降雨对非饱和黄土边坡含水量变化规律分析

首先确定了非饱和黄土的土-水特征曲线和渗透系数-基质吸力关系曲线及其参数,进一步通过非饱和渗流计算,研究了降雨对非饱和黄土边坡含水量的影响.揭示出坡高对边坡土体含水量分布影响较小,但边坡含水量增大区域随坡高的增加而增加.坡度越陡,降雨引起的坡体浸润区深度增加越大,不利于边坡稳定.土体干密度增加,降雨增湿区域变小,边坡稳定性较好.随初始含水量增加,降雨入渗区域和区内土体含水量增加.降雨历时越长,入渗深度不断增加,边坡土体湿软区不断增加,对边坡稳定性影响较大.在干旱半干旱地区,土体含水量较少,降雨入渗时边坡浸润区大小和含水量都增加缓慢,有利于边坡抵抗降雨入渗;在湿润地区,土体含水量较高,降雨入渗时边坡浸润区大小和含水量都增加较快,不利于边坡抵抗降雨入渗.     

考虑多参数空间变异性的降雨入渗边坡失稳机理及可靠度分析

降雨入渗边坡稳定可靠度研究大多只考虑了土体饱和渗透系数空间变异性的影响,而忽略了土体抗剪强度参数空间变异性的影响。以无限长边坡模型为例,发展了可确定不同降雨历时边坡土体含水率分布和湿润锋深度的修正Green-Ampt入渗模型,探讨了土体多参数(饱和渗透系数和抗剪强度参数)空间变异性与降雨入渗相互作用下的边坡失稳机理,进而比较了不同降雨历时下的边坡失效概率。结果表明:考虑土体饱和渗透系数变异性时,降雨初期较小的饱和渗透系数变异系数对应较小的边坡失效概率,但随着降雨历时的增加,较小的饱和渗透系数变异系数反而对应更大的边坡失效概率。此外,降雨初期土体抗剪强度参数空间变异性引起的滑动面位置不确定性是影响边坡稳定性的关键因素。边坡除了会沿湿润锋附近和不透水层处失稳,还可能沿因抗剪强度参数空间变异性引起的软弱带处失稳,故考虑抗剪强度参数空间变异性对应的边坡失效概率更大。相比之下,降雨后期湿润锋的推进是影响边坡稳定性的关键因素,此时不管是否考虑了抗剪强度参数空间变异性边坡都主要沿湿润锋附近失稳,获得的边坡失效概率相近。     

无限长均质斜坡降雨入渗解析解

降雨是导致边坡失稳的最主要的环境因素,研究降雨入渗对边坡稳定性的影响,关键和难点是计算降雨入渗在边坡土体中引起的渗流场。采用Fourier积分变换分别对降雨强度小于和大于土体饱和渗透系数情况下的斜坡入渗解析解进行了推导,并给出了这两种情况下斜坡入渗解析解的统一表达式。该解不仅能反映非饱和土特性及斜坡的影响,还能反映当降雨强度大于土体饱和渗透系数时,坡面边界由流量边界转化为水头边界的动态变化过程,便于更全面地对降雨情况下斜坡的入渗规律进行研究。通过与有限元软件的计算结果进行对比,证明该解是稳定可靠的,且其形式简单,计算效率高;该解可精确计算坡面积水时间及任意时间点及空间点的孔隙水压力值,并用于评估降雨入渗对非饱和土斜坡稳定性的影响。     

降雨强度对黄土边坡入渗性能影响的研究

降雨入渗使得非饱和土边坡内含水量增大,进而令土的抗剪强度降低,这是降雨诱发浅层边坡失稳的重要原因之一。通过有限元方法计算三种不同降雨强度下边坡内瞬态渗流场分析降雨入渗性能,数值计算结果表明：在历时3 h降雨以内,大于或等于土体饱和渗透性能的雨强条件下湿润锋深度相差不大,但饱和度明显不同;雨强越大,边界瞬时入渗量越大,稳定入渗量越大;在降雨期间,雨水不会以饱和入渗速度入渗,而是随时间呈非线性变化。     

