静荷载下剪应力之间,相关物理量之间具体的定量关系需要进一步研究。     

Effects of vegetation on flow and sediment transport: comparative analyses and validation of predicting models

The presence of vegetation modifies flow and sediment transport in alluvial channels and hence the morphological evolution of river systems. Plants increase the local roughness, modify flow patterns and provide additional drag, decreasing the bed‐shear stress and enhancing local sediment deposition. For this, it is important to take into account the presence of vegetation in morphodynamic modelling. Models describing the effects of vegetation on water flow and sediment transport already exist, but comparative analyses and validations on extensive datasets are still lacking. In order to provide practical information for modelling purposes, we analysed the performance of a large number of models on flow resistance, vegetation drag, vertical velocity profiles and bed‐shear stresses in vegetated channels. Their assessments and applicability ranges are derived by comparing their predictions with measured values from a large dataset for different types of submerged and emergent vegetation gathered from the literature. The work includes assessing the performance of the sediment transport capacity formulae of Engelund and Hansen and van Rijn in the case of vegetated beds, as well as the value of the drag coefficient to be used for different types of vegetation and hydraulic conditions. The results provide a unique comparative overview of existing models for the assessment of the effects of vegetation on morphodynamics, highlighting their performances and applicability ranges. Copyright © 2014 John Wiley & Sons, Ltd.     

Patterns of bedload entrainment and transport in forested headwater streams of the Columbia Mountains,Canada

We monitor bedload transport and water discharge at six stations in two forested headwater streams of the Columbia Mountains, Canada. The nested monitoring network is designed to examine the effects of channel bed texture, and the influence of alluvial (i.e. step pools and riffle pools) and semialluvial morphologies (i.e. boulder cascades and forced step pools) on bedload entrainment and transport. Results indicate that dynamics of bedload entrainment are influenced by differences in flow resistance attributable to morphology. Scaled fractional analysis shows that in reaches with high form resistance most bedload transport occurs in partial mobility fashion relative to the available bed material, while calibers finer than 16 mm attain full mobility during bankfull flows. Equal mobility transport for a wider range of grain sizes is achieved in reaches exhibiting reduced form resistance. Our findings confirm that the Shields value for mobilization of the median surface grain size depends on channel gradient and relative submergence; however, we also find that these relations vary considerably for cobble and gravel bed channels due to proportionality between dimensionless shear stress and grain size. Exponents of bedload rating curves across sites correlate most with the D_90s of the mobile bed, however, where grain effects are controlled (i.e. along individual streams), differences in form resistance across morphologies exert a primary control on bedload transport dynamics. Application of empirical formulae developed for use in steep alpine channels present variable success in predicting transport rates in forested snowmelt streams. Formulae that explicitly account for reductions in mobile bed area and high morphological resistance associated with woody debris provide the best approximation to observed empirical data. Copyright © 2014 John Wiley & Sons, Ltd.     

斜坡地基静力触探试验离散元分析

通过二维离散元模拟斜坡地基静力触探试验,研究试验过程中锥尖阻力的变化以及斜坡地基的宏微观力学响应。结果表明:归一化锥尖阻力峰值在坡角较小时无明显提升,在坡角足够大时提升明显,稳定值随着坡角增大略微减小,说明斜坡边界对表层土力学性能影响明显,而对深层土影响较弱;锥尖贯入过程中,斜坡地基探杆两侧土体变形不对称,坡上一侧膨胀,坡下一侧压缩,不对称性随着坡角增大而增大;贯入到表层时,探杆两侧微观接触力链不对称,坡上一侧具有更大应力,但当贯入到深层时力链趋向于对称,其临界深度随着坡角增大而增大,与坡角呈近似线性关系。     

漠河-加格达奇段多年冻土区中俄原油管道运营以来的次生地质灾害研究——以MDX364处的季节性冻胀丘为例

通过对中俄原油管道漠河-加格达奇段多年冻土区的现场勘查研究, 统计了管道运营以来出现的冻土次生地质灾害主要有冻胀、融沉、水毁、冻胀丘、冰椎等. 在研究区域特定的气候背景下, 管道的修建和季节性变化的正油温运营, 破坏了管道周围冻土的水热平衡, 使得管道周围土体出现差异性冻胀和融沉, 这种差异性位移量的累积对管道安全稳定长期运营造成了威胁. 以管道里程MDX364处的冻胀丘为例, 利用探地雷达进行了现场探测. 结果表明: 管道周围存在的融区为冻胀丘的发生和发展提供了水源补给通道, 管道的热影响加速了冻胀丘的发展和消融, 2014年3-10月管道周围地表产生的差异性位移超过了1.1 m. 针对该次生开放型季节冻胀丘, 提出了修筑或疏通管道附近的排水通道、钻孔放水和保温排水渗沟等防治措施. 研究成果能为中俄原油管道的安全稳定运营提供技术支撑, 为其他冻土区管道设计施工和运营维护提供参考和依据.     

