Suggestion of a method for landslide early warning using the change in the volumetric water content gradient due to rainfall infiltration

An early warning system can be an effective measure to reduce the damage caused by landslides by facilitating the timely evacuation of residents from a landslide-prone area. Early detection of landslide triggering across a broad range of natural terrain types can be accomplished by monitoring rainfall and the physical property changes of soils in real time or near-real time. This study involved the installation of a real-time monitoring system to observe physical property changes in soils in a valley during rainfall events. This monitoring included the measurement of volumetric water content, which was compared with the results of laboratory flume tests to identify landslide indicators in the soils. The response of volumetric water content to rainfall events is more immediate than that of pore-water pressure, and volumetric water content retains its maximum value for some time before slope failure. Therefore, an alternative method for landslide monitoring can be based on the observation of volumetric water content and its changes over time at shallow soil depths. Although no landslide occurred, the field monitoring results showed a directly proportional relationship between the effective cumulative rainfall and the gradient of volumetric water content per unit time (t/t _max). This preliminary study thus related slope failure to the volumetric water content gradient as a function of rainfall. Laboratory results showed that a high amount of rainfall and a high gradient of volumetric water content could induce slope failure. Based on these results, it is possible to suggest a threshold value of the volumetric water content gradient demarcating the conditions for slope stability and slope failure. This threshold can thus serve as the basis of an early warning system for landslides considering both rainfall and soil properties.     

降雨诱发堆积体滑坡水土响应与稳定性时空演化试验研究

降雨在松散堆积土中入渗引起内部水土力学的变化是影响稳定性的关键。目前研究多侧重考虑颗粒粒径、含量等因素对斜坡破坏的影响,但是针对斜坡体内部水土响应及稳定性时空演化方面的研究存在不足。基于野外滑坡案例,通过室内降雨滑坡模型试验、土力学试验和理论分析手段,研究了降雨触发松散堆积体斜坡变形破坏过程及模式,采用Van Genuchten模型（VG模型）重构了土体的土-水特征曲线,重点探究了斜坡内部水土力学变化以及稳定性时空演化规律。结果表明：（1）堆积体斜坡破坏经历了微裂隙发育-局部破坏-整体破坏3个阶段,呈现出“初期拉裂-坡面坍塌-塑性滑动”的破坏模式;（2）入渗过程斜坡体积含水率以及孔隙水压力急速增加,而土颗粒之间基质吸力下降甚至消散,促进了斜坡破坏发展;（3）土体力学强度随体积含水率升高呈指数下降,体积含水率为36.3%时,有效黏聚力和有效内摩擦角仅为0.27 kPa、3.39°;（4）基于极限平衡理论和斜坡土水特征监测数据,构建了斜坡稳定性时空演化图谱,与模型试验破坏特征有较好的一致性。研究结果对降雨作用下的堆积层斜坡监测预警与防灾减灾提供理论支撑。     

滑坡对降雨响应的多指标监测及综合预警探析:以赣南罗坳滑坡为例

降雨量和位移是当前降雨型滑坡监测预警最常用的指标。然而,降雨量和位移监测结果只能反映降雨作用下滑坡的变形情况,不能揭示滑坡内在物理力学性状对降雨的响应。因此,除降雨量和位移监测之外,建立包括体积含水率、基质吸力等反映滑坡动态演化过程的关键指标监测体系必将成为今后更真实地把握滑坡内在演化趋势、更准确地建立滑坡综合预警判据的最有效手段。笔者对赣南地区典型降雨型滑坡进行了多指标监测及综合预警示范研究。结果表明:(1)在降雨条件下滑坡土体内部体积含水率、基质吸力和温度等多指标均产生有规律的动态响应;(2)随着降雨的持续,滑体体积含水率与基质吸力的变化均具有显著的滞后现象;(3)体积含水率和基质吸力变化速率与滑体位移具有显著的正相关性;(4)滑体温度分布变化规律受大气温度和体积含水率的共同影响。以实测数据的滑坡稳定性分析为基准,在考虑实际降雨入渗深度与滑坡稳定性的关联度上,建立了包括日降雨量、体积含水率增加速率、基质吸力减小速率以及位移速度多元指标预警方法体系,提出了基于关键指标综合预警体系及确定方法,旨在为降雨滑坡准确预警提供新模式。     

