Problems in predicting the mobility of slow-moving landslides

This paper discusses problems in predicting the mobility of slow-moving landslides. Three case studies are presented here where research has been carried out by the Utrecht University: The La Valette landslide complex in the French Alps, the La Mure landslide in the French pre-Alps near Grenoble and the Hau landslide in Switzerland.To predict field velocities of these slow-moving landslides the viscosity parameters of the material of these landslides were determined by strain-controlled tests in a ring shear apparatus based on Bishop's design at Utrecht University. The viscosity parameters from the laboratory proved to be 10 to 1000 times lower than viscosities obtained from back analyses on the observed velocities in the field. This discrepancy may be explained by the development of negative pore pressures when the plastic material slides over a rigid, wavy slip surface and/or by convergent flow effects. The associated gain in strength results in a higher apparent viscosity.A more detailed analysis is made for the movements of the La Valette landslide. Observed velocities at the La Valette landslide are difficult to describe by one parameter set as the response to a change in groundwater level is not the same during a rise or fall in the piezometric level. This deviation may be explained by rapid changes in total stresses and consequently changes in pore pressure under (partly) undrained conditions. The emerging hysteresis with local and temporal variations in pore pressure makes it difficult to predict in detail the moving pattern of landslides.     

低速缓动滑坡滑带土剪切蠕变特性

滑带土的蠕变特性对低速缓动滑坡的形成演化过程起着控制性作用。本文选取典型的低速缓动滑坡—二庄科北区滑坡的滑带土为研究对象,在不同正应力水平作用下,通过对该滑带土的饱和试样进行直剪蠕变试验,研究了它的剪切蠕变特性。结果表明:该滑带土具有明显的蠕变特性,均包括瞬时变形、衰减蠕变和稳态蠕变3个阶段;随着剪切荷载的增大,衰减蠕变阶段及瞬时变形阶段的变形量均增大,衰减蠕变持续时间更长;其等时应力—应变曲线呈现出非线性特征,且曲线具有明显拐点;在长期荷载作用下出现强度损失,且正应力越大,强度损失越大;随着时间增大,衰减蠕变阶段的剪应变率越来越小,且剪应力越大,剪应变率越大;在蠕变破坏前的阶段,剪应变率随正应力增大而减小,即滑坡埋深越深,滑速越慢。     

A finite element approach for the analysis of active slow-moving landslides

In the present paper, a finite element approach is proposed to analyse the mobility of active landslides which are controlled by groundwater fluctuations within the slope. These landslides are usually characterised by low displacement rates with deformations that are essentially concentrated within a narrow shear zone above which the unstable soil mass moves with deformations of no great concern. The proposed approach utilises an elasto-viscoplastic constitutive model in conjunction with a Mohr-Coulomb yield function to describe the behaviour of the soil in the shear zone. For the other soils involved by the landslide, an elastic model is used for the sake of simplicity. A significant advantage of the present method lies in the fact that few constitutive parameters are required as input data, the most of which can be readily obtained by conventional geotechnical tests. The rest of the required parameters should be calibrated on the basis of the available monitoring data concerning the change in the piezometric levels and the associated movements of the unstable soil mass. After being calibrated and validated, the proposed approach can be used to predict future landslide movements owing to expected groundwater fluctuations or to assess the effectiveness of drainage systems which are designed to control the landslide mobility. The method is applied to back-predict the observed field behaviour of three active slow-moving landslides documented in the literature.     

Investigating slow-moving landslides in the Zhouqu region of China using InSAR time series

In the Zhouqu region (Gansu, China), landslide distribution and activity exploits geological weaknesses in the fault-controlled belt of low-grade metamorphic rocks of the Bailong valley and severely impacts lives and livelihoods in this region. Landslides reactivated by the Wenchuan 2008 earthquake and debris flows triggered by rainfall, such as the 2010 Zhouqu debris flow, have caused more than 1700 casualties and estimated economic losses of some US$0.4 billion. Earthflows presently cover some 79% of the total landslide area and have exerted a strong influence on landscape dynamics and evolution in this region. In this study, we use multi-temporal Advanced Land Observing Satellite and Phased Array type L-band Synthetic Aperture Radar (ALOS PALSAR) data and time series interferometric synthetic aperture radar to investigate slow-moving landslides in a mountainous region with steep topography for the period December 2007–August 2010 using the Small Baseline Subsets (SBAS) technique. This enabled the identification of 11 active earthflows, 19 active landslides with deformation rates exceeding 100 mm/year and 20 new instabilities added into the pre-existing landslide inventory map. The activity of these earthflows and landslides exhibits seasonal variations and accelerated deformation following the Wenchuan earthquake. Time series analysis of the Suoertou earthflow reveals that seasonal velocity changes are characterized by comparatively rapid acceleration and gradual deceleration with distinct kinematic zones with different mean velocities, although velocity changes appear to occur synchronously along the landslide body over seasonal timescales. The observations suggest that the post-seismic effects (acceleration period) on landslide deformation last some 6–7 months.     