高速铁路路基填料冻胀试验研究

路基冻胀是季节冻土区修建高速铁路必须面对的关键性技术难题. 国内外针对路基填料的冻胀机理和特性进行了很多研究, 但很少涉及路基填料的渗透性问题. 通过室内试验研究了细颗粒含量对路基填料含水量和冻胀率的影响, 通过现场试验研究了路基填料的渗透性能, 选取哈齐客专某试验段路基进行冻胀监测, 验证了防冻层填料的导水特征及其防冻胀性能. 结果表明: 采用填料防冻的技术路线是可行的, 通过合理控制填料组分、级配、细颗粒含量等设计参数, 严格把控施工质量, 路基填料能够满足高速铁路路基防冻胀要求.     

深季节冻土区路基防冻胀道砟碎石排水盲沟热状况的数值分析

用透水土工布包裹道砟碎石填筑的截排水纵向盲沟, 是哈尔滨-大连高速铁路(简称哈大高铁)在大开挖路堑段采用的重要的防冻胀工程措施. 由于碎石层在顶部温度较低时, 其中发生对流换热过程而强化对其下部土体的降温作用, 可能导致盲沟底部冻结而影响其防冻胀效果. 通过数值仿真软件分析了排水盲沟的温度状况, 结果表明: 对于哈大高铁的气温环境, 在不考虑积雪影响的条件下, 将发生盲沟底部排水管冻结积冰, 影响纵向盲沟防冻胀效果的问题. 在盲沟顶土层中加设0.1 m保温板能够提高盲沟底部的最低温度, 推迟起始冻结时刻, 缩短冻结时长, 但不能完全避免盲沟底部排水管的冻结问题. 在有表层保温板的条件下, 在盲沟底部换填砂层虽然不能提高盲沟碎石层的温度, 但却能有效提高盲沟底部的温度. 复合措施可以有效解决盲沟底部冻结的问题.     

Effects of hydrocarbon physical properties on caprock’s capillary sealing ability

A new mechanics formula of caprock’s capillary sealing ability has been established in this paper, in which the boundary layer resistance was considered and characterized by starting pressure gradient. The formula shows that capillary sealing ability of caprock is determined not only by the capillary force of rock and the buoyancy of hydrocarbon column, but also by the starting pressure gradient of hydrocarbons and the thickness of caprock. The buoyancy of hydrocarbon column, the starting pressure gradient of hydrocarbon, and the capillary force of caprock are affected by hydrocarbon density, hydrocarbon viscosity, and hydrocarbon-water interface tension respectively. Based on hydrocarbon property data of reservoirs of Jiyang Depression and equations from literature, the effects of hydrocarbon density, hydrocarbon viscosity, and hydrocarbon-water interface tension on the sealing ability of caprock are analyzed. Under formational conditions, the sealing ability of oil caprock can vary up to dozens times because of the variations of the oil density, oil viscosity, and oil-water interface tension. Thus, the physical characters of hydrocarbon should be considered when evaluating the capillary sealing ability of caprocks. Study of the effects of physical characters on sealing ability of caprock can provide guidance to exploring special physical property hydrocarbon resources, such as viscous oils, and hydrocarbon resources in special pressure-temperature environments.     

Modelling the dynamics of snow cover, soil frost and surface ice in Norwegian grasslands

Studying the winter survival of forage grasses under a changing climate requires models that can simulate the dynamics of soil conditions at low temperatures. We developed a simple model that simulates depth of snow cover, the lower frost boundary of the soil and the freezing of surface puddles. We calibrated the model against independent data from four locations in Norway, capturing climatic variation from south to north (Arctic) and from coastal to inland areas. We parameterized the model by means of Bayesian calibration, and identified the least important model parameters using the sensitivity analysis method of Morris. Verification of the model suggests that the results are reasonable. Because of the simple model structure, some overestimation occurs in snow and frost depth. Both the calibration and the sensitivity analysis suggested that the snow cover module could be simplified with respect to snowmelt and liquid water content. The soil frost module should be kept unchanged, whereas the surface ice module should be changed when more detailed topographical data become available, such as better estimates of the fraction of the land area where puddles may form.