Rain-triggered slope failure of the railway embankment at Malda,India

The common slope stability analysis is incapable of accurately forecasting shallow slides where suction pressures play a critical role. This realization is used for elaborate stability analyses which include soil suction to better predict rainfall-induced slides at railway embankment at Malda where three known cases of slope failures and train derailments occurred after heavy rainfall. The relationship between the soil–water content and the matric suction is established for the embankment soil. It is then used in the coupled analyses of seepage and slope stability to estimate performances of the embankment at different intensity and duration of rainfall. The numerical simulations are performed with the FE code Geo-Studio. The numerical results show significant reduction in the factor of safety of the railway embankment with the increase in the intensity and duration of rainfall. The effectiveness of the proposed mitigation measures including placement of 2 m-wide free draining rockfill across the slopes and drilling 5-m-long sheet pile wall at the toe of the embankment is studied numerically. The study confirms that the proposed mitigation measures effectively increase the factor of safety of the embankment and stabilizing it even in case of a heavy rainfall of 25 mm/h over 12 h.     

不同渗流边界条件下粉砂边坡失稳模型试验研究

研究不同渗流边界条件下粉砂边坡的稳定特性对滑坡安全评价及防灾减灾具有重要意义。开发研制了室内边坡模型试验系统,分别进行了在上部边界入渗、侧向边界入渗和底部边界入渗的边坡失稳模型试验。分析了各试验中边坡内部不同位置体积含水率、孔隙水压力和基质吸力的变化规律以及坡体破坏过程。结果表明,不同入渗边界条件下边坡土体局部由非饱和变为饱和状态,基质吸力消失,最后在坡面形成不稳定区域塌落破坏。上部边界入渗、侧向边界入渗和底部边界入渗所引起的破坏模式根据其特点可分为局部浅层破坏、多级后退式破坏和坡面滑动破坏。通过监测边坡不同位置的体积含水率,可以为渗流引发的边坡失稳预警机制提供参考依据。     

Detection of water infiltration and deformation of unsaturated soils by elastic wave velocity

Intense rainfall is the most important landslide trigger. In many mountainous environments of the world, heavy rainfall has caused many landslides and slope failures in a matter of seconds without warning. Therefore, an early warning system can be an effective measure to reduce the damage caused by landslides and slope failures by facilitating the timely evacuation of people from landslide-prone areas. In this study, we propose an idea to correlate soil moisture changes and deformations in slope surface by means of elastic wave propagation in soil. Constant shear stress drained triaxial tests where water was infiltrated from the bottom of specimen until failure, and slope model tests under artificial rainfall were performed to investigate the response of elastic wave velocities during pre-failure phases of rainwater infiltration and deformation. Analysis of the results has established that the elastic wave velocity continuously decreases in response of moisture content and deformation, and there was a distinct surge in the decrease rate of wave velocity when failure was initiated. Possible mechanisms were interpreted based on the test results. It is proposed that a warning be issued at switch of wave velocity decrease rate. This approach can thus serve as the basis of an early warning system for landslides and slope failure considering both moisture content and deformation.     

Comparison of single- and dual-permeability models in simulating the unsaturated hydro-mechanical behavior in a rainfall-triggered landslide

Landslide-prone slopes in earthquake-affected areas commonly feature heterogeneity and high permeability due to the presence of cracks and fissures that were caused by ground shaking. Landslide reactivation in heterogeneous slope may be affected by preferential flow that was commonly occurred under heavy rainfall. Current hydro-mechanical models that are based on a single-permeability model consider soil as a homogeneous continuum, which, however, cannot explicitly represent the hydraulic properties of heterogeneous soil. The present study adopted a dual-permeability model, using two Darcy-Richards equations to simulate the infiltration processes in both matrix and preferential flow domains. The hydrological results were integrated with an infinite slope stability approach, attempting to investigate the hydro-mechanical behavior. A coarse-textured unstable slope in an earthquake-affected area was chosen for conducting artificial rainfall experiment, and in the experiment slope, failure was triggered several times under heavy rainfall. The simulated hydro-mechanical results of both single- and dual-permeability model were compared with the measurements, including soil moisture content, pore water pressure, and slope stability conditions. Under high-intensity rainfall, the measured soil moisture and pore water pressure at 1-m depth showed faster hydrological response than its simulations, which can be regarded as a typical evidence of preferential flow. We found the dual-permeability model substantially improved the quantification of hydro-mechanical processes. Such improvement could assist in obtaining more reliable landslide-triggering predication. In the light of the implementation of a dual-permeability model for slope stability analysis, a more flexible and robust early warning system for shallow landslides hazard in coarse-textured slopes could be provided.     

