降雨促发黄土滑坡的启动机制模拟

黄土滑坡和降雨关系尤为密切。为深入研究降雨入渗对滑坡的促发作用,在对陕西西安地区“9·17”灞桥滑坡现场勘察的基础上,利用数值模拟方法系统研究了黄土边坡在降雨入渗条件下土体相关物理力学指标的变化响应特征及时空分布规律;从滑动面安全系数变化的角度分析了边坡的失稳过程,并揭示了该类滑坡的启动机制。结果表明：(1)降雨入渗首先引起坡面土体的基质吸力逐渐降低,而且不同分布位置的降幅不同;(2)滑坡启动前,坡体的高体积含水量范围随降雨明显扩大,且体积含水量表现出从古土壤层向邻近黄土层递减的规律;(3)边坡的水平方向位移自坡面中部向坡体的上下部呈放射状递减特征,垂直方向位移由上至下逐渐减小,而临界滑动面的安全系数也随降雨入渗过程逐步递减;(4)节理处土体的孔隙水压力和体积含水量的变化响应时间及幅度都早于且强于坡体其他区域,坡体内最大剪应变的区域分布与坡面基本平行,模拟结果与原型滑坡一致;(5)基于黄土独特的水敏性、地质构造和人类工程活动等诱因的影响,加上节理裂隙为水的入渗和运移提供了优势通道,降雨加速了黄土潜蚀和坡体结构破坏过程,改变了边坡内部应力场、位移场和水文地质条件,进而促发了滑坡。     

基于降雨作用下滑面抗剪强度动态变化的层状边坡稳定性评价

降雨入渗对边坡稳定性具有重要影响。鉴于层状边坡在自然情况中广泛存在，因此，开展在降雨情况下的多层状边坡入渗模型及其稳定性研究具有重要意义。考虑各层土体的渗透性能差异，利用约束函数求得各层土体的有效渗透系数，并基于Ｇｒｅｅｎ－Ａｍｐｔ模型建立层状边坡入渗模型。结合非饱和土强度经验公式，推导了考虑滑面抗剪强度动态变化的层状边坡稳定性方程。最后，对深圳某边坡在不同降雨强度和降雨历时下的稳定性系数进行了计算并对结果进行了分析。结果表明，当湿润锋未达到滑面处时，边坡稳定性系数与降雨历时和降雨强度呈线性关系。考虑边坡入渗能力，提出了降雨入渗临界时间概念。在任意降雨强度下，当降雨历时小于该时间时，边坡均不会失稳。本研究可以用于指导降雨情况下层状边坡的稳定性评价。      

软硬互层顺层岩质边坡破坏试验

软硬互层结构的顺层岩质边坡破坏类型复杂、难于防治, 针对此类边坡地质灾害易发、多发的问题, 从坡面角度、岩层倾向及组合形式、节理分布等方面进行了研究。边坡物理模型试验是揭示边坡变形破坏机理的重要手段, 基于相似理论, 以重庆市万州区孙家滑坡为工程依托, 根据滑坡区地质勘探报告设计了室内边坡物理模型试验; 试验通过顶升模型箱模拟重力加载来探究顺层岩质边坡发生破坏时, 前缘坡角和软弱夹层倾角之间的关系; 结合有限元分析软件Plaxis 2D对物理模型进行了多组数值模拟试验, 以验证软硬互层顺层岩质边坡破坏机制。试验结果表明: 对于顺层岩质边坡, 当软弱夹层的倾角在22°左右, 前缘开挖坡角58°左右时, 顺层岩质边坡容易发生滑动, 滑动面为后缘节理面和软弱夹层的贯通面。因此, 顺层岩质边坡稳定性受层面和节理面密度的控制, 当边坡含多层软弱层面时, 易沿层面和后缘节理贯通面发生破坏, 随着软弱面层数增加, 边坡稳定系数逐渐降低。研究成果可以为公路开挖切坡导致的顺层岩质边坡失稳机理研究及其稳定性评价提供理论依据, 为顺层岩质边坡失稳的预测预报提供支撑。      