降雨入渗条件下抗滑桩加固边坡稳定性分析

采用有限差分软件FLAC~(3D)内置FISH语言编制相关程序,将边坡非饱和区渗透系数和基质吸力随饱和度的变化关系嵌入到数值计算中,实现了降雨入渗条件下边坡非饱和渗流过程的模拟,结合基于Fredlund双应力状态变量理论的强度折减法,进一步实现了非饱和边坡的稳定性分析。基于某典型抗滑桩加固边坡算例建立数值计算模型,研究了降雨入渗对边坡孔隙水压力分布及边坡稳定性的影响,并对降雨强度、基质吸力及桩顶约束形式对边坡稳定性的影响进行了参数分析。研究结果表明:降雨入渗条件下,随降雨历时增加,湿润锋线不断向坡体内部推进,基质吸力明显减小,边坡安全系数显著降低;边坡安全系数随降雨强度增大而减小,当降雨强度超过土体饱和渗透系数后,边坡安全系数随降雨强度变化不大,并趋于稳定;基质吸力对边坡稳定影响显著,考虑基质吸力时边坡安全系数明显提高;在桩顶设置预应力锚索可使桩身位移明显减小,边坡安全系数提高。     

膨胀土路堑边坡降雨入渗稳定性分析

膨胀土遇水发生软化,导致土体强度降低,降雨入渗对膨胀土路堑边坡影响较大。依据岩土饱和/非饱和渗流理论,利用有限元数值模拟分析软件,通过降雨入渗模型分析,建立膨胀土路堑边坡饱和/非饱和降雨入渗模型,模拟了降雨过程中边坡地下水含水率、压力水头变化规律,分析研究降雨入渗对路堑边坡渗流场及稳定性的影响规律。结果表明:降雨入渗对膨胀土边坡渗透影响仅发生在浅部区域,坡面低渗透性影响雨水入渗效果,裂隙性坡面则入渗稍快;对于低透水性、高基质吸力,短期降雨不至于导致边坡大面积滑塌破坏,边坡滑面只出现在表层,表现为浅层滑动,对深部土体影响不大。为以后膨胀土路堑边坡施工与防护提供一定的理论依据和数据参考。     

增湿条件下黄土陡坡变形特性及稳定性分析

降雨入渗诱发的黄土边坡病害屡见报道,黄土遇水软化以及近坡表非饱和区基质吸力降低是边坡失稳的主要原因。本文采用有限差分数值计算软件,通过软件内置语言编制相关计算程序,考虑降雨过程中非饱和区基质吸力、非饱和渗透系数以及土体强度参数的变化情况,并采用自定义非饱和土强度折减法对降雨过程中黄土陡坡稳定性进行评价。研究结果表明:降雨入渗对非饱和黄土陡坡的稳定性影响很大,随降雨历时增加,湿润锋线不断向坡体内部推进,非饱和区基质吸力减小,边坡安全系数降低;边坡最大水平位移出现在坡脚处,潜在滑动面呈现出浅层滑动趋势。     

强降雨入渗下高大挡土墙渗流与稳定分析

分析了高大挡土墙的降雨入渗过程,提出了挡土墙的降雨模型,讨论了饱和—非饱和渗流场下的挡土墙的孔隙水压力分布及挡土墙稳定性变化,研究了不同原状边坡下挡土墙的渗流及整体稳定性,分析结果表明,墙后土体表面回填土区在原状土区径流的作用下降雨强度加大,加剧了回填土区渗流,土体内部回填土与原状土交界面逐步形成饱和带,而原状土坡更陡的挡土墙,其墙后渗流作用越显著,受降雨影响更明显,应提高其挡土墙稳定性富余度.     

交河故城土遗址边坡降雨非饱和入渗分析

降雨非饱和入渗是诱发边坡失稳滑动的主要原因之一。本文基于饱和-非饱和降雨入渗理论,建立饱和-非饱和降雨入渗模型,并进一步建立了其数值计算模型,发展了相应的数值模拟技术。在前人工作的基础上,编制完善了饱和-非饱和降雨入渗有限元计算程序。选取新疆交河故城土遗址一典型边坡,详细分析了一个较为完整降雨入渗过程中边坡内部渗流场的分布特征;结果表明:所建立的饱和-非饱和降雨入渗模型较好的描述了降雨过程中边坡内部的渗流场分布,为进一步分析边坡在降雨非饱和入渗条件下的稳定性提供了支持。     

