2009年65重庆武隆鸡尾山崩滑灾害基本特征与成因机理初步研究

2009年6月5日下午15时许,重庆市武隆县铁矿乡鸡尾山山体发生了大规模的崩滑,约500万m3被结构面切割成积木块状的灰岩山体,沿缓倾页岩软弱夹层发生整体滑动。高速运动的滑体物质在堵塞前部宽约200m,深约50m的铁匠沟沟谷后,形成平均厚约30m,纵向长度约2200m的堆积区,掩埋了12户民房和正在开采铁矿的矿井入口,造成10人死亡,64人失踪,8人受伤,成为近年来少有的一次崩滑灾难性事件。本文在对灾害现场进行大量地质调查的基础上,结合遥感、三维激光扫描等综合手段,对鸡尾山崩滑体特征进行了详细描述,对灾害发生原因进行了初步分析。结果表明,鸡尾山山体垮塌是在不利的地质结构条件下,并受到长期重力、岩溶等作用和采矿活动的影响,因前部起阻挡作用的关键块体被剪断突破而导致的一起大型山体崩滑事件。深入研究鸡尾山崩滑体的形成条件和成灾机理,对我国西南地区存在与鸡尾山崩滑体类似地质条件的灾害隐患点的减灾防灾工作,具有重要的指导意义。     

岩溶及水动力对鸡尾山滑坡影响作用研究

鸡尾山特大型滑坡是我国西南岩溶石山地区一种独特的大型岩质斜坡失稳模式,它得到工程地质等领域极大重视,从滑坡成因和机理、滑坡启动以及运动过程、堆积物分布、次生灾害等方面都开展了一系列研究,取得了众多成果;对于滑坡结构面和滑动面方面认为:垂直结构面由两组构成,水平滑动面则为软弱夹层.本文通过野外地质调查,样品测试、综合分析...     

Mechanism on apparent dip sliding of oblique inclined bedding rockslide at Jiweishan, Chongqing, China

Because of the existence of a front stable rockmass barrier, the failure pattern of an oblique inclined bedding slope is conventionally recognized as a lateral rockfall/topple, and then a transformation into a rockfall accumulation secondary landslide. However, the Jiweishan rockslide, Wulong, Chongqing, which occurred on June 5, 2009, illustrates a new failure pattern of massive rock slope that rockmass rapidly slides along apparent dip, and then transforms into a long runout rock avalanche (fragment flow). This paper analyzes the mechanism of the new failure pattern which is most likely triggered by gravity, karstification, and the processes associated with mining activities. A simulation of the failure processes is shown, using the modeling software, FLAC^3D. The results show that there are five principal conditions for an apparent dip slide associated with an oblique inclined bedding slope are necessary: (1) a block-fracture bedding structure. The rockmass is split into obvious smaller, distinct blocks with several groups of joints, (2) an inclined rockmass barrier. The sliding rockmass (i.e., the rockslide structure before movement) exists along a dip angle and is barricaded by an inclined stable bedrock area, and the subsequent sliding direction is deflected from a true dip angle to an apparent dip angle; (3) apparent dip exiting. The valley and cliff provide a free space for the apparent dip exiting. (4) Driving block sliding, which means the block has a push type of effect on the motion of the rockslide. The “toy bricks” rockmass is characterized by a long-term creeping that induces the shear strength reduction from peak to residual value along the bottom soft layer, and the sliding force is therefore increased. (5) The key block resistance and brittle failure. The pressure on the key block is increased by the driving rockmass and its strength decreases due to karstification, rainfall, and mining. The brittle failure of the karst zone between the key block and the lateral stable bedrock occurs instantaneously and is largely responsible for generating the catastrophic rockslide–rock avalanche. If there was not a pre-existing key block, the failure pattern of such the inclined bedding rockmass could be piecemeal disintegration or small-scale successive rockfall or topple. The recognition of catastrophic failure potential in such inclined bedding slopes requires careful search for not only structures dipping in the direction of movement, but also key block toe-constrained condition.     