赣南边坡变形破坏模式与防治对策

赣南地质环境复杂,沉积岩、岩浆岩与变质岩交叉并存;岩浆岩广泛分布,岩体破碎;褶皱、断裂构造发育;风化作用强烈,边坡稳定性差。研究这一地区的变形破坏模式,可以科学有效地对边坡进行防治。大量的野外调研显示,赣南边坡常见的类型主要有土质边坡、类土质边坡、岩质边坡和岩土二元结构边坡4类;赣南边坡常见的变形破坏模式主要有:沿原有结构面滑动、沿顶部拉裂-滑动、土体拉裂-崩塌、圆弧形滑动、坡面冲刷5类。沿原有结构面滑动破坏多见于岩质边坡、类土质边坡中;其余破坏模式多发生于土质边坡、类土质边坡、岩土二元结构边坡的土体部分。根据赣南边坡不同的变形破坏模式,分别提出了科学的防治对策;并针对赣南地区降雨量大、降雨持时长等特点,强调了在边坡的施工过程中应及时修建边坡排水系统,及时进行坡面防护。      

降雨条件下花岗岩残坡积土路堑边坡稳定性研究

基于多孔介质饱和 非饱和渗流理论和非饱和土抗剪强度理论,采用极限平衡法与降雨渗流有限元相结合的方法,选取大庆—广州高速公路江西里仁—龙南段某典型高路堑边坡,利用岩土仿真软件GeoStudio建立模型,研究降雨条件下花岗岩残坡积土路堑边坡失稳破环的形态和机理.结果表明:随着降雨的进行,边坡中地下水位从坡脚处开始抬升,边坡坡脚处容易产生“小弧”滑动破坏;土体体积含水量的增加使得基质吸力下降,从而降低土体强度和边坡的安全系数,且基质吸力与安全系数的下降幅度基本同步;降雨强度对边坡的安全系数有较大影响,小雨阶段,安全系数降幅较缓,而暴雨阶段,安全系数降幅加剧,降幅达24;4％;在强降雨条件下,滑面基质吸力大部分丧失,失稳边坡发生了从整体失稳向局部失稳、从深层失稳向浅层失稳的转变,即强降雨是浅层滑坡的主要外在因素.     

降雨与开挖作用下黄土滑坡失稳过程分析: 以关中地区长武县杨厂村老庙滑坡为例

降雨入渗和人工开挖是诱发黄土滑坡的重要因素, 为了研究在这2种诱因作用下关中地区黄土滑坡失稳过程及其对稳定性的影响, 以陕西省长武县杨厂村老庙滑坡为研究对象, 通过现场调查、地质测绘和钻孔勘探, 查明了该滑坡变形特征, 定性分析了滑坡变形演变过程; 基于滑坡变形前15 d内日降雨量实测值, 采用有限元软件, 对坡脚开挖后连续降雨作用下滑坡形成过程进行了仿真模拟; 基于强度折减法对该滑坡稳定性变化规律进行了研究。结果表明: ①关中地区特殊的地层结构是滑坡变形的内因, 降雨是最主要的诱发因素; ②滑坡失稳演化过程表现为: 坡体处于蠕滑状态, 坡脚开挖后, 坡体前缘失稳, 牵引中后缘坡体向下错动而产生张拉裂缝, 在降雨作用下, 雨水沿裂缝渗入坡体深部, 滑坡中部岩土体浸水后抗剪强度降低, 从而导致黄土层与红黏土层接触面饱水形成贯通滑带, 诱发深层滑坡; ③滑坡开挖后较初始状态, 稳定性系数降幅为0.102, 此后受连续降雨影响, 稳定性系数在前10 d以平均0.010/d的速率缓慢下降, 第10~13 d以0.034/d的速率快速下降至最低, 第13 d以后开始回升。研究结果可以为该类滑坡防治提供有效依据。      

太原市杏花岭区小返乡后李家山村边坡稳定性分析

通过对太原市杏花岭区小返乡后李家山村的两处边坡进行勘察,主要查明岩土类型、成因、工程特性以及岩土的透水性和地下水的出露情况,查明坡体岩土的物理力学性质和软弱结构面的抗剪强度,对边坡的稳定性进行了分析,稳定性分析采用圆弧滑动法进行,两处边坡均处于不稳定状态,在降水条件诱发下极易发生滑动,必须尽快进行综合整治.     