媒体重头文章概览

<正>《土木建筑与环境工程》02/2017降雨条件下坡积土边坡暂态饱和区形成机理及分布规律降雨是引起坡积土边坡失稳的最常见外部因素之一,开展雨水在边坡内部的渗流过程研究已成为分析边坡在降雨条件下稳定性的前提。本文基于有限元数值模拟方法,进行了雨水在土体中渗流过程的模拟,着眼于降雨条件下边坡暂态饱和区的形成、分布及消散特征,描述了该过程中边坡内部含水率、基质吸力、水力梯度的变化规律。结果表明:暂态饱和区形成的主要原因是土体中向湿润锋下方渗出的雨水量小于降雨入渗补给量,从而使     

渗透侵蚀下砂性土细颗粒流失率预测方法

渗流过程中内部不稳定砂性土细颗粒易于流失而形成渗透侵蚀,将对土工建筑物或地基造成不良影响。细颗粒流失率是土体力学性能劣化及稳定性分析的关键参数,为了对渗透侵蚀下砂性土细颗粒的流失率进行预测,笔者讨论了判断土体是否会发生渗透侵蚀的内部稳定性评价准则;介绍了细颗粒起动临界坡降,并对应力折减系数进行修正,以考虑土颗粒受力的差异性;提出了“细颗粒起动概率”,对细颗粒起动与否进行量化;分析细颗粒在土体孔隙网络中的迁移过程,给出了细颗粒通过收缩的概率和渗透迁移距离;根据细颗粒起动和迁移规律给出了细颗粒侵蚀和沉积定律,基于质量守恒定律形成细颗粒流失率预测方法。采用预测方法进行流失率计算,计算结果和试验结果吻合较好,误差基本在15%以内。     

非饱和原状黄土边坡浸水试验研究

 非饱和黄土的渗流问题,由于其理论的复杂性和测试手段的滞后,至今发展很慢。通过大型现场试验,得到水在边坡中的渗透范围,分析在浸水试坑下部2～3 m范围内土体每日含水率的变化规律,讨论暂态饱和区的形成和水在边坡土体中的运移过程,得出本次试验水在非饱和原状黄土边坡中渗透范围的形成机制和运移规律。结果表明：水在非饱和黄土中的渗流过程是暂态饱和区不断下移扩大的过程;浸润角不是定值,随着水入渗总量增加而增大;水在非饱和黄土中的渗透范围分为四部分：第一、第二部分为水的横向运移段,第三部分为水的竖向渗透段,第四部分为暂态饱和区的湿润锋面;水在非饱和黄土中的渗透范围是一个闭合的近似椭圆形区域。     

降雨触发不同级配堆积体滑坡模型试验研究

深入研究降雨条件下堆积体坡失稳规律对滑坡预测预报和防灾减灾具有重要的理论意义和工程实用价值。开发研制了降雨滑坡室内模型试验系统,对3种配制的堆积体土样进行了模型试验。研究了降雨条件下堆积体土坡的渗流、变形、破坏和颗粒运移的规律,探讨颗粒级配对堆积体土坡稳定性的影响。结果表明:坡内土体体积含水率、孔隙水压力和吸力随降雨历程响应明确;湿润锋到达后体积含水率和孔隙水压力持续增加而吸力持续减小,达到峰值后稳定;降雨停止后体积含水率和孔隙水压力立即降低而吸力逐渐增大。坡体破坏瞬时土体位移有一个加速过程。颗粒级配(含石量)对土坡破坏模式具有显著的影响;堆积体含石量为13%,19%,41%的土坡破坏模式分别为多级后退式破坏、冲蚀引起的局部浅层滑动破坏和块状滑动破坏;含石量越小,滑裂面越明显;含石量对细颗粒流失也有影响,含石量越大细颗粒流失越显著,坡脚细颗粒含量越大。     

Theoretical analysis and model test for rainfall-induced shallow landslides in the red-bed area of Sichuan