重庆武隆鸡尾山滑坡视向滑动机制分析

重庆武隆鸡尾山山体为典型的斜倾厚层灰岩山体,其破坏模式不同于常见侧向崩塌-堆积层滑坡,属于真倾向滑移变形转为视向整体滑动的特殊失稳模式。在现场地质调查的基础上,从地层岩性、岩体结构、岩溶及地下水作用、软弱夹层等影响因素分析重庆武隆鸡尾山滑坡形成原因;根据滑坡破坏机制,基于关键块体控制理论,对鸡尾山滑坡进行三维稳定性极限平衡分析;利用三维离散元软件模拟鸡尾山滑坡的初始变形破坏过程,分析了鸡尾山滑坡不同影响因素条件下的视向滑动形成机制和变形破坏特征,并探讨了节理化和溶蚀岩体的参数取值。结果分析认为,在重力的长期作用下,鸡尾山山体初始沿真倾向方向滑移,沿岩溶发育的陡倾节理裂隙逐渐产生后缘及侧向裂缝,形成块状后部驱动块体,由于地下水等因素使软弱夹层软化,驱动块体下滑力增大,前缘阻滑关键块体内部应力积累,并最终沿强度较低的岩溶发育带发生剪切破坏,从而导致整体滑动;在进行滑坡稳定性极限平衡分析时,考虑实际的滑坡机制,将滑体分为驱动块体和关键块体分别进行力的解析,并以关键块体的安全系数代表滑坡的安全系数更加合理。数值模拟显示,软弱夹层强度降低、岩溶发育带剪切破坏后,滑体进入大变形阶段,表明关键块体控制和阻滑作用明显,软弱夹层强度降低是滑坡发生的关键因素。采矿形成的采空区对山体的影响主要是使上覆岩体压应力增大,但对滑体的变形无影响。     

Mechanisms of shear failure in artificial fractures of sandstone and their implication for models of hydromechanical coupling

Summary Direct shear tests, in which the behaviour of the rock surfaces during shear could be continuously observed, were used to study the shear failure of a profiled and clean discontinuity artificially prepared from natural sandstone. Displacement transducers were used to measure the normal and shear displacements. A series of strain gauges glued on the sides of the upper block provided information on the change of the stress field occurring close to the discontinuities whilst shear displacement increased, and these changes were then compared with the behaviour of the profiled surface. The results of the laboratory tests, the sequence of photographs taken for most of them, and the results conducted with a sample of similar shape made from the same rock material and tested in a rotary shear machine, allowed several stages and mechanisms of failure to be defined: static friction and mobilization of initial shear stiffness; mobilization of sliding; mobilization of brittle fracture; post-peak failure of the teeth; descent of the teeth; gliding and ploughing; commencement of second cycle of shearing. In many respects these stages are similar to those occurring between sliding surfaces of metal and suggest that the analyses developed in tribology may be relevant to the development of constitutive models for predicting the hydromechanical coupled behaviour of a discontinuity with shear displacement. Such models will have to consider these different stages of shear, because the original discontinuity changes its geometry with displacement and is filled with gouge which changes its grain size with displacement.     

A rigorous solution for the stability of polyhedral rock blocks

Block theory has been widely applied to stability analysis of rock engineering due to its clear concept and elegant geometrical theory. For a general block with multiple discontinuity planes, it is assumed that contact is only maintained on a single plane (single-plane sliding) or two intersecting planes (double-plane sliding) in block theory analysis. Since the normal forces and shear resistances acting on the other discontinuity planes are omitted, it can cause unreasonable estimations of block failure modes and incorrect calculation of factors of safety. In this study, a new method is presented that permits to consider the contribution of the normal forces and shear resistances acting on each discontinuity plane to the block stability. The proposed method meets all of the force-equilibrium and moment-equilibrium conditions and provides a rigorous solution for stability of general blocks with any number of faces and any shape. Some typical polyhedral blocks in rock slopes are analyzed using block theory and the proposed method. The results indicate that the traditional block theory may give a misleading conclusion for the predictions of stability and sliding direction of rock blocks when contact occurs on more than two discontinuity planes.     

基于单个钻孔孔壁电视图像确定地下岩体结构面产状的普适数学方法

岩体结构面产状是影响岩体力学性质及工程稳定性的重要地质因素.从钻孔孔壁电视平展图上可较精确地提取结构面迹线上不共线三点的相对二维坐标(d, h),再利用空间解析几何理论确定其三维空间相对坐标(x, y, z),并求得结构面法向矢量.根据钻孔孔壁与坡面及结构面产状的空间几何关系,在保持原点不变的情况下,采用空间坐标变换的方法,将三维坐标系z轴和y轴分别转化为竖直方向z′和正北地理方向y′,并确定转化后坐标系空间的结构面法向矢量.再从结构面产状的定义出发,根据结构面法向矢量与结构面倾向、倾角的相关性,建立了一个空间几何模型,通过该模型可求得任意平直坡面上任一钻孔内局部岩体结构面的产状.      