考虑降雨渗流影响的露天煤矿黄土开挖边坡分析

黄土开挖边坡在降雨作用下坡面破坏可分为片蚀破坏、沟状侵蚀、坍塌破坏3个阶段。通过野外现场地质调查,建立调查区域露天煤矿某段运煤大道处黄土开挖边坡三维数值分析模型,采用V G模型设定渗流边界函数,并以此根据室内外实验设定初始负孔隙水压力;在有限差分软件中运用fish语言将野外现场测得的稳定渗透系数与边坡开挖高度关系编入渗流计算命令流中,模拟了不同降雨强度下孔隙水压力、开挖边坡位移、坡面冲刷程度、最大主应力变化情况,提出黄土区开挖边坡在降雨作用下沟状侵蚀在第三级台阶(坡高60 m)左右破坏严重,坡高高于此高度应注意沟状侵蚀的剧烈扩展并加以防治;应在开挖边坡顶部以及台阶坡顶、坡肩处加强监测与防护治理工作。      

山西乡宁—吉县地区黄土高边坡可靠度研究

在山西乡宁—吉县地区实测了7个具有代表性的自然极限状态黄土高边坡断面,建立边坡地层结构模型;选取研究区合理的黄土物理力学参数,并分地层年代对10个典型工点的黄土强度参数内聚力和内摩擦角的变异性进行分析;选取6组典型的强度参数变异系数组合,基于Monte Carlo法进行可靠度模拟,评价该区边坡稳定性;采用自然类比法进行边坡设计.结果表明:边坡失效概率随坡高的变化趋势一致,均是中等坡高(49.8m)的边坡失效概率较大,低坡和高坡的失效概率较低;当变异系数较小时,边坡失效概率对坡高和坡度的变化敏感,而当变异系数较大时,边坡失效概率对坡高和坡度的变化不敏感;当坡型一定、强度参数变异系数较小时,边坡的稳定系数基本不变,而当参数变异系数超过某一界限时,稳定系数随变异系数的增大而增大,二者存在非线性相关关系;研究区黄土边坡处于基本稳定状态,但其失效概率最大达69.3%,平均21.1%,介于20%~30%的比例为33.3%,大于30%的比例为14.3%,可靠度指标介于-0.5~8.5,其中小于2.7的比例为88.1%;对于坡高约为50m的黄土高边坡,若取30%作为可接受失效概率,边坡坡度需降至45°以下,如果期望可接受失效概率在10%以内,则坡度不宜超过34°.     

降雨作用碎石土堆积层滑坡变形规律

降雨诱发碎石土堆积层滑坡十分普遍,野猫面滑坡则是典型的碎石土堆积型滑坡,也是三峡库区内距三峡大坝最近的
特大型滑坡体,研究其在不同降雨条件下的变形规律具有示范性和实用价值。首先,在滑坡区域地质分析的基础上,研究了碎石
土堆积层滑坡的组成结构;然后应用Modflow软件建立了滑坡地下水渗流场数值模型,对不同降雨强度和降雨历时条件下,降雨
入渗补给地下水的规律进行了模拟;最后,运用FLAC3D软件对不同降雨工况条件下,渗流场变化引起的滑坡变形规律进行了模
拟。研究结果表明:①野猫面滑坡碎石土堆积层表现为三层结构,上部为碎块石土,中下部为块石土、钙质胶结碎块石,下部滑面
为碎石土和黏土夹碎石;②降雨强度和降雨历时增加均使滑坡地下水头抬升,在斜坡前缘临近库岸部分的水力坡度明显增大;③
降雨作用下,滑坡位移量增加,变形由地表向深部、由前缘向后缘发展,在滑面处剪应变集中;极端大暴雨条件下,前缘存在整体变
形区;④碎石土堆积层滑坡地表径流疏导与生态防渗可有效防治滑坡。      