Heavy rainfall is a key cause of shallow landslides in red-bed terrains with steep topography and residual soils of degradable strength. In this study, laboratory model tests were carried out to examine the characteristics of rainfall infiltration, deformation, and failures of slopes in the red-bed area of Sichuan. The hydrological response and deformation of the slope soil during rainfall are addressed. Based on a modified Green–Ampt infiltration model, ponding along the bedrock surface is incorporated. A physically based model for shallow landslides caused by rainfall is developed. The theoretical analysis and the model test results indicate that the slope failures are related to erosion in the shallow soil layer and rainwater infiltration, particularly along preferential seepage channels. The process of rainfall-induced shallow landslides can be separated into three stages: erosion at the slope toe, tension crack formation at the slope crest, and shallow sliding. When initial underground water level is located at the bedrock surface or the preferential seepage flow quickly reaches the bedrock surface, it is easier for the soil slopes to slide along the bedrock surface than along the wetting front.     

Effects of Dry Density and Grain Size Distribution on Soil-Water Characteristic Curves of Sandy Soils

The soil-water characteristic curve (SWCC) of soil plays the key roll in unsaturated soil mechanics which is a relatively new field of study having wide applications particularly in Geotechnical and Geo-environmental Engineering. To encourage the geotechnical engineers to apply unsaturated soil mechanics theories in routine practice, numerical methods, based on the SWCC and saturated soil properties, have been developed to predict unsaturated permeability function and unsaturated shear strength properties which are expensive and time consuming to measure in laboratories. Further, several methods have been proposed to predict the SWCC in order to avoid difficulties in measuring the SWCC in laboratories. It is time consuming and it may require special techniques or apparatus to measure the SWCC in laboratories. However, it is important to have laboratory measured data of SWCCs to enhance and verify the proposed numerical methods. Hence, employing a Tempe pressure cell apparatus, the present study aims to investigate the effects of dry density and grain-size distribution on the SWCCs of sandy soils. Drying and wetting SWCCs were obtained for four sandy soils with different dry densities. The test data were best-fitted using the Fredlund and Xing (1994) equation and found that the fitting parameter, a, increases linearly with increasing the air-entry value of the SWCC and the fitting parameter, m, decreases with increasing the residual suction of the SWCC. The results revealed that soils with a low density have lower air-entry value and residual suction than soils with a high dry density. Further, the maximum slope of drying SWCC and hysteresis of drying and wetting SWCCs decrease with increasing density of soil. The air-entry value, residual suction, and hysteresis (the difference between the drying and wetting SWCCs) tends to decrease when the effective D₁₀ of the soil increases. A soil with uniform grain-size distribution (the steeper slope in grain-size distribution) has a less hysteresis and a greater slope of drying SWCC than those of a non-uniform soil.     

旱涝急转下斜墙坝渗流特性试验研究

对斜墙坝渗流安全,关注点多在于是否形成稳定渗流场,浸润线高低以及渗流量多少等,渗流分析基本为饱和渗流,而在旱涝急转情况下,受旱土体处于非饱和状态,土体表面分布着裂缝,渗流情况复杂,常规渗流分析难以进行。基于室内坝坡模型,模拟旱涝急转工况,研究斜墙坝渗流特性。研究结果表明:对于受旱后裂缝发育程度低的黏土防渗体防渗性能良好,其渗透系数要小于饱和渗透系数,而对于裂缝较发育区域,由于其良好的透水性,渗透性远大于土体饱和时的渗透性;降雨及水位上涨时,裂缝会自行愈合,但愈合仅限于表面,土体完整性无法再回到产生裂缝前的状态,此时坝坡土压力值相对试验初期要小许多,裂缝的产生增大了土体的孔隙率,改变了土体渗透性,永久削弱了黏土防渗体防渗性能。试验得出了旱涝急转下斜墙坝渗流演变规律,可据此对实际工程中的渗流薄弱部位进行重点监测与保护。     

Strength reduction of cohesionless soil due to internal erosion induced by one-dimensional upward seepage flow

Suffusion, one of the modes of internal erosion, has been widely detected in both natural deposits and filled structures. It is the phenomenon that the fine particles in soil gradually migrate through the voids between the coarse particles, leaving behind the soil skeleton. In this paper, the main focus is on the changes in soil strength due to internal erosion. A series of one-dimensional upward seepage tests at a constant water head is performed to cause internal erosion in a soil sample by controlling the three variable parameters, namely (a) the fine content, (b) the relative density of the soil, and (c) the maximum imposed hydraulic gradient on the specimen. The mechanical consequences of the internal erosion are examined by cone penetration tests. The internal erosion indicated by the loss of fine particles causes changes in the void ratio and a significant increase in hydraulic conductivity, resulting in a decrease in the soil strength from its initial value.     