基于失效概率的边坡降雨阈值曲面探讨

降雨是诱发边坡变形失稳的主要因素,而针对降雨型边坡的预警预报也一直是工程领域的核心问题。本文将蒙特卡罗方法引入降雨型滑坡的预警预报,首先基于正态分布的岩土体物理力学参数,建立了边坡的有限元数值计算模型,并分析了9种不同型式降雨下边坡稳定性系数的变化情况。结果显示递增型降雨对边坡的稳定性尤为不利,均匀型降雨次之,递减型降雨影响最小。其次,将降雨过程划分为前期降雨+当期降雨,并确定了前期降雨对于当期降雨的有效时间为6 d。最后,论文结合可靠度理论,选取失效概率P_f=10%作为预警指标,通过把前期降雨引入降雨强度-降雨历时关系曲线并作为第三坐标轴,最终将该曲线扩展成为前期降雨(A)-当期降雨(I)-降雨历时(D)曲面(A-I-D阈值曲面),研究结果对于降雨型边坡的预警预报具有一定的指导意义。     

Modeling uncertainty in stability analysis for design of embankment dams on difficult foundations

A probabilistic 3-D slope stability analysis model (PTDSSAM) is developed to evaluate the stability of embankment dams and their foundations under conditions of staged construction taking into consideration uncertainty, spatial variabilities and correlations of shear strength parameters, as well as the uncertainties in pore water pressure. The model has the following capabilities: (1) conducting undrained shear strength analysis (USA) and effective stress analysis (ESA) slope stability analysis of staged construction, (2) incorporation of field monitored data of pore water pressure, and (3) incorporation of increase of undrained shear strength with depth, effective stress, and pore water pressure dissipation. The PTDSSAM model is incorporated in a computer program that can analyze slopes located in multilayered deposits, considering the total slope width.

The main outputs of the program are the geometric parameters of the most critical sliding surface (i.e., center of rotation/radius of rotation and critical width of failure), mean 2-D safety factor, mean 3-D safety factor, squared coefficient of variation of resisting moment, and the probability of slope failure. The program is applied to a case study, Karameh dam in Jordan. Monitored data of induced pore water pressure in the dam embankment and soft foundation were gathered during dam construction.

The stability of Karameh dam embankment and foundation was evaluated during staged construction using deterministic and probabilistic analysis. Foundation stability was evaluated based on the monitored data of pore water pressure.

The study showed that the mean values of the corrective factors which account for the discrepancies between the in situ and laboratory-measured values of soil properties and for the modeling errors have significant influence on the 2-D safety factor, 3-D safety factor, slope probability of failure, and on the expected failure width.

The degree of spatial correlation associated with shear strength parameters within a soil deposit also influences the probability of slope failure and the expected failure width. This correlation is quantified by scale of fluctuation. It is found that a larger scale of fluctuation gives an increase in the probability of slope failure and a reduction in the critical failure width.     

基于无限边坡算法的降雨型滑坡预警系统的模型试验研究

滑坡预警系统是减少降雨型滑坡灾害的重要手段,其中针对具体单个滑坡失稳的预警系统,尽管具有较好的物理判断依据,但由于其构造机制较为复杂,目前仍然比较少见。基于无限边坡算法构建了较为简单的实验室降雨型滑坡技术性预警系统,并详细介绍了该系统的预警思路、预警时间计算原理、信息交流及反馈的实现。通过模型试验结果考察该预警系统在估算不同降雨强度及不同初始含水状态下滑坡失稳时间的表现,并对模型试验结果、预警能力进行分析。结果表明,基于无限边坡算法的预警系统的预警能力尽管受到降雨侵蚀、滑动面位置、初始含水状态的影响,但仍然具有较好的准确性。     

考虑含水率影响的压实黄土路堤稳定性研究

压实黄土路堤是主要的路基型式之一,由降雨引起的压实黄土含水率增大导致的高填方路堤边坡失稳时有发生。通过对山－平（山阴至平鲁）高速公路黄土路堤填料进行室内击实试验和三轴压缩试验,分析了填土含水率对填土密度、干密度及填土抗剪强度的影响。结果表明,含水率对黄土填料密度和干密度的影响都较大;随着含水率的变化,填土黏聚力和内摩擦角呈现一增一减的变化规律。根据试验结果,采用FLAC3D建立现场试验段加筋路堤数值模型,基于强度折减法分析4种加筋工况下含水率对黄土填土路堤稳定性的影响表明,随着填土含水率的增大,路堤安全系数不断减小,路堤稳定性降低;路堤安全系数随土工格栅铺设层数的增加而增大,在路堤中铺设土工格栅能够提高路堤整体性,很好地防止路堤失稳。     