Hydro-mechanical analysis of rainfall-induced fines migration process within unsaturated soils

Seepage-induced fines migration under rainfall infiltration is a main cause leading to shallow failures in loose colluvial slopes. To describe the full process of fines migration within unsaturated soils during rainfall infiltration and the associated hydro-mechanical behaviors, a seepage-erosion-deformation coupled formulation is proposed in this paper. The governing equations proposed are implemented into a finite element code and used to investigate the influences of skeleton deformation on the rainfall infiltration process through unsaturated soil columns. The numerical results were presented in detail for a better understanding of the rainfall-induced fines migration process within unsaturated soils. Further, the obtained results are integrated into an infinite slope model for slope stability analysis. The results show that, the skeleton deformation will affect the rainfall infiltration rate and hence the timing of slope failures; meanwhile their influences are more evident if the fines deposition process is taken into account. Moreover, the slope stability could be reduced gradually due to the soil strength loss along with loss of fine particles. Therefore, particular attentions should be paid to analyzing the stability of soil slopes susceptible to internal erosion.     

Seismic stability analysis of slopes with pre-existing slip surfaces

In analyzing seismic stability of a slope with upper bound limit analysis method, the slip surface is often assumed as a log-spiral or plane slip surface. However, due to the presence of a weak layer and unfavorable geological structural surface or a bedrock interface with overlying soft strata, the preexisting slip surface of the slope may be irregular and composed of a series of planes rather than strictly log-spiral or plane shape. A computational model is developed for analyzing the seismic stability of slopes with pre-existing slip surfaces. This model is based on the upper bound limit analysis method and can consider the effect of anchor bolts. The soil or rock is deemed to follow the Mohr-Coulomb yield criterion. The slope is divided into multiple block elements along the slip surface. According to the displacement compatibility and the associated flow rule, a kinematic velocity field of the slope can be obtained computationally. The proposed model allows not only calculation of the rate of external work owing to the combined effect of self-weight and seismic loading, but also that of the energy dissipation rate caused by the slip surface, interfaces of block elements and anchorage effect of the anchors. Considering a direct relationship between the rate of external work and the energy dissipation rate, the expressions of yield acceleration and permanent displacement of anchored slopes can be derived. Finally, the validity of this proposed model is illustrated by analysis on three typical slopes. The results showed that the proposed model is more easily formulated and does not need to solve complex equations or time consuming iterations compared with previous methods based on the conditions of force equilibrium.     

Debris flows introduced in landslide deposits under rainfall conditions: The case of Wenjiagou gully

Debris flows often occur in landslide deposits during heavy rainstorms. Debris flows are initiated by surface water runoff and unsaturated seepage under rainfall conditions. A physical model based on an infinitely long, uniform and void-rich sediment layer was applied to analyze the triggering of debris-flow introduced in landslide deposits. To determine the initiation condition for rainfall-induced debris flows, we conducted a surface water runoff and saturated-unsaturated seepage numerical program to model rainfall infiltration and runoff on a slope. This program was combined with physical modeling and stability analysis to make certain the initiation condition for rainfall-introduced debris flows. Taking the landslide deposits at Wenjiagou gully as an example, the initiation conditions for debris flow were computed. The results show that increase height of surface-water runoff and the decrease of saturated sediment shear strength of are the main reasons for triggering debris-flows under heavy rainfall conditions. The debris-flow triggering is affected by the depth of surface-water runoff, the slope saturation and shear strength of the sediment.     