端部岩桥直剪破坏试验及断裂条件

以重庆鸡尾山岩质滑坡为代表的前缘“关键块体”型滑坡具有较强的隐蔽性,研究“关键块体”前缘锁固段岩桥的破坏性质对滑坡预警具有重要意义。以鸡尾山滑坡地质条件为基础,在岩样端部预制不同长度边缘裂缝,填入软弱材料,形成软弱夹层节理,在不同法向压力下进行直剪试验,分析了前缘端部岩桥的裂纹扩展规律、块体剥落特征信息和岩桥断裂条件,提出了临滑阶段剪应力变化率k值。试验结果表明：软弱夹层节理长度对岩桥破坏模式和块体剥落剧烈程度有重要影响,且端部岩桥越长,破坏前临滑阶段剪应力增速k值越小。端部岩桥破坏模式为剪切破坏、拉剪破坏和张拉破坏,且不同破坏模式决定了相同节理岩桥块体剥落的剧烈程度。岩桥块体剥落点与破坏点剪应力比值百分数平均值为79.5%～92.2%,且端部岩桥临滑阶段时间快慢依次为短节理慢、中间节理居中和长节理快。端部岩桥3种破坏模式满足一定断裂条件,且呈现出3个阶段和裂纹稳定扩展。通过本直剪试验研究揭示的端部岩桥破坏特征信息和断裂条件,可对前缘“关键块体”锁固段型岩质边坡失稳破坏评价提供理论依据。     

基于岩体结构特征的高速远程滑坡致灾范围评价

高速远程滑坡的演化机制与运动过程受控于岩体结构.以重庆市武隆县鸡尾山滑坡为例,采用地面激光扫描技术,获取岩体结构面几何信息,基于离散元数值模拟方法,考虑岩体结构特征,分析高速远程滑坡演化过程及其致灾范围.研究结果表明:(1) 岩体结构特征由岩体内部发育的结构面所决定,针对点云数据开展空间几何计算与聚类分析,可以快速精细地获取岩体结构面产状信息,从而进行岩体结构面识别与组别划分;(2) 将岩体结构特征评价结果导入离散元模型中,能够实现高速远程滑坡致灾范围的快速评价目的,并且取得了与实际滑动距离较为吻合的结果.      

Rain-induced slope instability in Hong Kong

Catastrophic geological disasters frequently occur on slopes with obliquely inclined bedding structures (also referred to as obliquely inclined bedding slopes), where the apparent dip sliding is not readily visible. This phenomenon has become a focal point in landslide research. Yet, there is a lack of studies on the failure modes and mechanisms of hidden, steep obliquely inclined bedding slopes. This study investigated the Shanyang landslide in Shaanxi Province, China. Using field investigations, laboratory tests of geotechnical parameters, and the 3DEC software, this study developed a numerical model of the landslide to analyze the failure process of such slopes. The findings indicate that the Shanyang landslide primarily crept along a weak interlayer under the action of gravity. The landslide, initially following a dip angle with the support of a stable inclined rock mass, shifted direction under the influence of argillization in the weak interlayer, moving towards the apparent dip angle. The slide resistance effect of the karstic dissolution zone was increasingly significant during this process, with lateral friction being the primary resistance force. A reduction in the lateral friction due to karstic dissolution made the apparent dip sliding characteristics of the Shanyang landslide more pronounced. Notably, deformations such as bending and uplift at the slope’s foot suggest that the main slide resistance shifts from lateral friction within the karstic dissolution zone to the slope foot’s resistance force, leading to the eventual buckling failure of the landslide. This study unveils a novel failure mode of apparent dip creep-buckling in the Shanyang landslide, highlighting the critical role of lateral friction from the karstic dissolution zone in its failure mechanism. These insights offer a valuable reference for mitigating risks and preventing disasters related to obliquely inclined bedding landslides.     