降雨条件下体积含水率对堆积体边坡渗流特性的影响研究

锦屏水电站对外交通专用公路K48+722~800段滑坡受降雨影响明显,其深层稳定性关系到工程安全。本文依据实验数据,采用Fredlund-Xing方法拟合出滑坡体材料的土水特征曲线,以体积含水率反应土体初始湿润状态,研究了降雨条件下边坡压力水头的演化规律,体积含水率对压力水头以及边坡稳定性的影响。结果表明,降雨初期,坡体表面以及滑面处压力水头先增大,并向坡体内部逐渐扩展。雨停一段时间后,坡体表面以及前缘区压力水头逐渐消散,滑面后缘局部地区压力水头会有所升高,压力水头随着体积含水率的增大而增大。体积含水率较小时,在一定的时间内,边坡的稳定性系数先保持稳定,后缓慢降低;当体积含水率较大时,雨停一定时间后边坡的稳定性系数会出现上升的趋势。     

滑坡对降雨的动态响应及其监测预警研究

以福建德化县彭坑滑坡为例,基于非饱和土力学理论,结合滑坡实际监测数据,利用有限元法对坡体在雨水入渗条件下的渗流、变形特性和稳定性进行计算和分析,研究滑坡对降雨的动态响应和不同工况下安全系数和位移的关系。结果表明:滑体结构松散,易于雨水入渗,改变基质吸力空间分布,弱化岩土体参数; 持续降雨会使地下水位上升,坡脚发生变形,牵引坡体后缘产生拉裂,直至在降雨期间产生以非饱和区域为主的滑坡。强降雨初期,边坡安全系数下降较快,易发生滑坡; 雨停后安全系数会因雨水持续下渗而进一步降低,应注意雨后滑坡的产生; 同样水分在不同坡体部位对稳定性影响不同,安全系数有消有涨,存在时空效应。最后为反映地下水位线和降雨条件变化对边坡产生危害的程度,提出危险系数概念,建立危险系数与增量位移的关系,实现边坡测点位移变化的阶段式预警。研究结果和方法可为类似工程的治理和监测预警提供有意义的参考。     

Numerical model of antecedent rainfall effect on slope stability at a hillslope of weathered granitic soil formation

Modeling rainwater infiltration in slopes is vital to the analysis of slope failure induced by heavy rainfall. Although the significance of rainwater infiltration in causing landslides is widely recognized, there have been different conclusions as to the relative roles of antecedent rainfall to slope failure. In this study, a numerical model was developed to estimate the effect of antecedent rainfall on an unsaturated slope, the formation of a saturated zone, and the change in slope stability under weak rainfall and rainstorm event. Results showed that under a rainstorm event, slope failure occurred at comparably similar time although the antecedent rainfall drainage periods prior to the major rainfall were different (i.e., 24-h, 48-h and 96-h). However, under weak rainfall condition, differences of the antecedent rainfall drainage periods have significant effect on development of pore-water pressure. A higher initial soil moisture conditions caused faster increase in pore water pressure and thus decreasing the safety factor of the slope eventually increasing likelihood of slope failure.     

Probabilistic failure analysis of infinite slopes under random rainfall processes and spatially variable soil

Rainfall-induced landslides occur during or immediately after rainfall events in which the pore water pressure builds up, leading to shallow slope failure. Thereby, low permeability layers result in high gradients in pore water pressure. The spatial variability of the soil permeability influences the probability such low permeability layers, and hence the probability of slope failure. In this paper, we investigate the influence of the vertical variability of soil permeability on the slope reliability, accounting for the randomness of rainfall processes. We model the saturated hydraulic conductivity of the soil with a one-dimensional random field. The random rainfall events are characterised by their duration and intensity and are modelled through self-similar random processes. The transient infiltration process is represented by Richards equation, which is evaluated numerically. The reliability analysis of the infinite slope is based on the factor of safety concept for evaluating slope stability. To cope with the large number of random variables arising from the discretization of the random field and the rainfall process, we evaluate the slope reliability through Subset Simulation, which is an adaptive Monte Carlo method known to be especially efficient for reliability analysis of such high-dimensional problems. Numerical investigations show higher probability of slope failure with increased spatial variability of the saturated hydraulic conductivity and with more uniform rainfall patterns.     