京广铁路K1219路基土质边坡深层滑移失稳机制与整治对策

为研究土质边坡深层滑移失稳机制, 以京广铁路下行线K1219+000处路基边坡失稳为例, 通过现场调查测绘、工程地质钻探、原位试验和室内试验、深部位移监测和数值模拟等手段, 详细研究了该土质边坡变形破坏特征、地质力学过程和失稳模式。结果表明: 路基边坡表面裂缝宽度及深度呈坡顶至坡脚逐渐变浅变窄, 变形具有一定的旋转性, 牵引式特征明显, 属于深层滑移拉裂式失稳; 土质边坡经历了因坡脚开挖、抽水引起的应力场和渗流场重新分布阶段、雨水入渗软化导致下滑力不断增大而滑面逐渐迁移扩大加深阶段以及支挡结构抗力失效阶段3个地质力学过程, 其失稳模式包括浅层滑移、浅层滑面向深层迁移、动荷载触发深层滑移失稳3个阶段。在此基础上, 综合确定了滑面位置, 并通过反演方法确定了滑面力学参数。研究采用了刚架式双排抗滑桩的整治方案, 通过理论计算和数值分析, 边坡变形与抗滑桩变形基本一致, 且与监测结果整体吻合, 这表明土质边坡深层失稳理论分析准确且计算的力学参数科学, 整治对策稳妥、可靠。      

平面剪切岩质边坡滑裂面的确定及稳定性分析

由于传统搜索方法对岩质边坡滑裂面的确定无法兼顾效率与精度, 如何迅速准确确定潜在滑裂面仍然是个难题。极限平衡法在岩质边坡稳定性分析中备受认可, 采用岩质边坡平面剪切滑动模型, 以滑裂面的倾角来表征潜在滑裂面的位置; 基于极值法, 推导了极限平衡条件下平面剪切破坏型岩质边坡潜在滑裂面的解析解, 并结合香港秀茂坪路边坡对其准确性进行了验证, 进一步对四川宜宾打营盘山公路多级边坡进行了整体稳定性分析。结果表明: 香港秀茂坪边坡采用本文方法确定的边坡潜在滑裂面倾角与实际滑坡倾角基本一致。实际工程应用中, 采用Slide软件中布谷鸟搜索法和模拟退火法两种搜索方法得到的滑裂面倾角分别为38.0°和37.0°, 本解析法所得倾角为34.8°; 选用Janbu法、Morgenstern-Price法和Sarma法分别计算对应的稳定系数, 结果均为1.04左右, 本文所得稳定系数为1.15, 可见本文方法所得结果基本准确。通过参数敏感性分析发现, 随着黏聚力的增加, 边坡滑裂面倾角越来越小, 稳定系数也随之增加; 而当内摩擦角增大时, 边坡滑裂面倾角和稳定系数也随之增大。      

沿河县上覆第四系堆积物下伏泥岩类型滑坡失稳机制分析

建立小寨滑坡水文地质模型,模拟其在不同降雨条件下的渗流场分布及稳定性变化。结果表明,该类型滑坡的失稳演化过程主要分为上覆Q4el+dl堆积阶段、接触带泥岩富水风化阶段和风化带富水软化阶段,风化带富水软化程度与滑体的高渗透性和滑床的隔水特性密切相关,是滑坡失稳破坏的决定性因素;滑坡的发生是因地表降雨下渗受阻,在上覆堆积层及下伏泥岩层间形成富水带,富水加速泥岩风化,并在坡体内形成强风化泥岩软弱层,随地质过程持续,泥岩强风化层逐渐加厚形成软弱带,在极端暴雨气候条件下,软弱带发生蠕变,地表开裂,滑坡形成。     

基于修正Green-Ampt模型的降雨诱发区域浅层斜坡失稳灾害分析

由于具有类似的工程地质和水文地质条件, 在高度相关的降雨作用下, 同一个区域中的降雨诱发浅层斜坡失稳灾害常成群出现。在区域尺度预测浅层斜坡失稳灾害对滑坡灾害的防灾减灾工作具有重要的意义。为此, 提出了一种基于力学原理的降雨诱发浅层斜坡失稳灾害预测新模型RARIL。该模型采用修正Green-Ampt模型进行降雨入渗分析, 采用无限体边坡模型进行安全系数计算, 利用可靠度原理考虑区域斜坡稳定性分析中的参数不确定性。该模型具有可考虑降雨诱发浅层斜坡的失稳力学机理、可考虑区域内斜坡土体参数不确定性, 以及计算效率高、易于在GIS平台上实现等优点。案例分析表明, RARIL模型较为准确地预测了2010年8月12日11∶00至2010年8月14日9∶00期间强降雨在四川省汶川县映秀镇附近的303省道K0-K20段沿线区域引发的滑坡灾害, 研究结果证明RARIL模型在预测降雨诱发区域斜坡失稳灾害方面有很好的应用前景。      