缓倾滑坡地质力学模型研究——以冯店滑坡为例

在国内外缓倾滑坡研究的基础上,从滑坡形成机制出发,建立缓倾滑坡地质力学模型,采用数字滑坡技术结合地面调查获取滑坡规模、滑动面倾角、后缘拉裂槽积水深度等参数,代入模型表达式可获得滑坡的临界摩擦系数,进而获取滑坡的总下滑力和总阻滑力。提出临界摩擦系数的概念及求取方法,临界摩擦系数与缓倾滑坡的易滑性（或稳定性）直接相关,分析表明滑坡的临界摩擦系数与滑体规模（滑动面长与宽）、滑动面倾角正相关,与滑体重量反相关,滑坡后壁倾角在60°-90°范围的变化对缓倾滑坡易滑性的影响很微弱。      

Physical Modeling of Landslide Mechanism in Oblique Thick-Bedded Rock Slope: A Case Study

The Jiweishan landslide illustrates the failure pattern of an apparent dip slide of an oblique thick-bedded rockslide. Centrifugal modeling was performed using a model slope consisting of four sets of joints to investigate the landslide initiation mechanism. Crack strain gauges pasted between the slide blocks and the base failed in sequence from the rear to the front as the centrifugal acceleration increased. When the acceleration reached 16.3g, the instantaneous failure of the key block in the front triggered the apparent dip slide of all blocks. The physical modeling results and previous studies suggest that the strength reduction in the weak layer and the failure of the key block are the main reasons for the Jiweishan landslide. The centrifuge experiment validated the previously proposed driving-blocks–key-block model of apparent dip slide in oblique with inclined bedding rock slopes. In addition, the results from limit equilibrium method and centrifuge test suggest that even though the failure of the key block in the front is instantaneous, a progressive stable–unstable transition exists.     

酸雨对重庆武隆鸡尾山滑坡滑带页岩物理力学性质的影响

为了研究酸雨对重庆武隆鸡尾山滑坡滑带页岩物理力学性质的影响,在鸡尾山滑坡滑源区滑面向下游延伸处取样,制成粉末和块体试样,在实验室内使用pH为3的稀盐酸溶液浸泡,测试其物质成分的变化。结果显示:页岩与稀盐酸溶液作用的过程中有较为明显的钙质出溶现象,这一反应在粉末中发生的速率更快。测试从鸡尾山滑坡坡脚流出的泉水的离子成分,结果显示相对于酸雨,泉水中钙离子浓度显著增高,进一步证实滑带在酸雨作用下钙质出溶现象较为显著。钙质页岩中方解石成分的溶出,导致滑带强度减小。     

Regional landslide susceptibility following the Mid NIIGATA prefecture earthquake in 2004 with NEWMARK’S sliding block analysis

This study proposes a calculation method for regional earthquake-induced landslide susceptibility that applies the permanent seismic displacement calculated using Newmark’s sliding block analysis with estimated vertical and horizontal seismic motions. The proposed method takes into account the direction of slope failure based on the specified slope azimuth. The study results reveal the importance of predominant slope failure direction using a simple infinite slope model subjected to earthquakes. The target area for the earthquake-induced landslide susceptibility analysis constituted a region of more than 2000 km² surrounding the epicenter of the Mid Niigata prefecture earthquake in 2004. An earthquake-induced landslide susceptibility map was created based on the proposed method with a specific combination of friction angle and cohesion, and the resulting data were compared to the landslide inventory map produced from aerial photographs following the Mid Niigata prefecture earthquake in 2004. To create the susceptibility map, geomaterial cohesion values for the slope were back-calculated to satisfy the minimum safety factor in the static state. This study also proposes a calculation method for the prediction rate and determines the back-calculated strength parameters of geomaterials. The proposed regional landslide susceptibility map will be useful for understanding potential slope failure locations and magnitude of damage, as well as for planning field investigation and preventing secondary disasters immediately after earthquakes.     