GIS-based three-dimensional slope stability analysis considering rainfall infiltration

Rainfall-induced landslides are a major cause of slope failure in mountainous areas. As rainfall begins to infiltrate a slope the wetting front advances into the soil and reduces its shear strength. Slope failures occur when the reduced shear strength becomes less than the resisting shear strength needed for equilibrium. These areas of instability are usually located near the ground surface where pore-water pressure changes rapidly during infiltration. The wetting front depth in a slope plays an important role in slope stability. In this study a well-known infiltration model, the Green and Ampt model, is integrated into three GIS-based three-dimensional limit equilibrium methods to assess the impact of rainfall on slope stability. This infiltration model can predict the depth of the wetting front during steady and unsteady rainfall. The applied three-dimensional methods are modified according to different positions of the wetting front to reflect the influence of rainfall on slope stability. This approach is capable of calculating safety factors corresponding to individual rainfall events and is also capable of predicting the corresponding failure time. The accuracy of the presented study has been verified by simulating the failure process of a real landslide triggered by a rainstorm.     

含浅层强透水层堤基的上覆砂层管涌破坏试验研究

堤基中往往存在局部浅层强透水层并形成渗流优先通道,该通道不能大幅度削减流体的水头势能,易引起堤基管涌破坏,此类堤基管涌破坏机理的研究尚不明朗,仍需进一步研究。采用砂槽试验模拟堤基渗流,试验中通过抬升水箱水位,观察砂土中细颗粒流失现象,并分析渗流量、渗透坡降、测压管水头、砂土颗粒级配、锥头阻力、沉降量等关键参数。试验结果表明,水箱水位增大至48cm,浅层强透水层上覆砂层被"击穿"发生管涌破坏,管涌破坏分为稳定渗流阶段、细颗粒流失阶段(0.05d≤0.075粒级砂土流失)、较细颗粒流失阶段(0.075d≤0.1粒级砂土流失)、管涌破坏扩大阶段(0.1d≤0.25粒级砂土流失)。管涌破坏过程中,细颗粒砂土流失,锥头阻力降低,砂土层发生沉降,且较细颗粒流失阶段的沉降较为突出。细颗粒砂土流失导致砂土层孔隙率和渗透系数上升,渗流量和渗透坡降随之增大。     

强降雨条件下含软弱夹层粘性土坡稳定性分析

为了分析含软弱夹层粘性土坡在强降雨条件下的稳定性,结合某失稳边坡工程现场实际调研资料,基于饱和非饱和渗流与非饱和抗剪强度理论,提出了一种新的含软弱夹层粘性土坡稳定性分析方法,并利用该方法分析了强降雨条件下边坡的渗流特性及安全系数变化规律。分析结果表明:降雨入渗先在边坡软弱夹层内形成暂态饱和区,且当坡顶入渗的雨水未渗流至软弱夹层时,夹层内暂态饱和区中的雨水会沿夹层上表面向着坡顶方向渗流;降雨入渗过程中,边坡基质吸力与铅直有效应力之间存在严格的正相关变化关系;随着降雨历时的增加,塑性区首先在软弱夹层内部贯通,然后向坡顶扩展,边坡安全系数逐渐降低;降雨停止一段时间后,由于坡顶入渗雨水的补给,软弱夹层内局部将仍存在暂态饱和区,此时,塑性区面积会由坡顶向软弱夹层内部减小,再由夹层内部至坡面逐渐缩减,但边坡安全系数并未明显上升;根据数值计算结果可将粘性土坡失稳过程分为夹层软化、夹层挤压、拉伸裂缝、坡顶沉降和断裂滑移等5个阶段。因此,为了降低强降雨对边坡稳定性的影响,在含软弱夹层粘性土坡支护设计时应着重考虑边坡排水系统的合理布设。