西安白鹿塬北缘黄土边坡稳定的可靠度分析

在西安白鹿塬北缘实测了14个黄土自然边坡断面,建立了地质模型,通过采样测试并收集已有的测试资料,获得了各时代黄土地层的物理力学参数,将Morgenstern-Price法稳定性计算公式作为极限状态方程,分别采用Monte-Carlo法和Duncan法进行了边坡稳定的可靠度分析。结果表明:采用Monte-Carlo法和Duncan法得到的14个边坡稳定系数分别为1.11～1.41和1.09～1.33,显示这些边坡处于基本稳定—稳定状态;采用上述两种方法得到的边坡可靠指标分别为0.56～1.79和0.39～1.60,得到的失效概率分别为3.4%～29.0%和5.5%～34.8%,并且共有75%的边坡失效概率大于10%,失效可能性较大。统计结果表明:白鹿塬区边坡的坡高和坡度存在负相关性。低而陡的边坡潜在最危险滑面剪出口较高,失效概率较小;坡高达到50m及以上时,边坡高而缓,剪出口较低,失效概率较大,其中河流下切深,有N2泥岩出露的极高边坡,稳定性最差。Duncan法求解的稳定系数略小于Monte-Carlo法求得的结果,而前者求解的失效概率略大于后者,两种方法计算结果较为接近。由于Duncan法理论简单,计算量小,更宜于在实际工程中应用。     

Stability characteristics of shallow landslide triggered by rainfall

Rainfall induced shallow landslides are known to be extremely dangerous since the sliding mass can propagate quickly and travel far from the source. Although the sliding mechanism in sloping ground is simple to understand, the problem may be complicated by unsaturated transient water flow. The flow behavior of rainwater in unsaturated sloping ground and the consequent factor of safety must be clearly understood to assess slope stability under rainfall conditions. A series of laboratory experiments was conducted to examine the critical hydrological states so that assessment of slope stability under rainfall condition can be performed. Based on the test results, a unique relationship between critical hydrological states, rainfall intensity, and soil properties was formulated. Sequential stability analysis provided insights into the stability of slopes subjected to variations in soil properties, slope angles and rainfall intensities, and the consequent variation in the depth of the failure plane, vital in landslide risk assessment, was determined through this analysis.The variation of rainfall intensity was found to strongly affect the depth of the failure plane in cohesionless sloping ground. Furthermore, the influence of rainfall intensity on the depth of the failure plane may be alleviated by a small magnitude of cohesive strength. The results of this study will reinforce knowledge of landslide behavior and help to improve mitigation measures in susceptible areas.     

Active earth pressure acting on retaining wall considering anisotropic seepage effect

This paper presents a general solution for active earth pressure acting on a vertical retaining wall with a drainage system along the soil-structure interface. The backfill has a horizontal surface and is composed of cohesionless and fully saturated sand with anisotropic permeability along the vertical and horizontal directions. The extremely unfavourable seepage flow on the back of the retaining wall due to heavy rainfall or other causes will dramatically increase the active earth pressure acting on the retaining walls, increasing the probability of instability. In this paper, an analytical solution to the Laplace differential governing equation is presented for seepage problems considering anisotropic permeability based on Fourier series expansion method. A good correlation is observed between this and the seepage forces along a planar surface generated via finite element analysis. The active earth pressure is calculated using Coulomb’s earth pressure theory based on the calculated pore water pressures. The obtained solutions can be degenerated into Coulomb’s formula when no seepage exists in the backfill. A parametric study on the influence of the degree of anisotropy in seepage flow on the distribution of active earth pressure behind the wall is conducted by varying ratios of permeability coefficients in the vertical and horizontal directions, showing that anisotropic seepage flow has a prominent impact on active earth pressure distribution. Other factors such as effective internal friction angle of soils and soil/wall friction conditions are also considered.