用拐点原理计算滑坡后沿张裂缝深度的新公式

当坡体后沿产生张裂缝时,通常意味着坡体接近极限平衡状态,滑坡可能发生,若遇降雨,张裂缝充水产生的水压往往是滑坡发生的启动因素,因为水压的大小与张裂缝的深度估算有关,是边坡稳定分析的重要条件之一。建立了一个坡体后部潜在三角形极限平衡体拉张模型,导出裂缝深度公式。应用拐点的数学性质原理,解出裂缝处与内摩擦角相应的滑面倾角,计算张裂缝深度系数,绘制了应用于实际的张裂缝深度系数的数值解曲线图。解释了张裂缝发生条件:裂缝处的滑面倾角必须小于裂缝深度系数的一阶导数的极值点确定的与土层内摩擦角对应的倾角α。给出了圆弧滑面和一种非圆弧滑面发生张裂缝位置的公式。与Spencer裂缝公式进行了比较,表明本文方法具有实用价值。     

大型层状基岩滑坡软弱夹层演化特征研究——以重庆武隆鸡尾山滑坡为例

软弱夹层经过长期地质历史演化,性质劣化后形成滑带,对大型层状基岩滑坡的稳定性起重要的控制作用。为了查清软弱夹层形成滑带的演化过程,以重庆武隆鸡尾山滑坡为例,对比研究了山体内软弱夹层的发育规律,将其划分为原生软岩、层间剪切带和滑带3个阶段,并通过岩矿组分含量、物理性质、微结构、物理化学性质和蠕变力学性质试验分析了3个阶段的演化特征。结果表明:从矿物成分演化过程来看,黏土矿物含量增加趋势明显,均值从4.4%增加到16.9%;从微结构演化过程来看,微结构由致密变得疏松,孔隙及节理裂隙增多,密度降低了5%~6%,孔隙率升高了108%;从物理化学性质演化过程来看,交换性盐基总含量在原岩中最高,其次是滑带,层间剪切带最低,有机质含量逐渐增大,整个演化环境呈弱碱性。从蠕变剪切强度演化过程来看,软弱夹层的内摩擦角由57.58°降低到29.63°,黏聚力由585 kPa降低到96 kPa。在此基础上,对鸡尾山滑坡驱动块体最大主剖面的下滑推力进行分析,下滑推力随着长期剪切强度参数的降低而增大,当内摩擦角φ < 25°,黏聚力c < 129 kPa时,下滑推力大于0,驱动块体失稳。该研究对受软弱夹层控制的层状基岩滑坡的发育发展过程、失稳机理研究提供了重要的借鉴意义。     

A numerical method for the study of shear band propagation in soft rocks

This paper investigates the possibility of interpreting progressive shear failure in hard soils and soft rocks as the result of shear propagation of a pre‐existing natural defect. This is done through the application of the principles of fracture mechanics, a slip‐weakening model (SWM) being used to simulate the non‐linear zone at the tips of the discontinuity. A numerical implementation of the SWM in a computation method based on the boundary element technique of the displacement discontinuity method (DDM) is presented. The crack and the non‐linear zone at the advancing tip are represented through a set of elements, where the displacement discontinuity (DD) in the tangential direction is determined on the basis of a friction law. A residual friction angle is assumed on the crack elements. Shear resistance decreases on elements in the non‐linear zone from a peak value at the tip, which is characteristic of intact material, to the residual value. The simulation of a uniaxial compressive test in plane strain conditions is carried out to exemplify the numerical methodology. The results emphasize the role played by the critical DD on the mechanical behaviour of the specimen. A validation of the model is shown through the back analysis of some experimental observations. The results of this back analysis show that a non‐linear fracture mechanics approach seems very promising to simulate experimental results, in particular with regards to the shear band evolution pattern. Copyright © 2009 John Wiley & Sons, Ltd.     

Mechanism of a long-runout landslide triggered by the August 1998 heavy rainfall in Fukushima Prefecture,Japan

Heavy rainfall from 26 to 31 August 1998 triggered many landslides in Nishigo Village of southern Fukushima Prefecture, Japan. The Hiegaesi landslide, a long-runout landslide with travel angle of 11°, which occurred in loamy volcanic-ash/pumice layer and was deposited in a nearby rice paddy, was investigated. In an observation pit dug in the middle part of the landslide deposit, the sliding zone just above the deflected rice plants was observed, and it was confirmed that grain crushing occurred in the sliding zone. The triggering and sliding mechanisms of this landslide then were investigated by ring-shear tests in laboratory. For the triggering mechanism, one saturated naturally drained test (test A: torque-controlled test) and one saturated undrained test (test B: speed-controlled test) were conducted on the samples taken from the source area of the landslide. Even in the naturally drained test opening the upper drain valve of the shear box, a temporary liquefaction occurred. In the undrained test, excess pore-pressure was generated along with shearing, and “sliding-surface liquefaction” phenomenon was observed. The effective stress and shear resistance finally decreased to near zero. These results can explain the observed phenomenon of small friction resistance like a flow of liquid when the sliding mass slid out of the source area. For the sliding mechanism of the landslide in the rice paddy, saturated undrained test (test C: speed-controlled test) was performed on soil sample above the deflected rice plants. The apparent friction angle obtained in this test was 8°. In addition, the residual friction angle measured after test B and test C was the same value of 41°. Combining with the observation on the shear zone in the ring-shear box after test C, it is concluded that, during the sliding in rice paddy, the undrained shear strength of the soil layer itself mainly influenced the high mobility of the landslide, probably because the friction between rice plants and soils is greater than the undrained shear strength inside the soil mass.     