基于弹性波波速的降雨型滑坡预警系统

降雨是诱发滑坡最主要的因素。为减少滑坡灾害造成的人员伤亡和经济损失,滑坡早期预警系统成为了最佳选择之一。根据弹性波传播的基本原理和基于降雨型滑坡变形破坏的特点,提出利用弹性波波速来反映边坡表面土体含水率和位移的变化。开发研制了一套三轴渗流-弹性波测试三轴仪和相关系统。该装置能让水从底部渗入土体,模拟降雨入渗土体的过程,同时能测试弹性波波速。试验过程中同步测试含水率、变形和弹性波波速的变化。还进行了降雨滑坡模型试验。利用三轴弯曲元注水试验和降雨滑坡模型试验,深入分析和研究降雨引起的土体滑坡过程与弹性波波速演化规律,揭示波速与含水率和变形的耦合关系。研究结果表明,弹性波波速随着含水率的增大而缓慢减小,随着变形的增大而急剧减小,临近失稳时,波速骤然减小。根据试验结果对含水率和变形导致弹性波波速减小可能的机制进行了解释,提出弹性波在波速骤然减小时发出滑坡预警。研究结果为滑坡防灾减灾和预测预报提供新的方法和可靠的依据。     

降雨对路堤斜坡稳定性影响的时间效应

以某高速公路填筑试验路堤为例,基于非饱和土渗透及强度理论,采用渗流场与应力场、稳定性计算耦合的方法,对两种不同透水特性的路堤在降雨过程中孔隙水压力和稳定性系数随时间变化的特点及规律进行较为深入的探讨。结果表明,（1）透水性能较好的路堤斜坡下部降雨入渗要强于斜坡上部,透水性能较差的路堤斜坡,雨水往往以近似于均匀的形式入渗;（2）透水能力强的路堤斜坡在短暂的超强降雨或者强度较低但持续时间超长的降雨过程中,其最小稳定性系数易于出现滞后现象,一般滞后0.5～3 d,透水能力差的路堤斜坡最小稳定性系数有显著滞后特点,一般滞后7～15 d;（3）总降雨量一定、雨强大于等于50 mm/d条件下透水能力强斜坡能达到的最小稳定性系数随降雨时间增长而降低,而透水能力差的斜坡最小稳定性系数受雨强和降雨时间共同影响。     

降雨条件下浅层滑坡稳定性探讨

降雨条件下浅层滑坡是一种常见、多发的地质灾害现象,为了解边坡稳定性随降雨入渗过程的变化情况,以Green-Ampt入渗模型为基础,并考虑了动水压力的作用,建立了降雨入渗条件下浅层滑坡的概念模型,分别推导了降雨前有、无地下水位条件下的边坡安全系数与降雨时间的关系表达式。从分析结果中可以看出,对于这两种情况下边坡稳定性发生突变的主要原因归结于：前者为在湿润锋与地下水位面接触的短时间内,滑带处的孔隙水压力迅速增高;后者为滑带在浸水饱和情况下,岩土体的强度迅速降低。在此基础上,根据降雨过程中边坡是否达到饱和,提出边坡饱和临界时间的概念,考虑了初始降雨强度小于土壤入渗能力的情况。这个时间可以作为一个参数指标用于浅层滑坡的预警。     

降雨过程中边坡临界滑动场

降雨入渗过程中孔隙水压力的升高与基质吸力的降低引起边坡稳定性的下降,是导致边坡滑塌的主要诱导因素。利用饱和-非饱和渗流有限元计算得到的孔隙水压力场,基于Fredlund提出的非饱和土抗剪强度理论,对边坡临界滑动场进行改进,提出可以考虑降雨过程的边坡临界滑动场数值模拟方法,能够方便、快速地计算出边坡局部、整体安全系数和相对应的临界滑动面在降雨过程中的变化历程。将该法用于一个典型均质边坡和一个非均质边坡在降雨过程中的稳定性计算,分析降雨持续时间、降雨强度和非饱和强度参数取值等因素对边坡稳定性的影响,并将计算结果与其他方法进行比较,结果表明临界滑动场方法能搜索任意形状最危险滑面,计算的安全系数合理。     

Failure Mechanism of a Shallow Landslide at Tun-Sardon Road Cut Section of Penang Island,Malaysia

The occurrences of shallow landslides in residual soils of Penang hilly areas are common in rainy days. The failure mechanisms of a shallow landslide occurred at km 3.9 road in Tun-Sardon area of Penang Island have been simulated using two different methods of slope stability analysis. The results indicate that the failure was initiated locally inside the slope and then propagated further to induce total failure. The failure propagation was started from initial local failure zone and was driven by mobilized shear strength along the shear plane. The slope was marginally stable with an overall factor of safety of 1.32 before it failed to a rainfall event on September 6, 2008. It is found from back calculation that the rain infiltration raised the temporary water level and reduced the shearing strength of soil to a minimum level with increased pore water pressure to trigger the failure. This paper suggests further research on shallow landslide of Penang Island considering the direct rainfall infiltration effect in terms of groundwater pressure-head distribution inside the slope.