基于物质点法的土体流动大变形过程数值模拟

土体滑坡作为一种自然地质灾害,受自然因素和人类活动的影响在我国时有发生,给周围居民的生命和财产安全带来了很大威胁,日益受到人们的广泛关注。滑坡防治也逐渐成为工程研究的热点之一。土体本质上是一种具有复杂组成结构的颗粒材料堆积体,通过对颗粒流动的模拟可以深入理解自然界中的土体流动现象,如滑坡、泥石流等,进而预测灾害破坏范围及改进相应工程防护措施。但由于土体流动是一个涉及大变形及大位移的复杂流动过程,传统的基于网格的有限元法（FEM）由于网格畸变,并不适合这类问题的研究。本文采用物质点法（MPM）模拟土体流动大变形问题。作为一种源于particle-in-cell（PIC）法的无网格法,兼具欧拉法和拉格朗日法的优点,因而,物质点法在处理大变形问题上具有独特的优势。目前,国内外利用物质点法模拟边坡滑动问题已有不少研究,但对相关参数进行敏感性分析的较少。本文基于物质点法模拟了黏性土体及无黏性土体流动大变形问题,并进行了参数敏感性分析,包括土体材料的内摩擦角、黏聚力、高宽比、底面坡度对土体滑动距离的影响规律。本文计算中采用Drucker-Prager（DP）弹塑性本构模型描述土的非线性特性。研究结果表明:（1）基于物质点法得到的土体的流动形态、滑动距离以及自然休止角等数值模拟结果均与文献中的实验结果基本吻合,验证了物质点法模拟土体大变形力学行为的精度及有效性;（2）随着内摩擦角、黏聚力的增大,滑动距离相应减小;（3）坡度对边坡稳定的影响是显著的,随着底面坡度的增大,滑动距离相应增大;（4）当土柱高宽比较小时,与滑动距离呈线性增长关系。其中,内摩擦角和黏聚力反映了土体的抗剪切性能,因此通过工程措施提高土体的抗剪能力可以降低土体滑坡带来的危害。研究结果为探索土体滑动破坏规律,降低滑动破坏范围提供了可靠的参考。     

Analysis of an anti-dip landslide triggered by the 2008 Wenchuan earthquake in China

The Guantan landslide, with a total displaced mass of about 468 × 10⁴ m³, was triggered by the 2008 Wenchuan earthquake and succeeding rainfall in Jushui Town, Sichuan Province, China. The landslide occurred on an anti-dip hard rock slope with a weak rock founding stratum of 200 m in thickness. To investigate the failure mechanism of the Guantan landslide, dynamic behaviors of hard and soft rock slopes were investigated by means of large scale shaking table tests. The laboratory models attempted to simulate the field geological conditions of the Guantan landslide. Sinusoidal waves and actual seismic waves measured from the Wenchuan Earthquake were applied on the slope models under 37 loading configurations. The experimental results indicated that deformation mainly developed at a shallow depth in the upper part of the hard rock slope and in the upper (near the crest) and lower (near the toe) parts of the soft rock slope. An equation for predicting the depth of sliding plane was proposed based on the location of the maximum horizontal acceleration. Finally, it was concluded that the failure process of the Guantan landslide occurred in three stages: (1) toppling failure caused by compression of the underlying soft rock strata, (2) formation of crushed hard rock and sliding surface in soft rock as the result of seismic shocks, particularly in the horizontal direction, and (3) aftershock rainfall accelerates the process of mass movement along the sliding plane.