Quantitative hazard and risk assessment for slow-moving landslides from Persistent Scatterer Interferometry

Preparation of reliable landslide hazard and risk maps is crucial for hazard mitigation and risk management. In recent years, various approaches have been developed for quantitative assessment of landslide hazard and risk. However, possibly due to the lack of new data, very few of these hazard and risk maps were updated after their first generation. In this study, aiming at an ongoing assessment, a novel approach for updating landslide hazard and risk maps based on Persistent Scatterer Interferometry (PSI) is introduced. The study was performed in the Arno River basin (central Italy) where most mass movements are slow-moving landslides which are properly within the detection precision of PSI point targets. In the Arno River basin, the preliminary hazard and risk assessment was performed by Catani et al. (Landslides 2:329–342, 2005) using datasets prior to 2002. In this study, the previous hazard and risk maps were updated using PSI point targets processed from 4 years (2003–2006) of RADARSAT images. Landslide hazard and risk maps for five temporal predictions of 2, 5, 10, 20 and 30 years were updated with the exposure of losses estimated in Euro (€). In particular, the result shows that in 30 years a potential loss of approximate €3.22 billion is expected due to these slow-moving landslides detected by PSI point targets.     

Combined use of statistical and DInSAR data analyses to define the state of activity of slow-moving landslides

Statistical analyses have been often used for landslide susceptibility zoning at small to medium scale when relevant base and thematic maps are available. Since the beginning of the last decade, images remotely acquired by spaceborne Synthetic Aperture Radar (SAR) and processed via Differential SAR Interferometry (DInSAR) proved extremely useful for non-invasive and non-destructive monitoring of displacements of the topographic surface. The present paper proposes an original procedure for the definition of the state of activity of slow-moving landslides via the combined use of multivariate statistical analyses and DInSAR data. The procedure is based on the following essential elements: distinction between terrain units used for computational purposes and the final zoning units; independent statistical and DInSAR analyses and activity models leading to first-level state of activity zoning maps; a consistency model between statistical and DInSAR analyses; two confidence and combination models leading, respectively, to second- or third-level state of activity zoning maps. The application in a test area including 19 municipalities in southern Italy, where slow-moving landslides are widespread and accurately mapped by using geomorphological criteria, allowed the generation of the three above-mentioned levels of zoning maps. The results were successfully crosschecked by exploiting a different DInSAR dataset and the results of previous works based on the use of slow-moving landslide-induced damage to facilities surveys.     

Site scale modeling of slow-moving landslides,a 3D viscoplastic finite element modeling approach

This paper presents an advanced 3D numerical methodology to reproduce the kinematics of slow active landslides, more precisely, to reproduce the nearly constant strain rate (secondary creep) and the acceleration/deceleration of the moving mass due to hydrological changes. For this purpose, finite element analyses are performed in a large area covering a long time-span (12 years), in order to exhibit different interacting slope movements. First, we perform a stability analysis using the shear strength reduction (SSR) technique with a Mohr-Coulomb failure criteria. It is done in order to compute factors of safety (FS) and to identify two different scenarios, the first one being stable (FS > 1) and the second one being unstable (FS < 1). In the studied test case, the Portalet landslide (Central Spanish Pyrenees), the first scenario corresponds to an initial stable configuration of the slope and the second one to an unstable excavated configuration. Second, taking the first scenario as an initial condition, a time-dependent analysis is performed using a coupled formulation to model solid skeleton and pore fluids interaction, and a simplified ground water model that takes into account daily rainfall intensity. In this case, a viscoplastic constitutive model based on Perzyna’s theory is applied to reproduce soil viscous behavior and the delayed creep deformation due to the excavation. The fluidity parameter is calibrated to reproduce displacements measured by the monitoring systems. Our results demonstrate that 3D analyses are preferable to 2D ones for reproducing in a more realistic way the slide behavior. After calibration, the proposed model is able to simulate successfully short- and medium-term predictions during stages of primary and secondary creep.     

Empirical fragility and vulnerability curves for buildings exposed to slow-moving landslides at medium and large scales

Slow-moving landslides yearly induce huge economic losses worldwide in terms of damage to facilities and interruption of human activities. Within the landslide risk management framework, the consequence analysis is a key step entailing procedures mainly based on identifying and quantifying the exposed elements, defining an intensity criterion and assessing the expected losses. This paper presents a two-scale (medium and large) procedure for vulnerability assessment of buildings located in areas affected by slow-moving landslides. Their intensity derives from Differential Interferometric Synthetic Aperture Radar (DInSAR) satellite data analysis, which in the last decade proved to be capable of providing cost-effective long-term displacement archives. The analyses carried out on two study areas of southern Italy (one per each of the addressed scales) lead to the generation, as an absolute novelty, of both empirical fragility and vulnerability curves for buildings in slow-moving landslide-affected areas. These curves, once further validated, can be valuably used as tools for consequence forecasting purposes and, more in general, for planning the most suitable slow-moving landslide risk mitigation strategies.     

Effect of gravel content on creep behavior of clayey soil at residual state: implication for its role in slow-moving landslides

Soil creep is mostly manifested in slow-moving landslides. It is often the case for active slow-moving landslide with slip zone comprising clayey soil where creep would develop in the residual condition. If gravel is presented in clayey soil, this will have considerable impact on creep behavior of clayey soil. However, knowledge about creep behavior of the clayey soil containing gravel particles is scarce. This paper discusses creep behavior of natural clayey soil with gravel at residual state through a series of creep shear tests. Soil samples for this testing program were collected from the slip zones of two large slow-moving landslides in China. The collected soil samples consisted of clayey soil containing various amounts of gravel particles. The test results show that the soil specimens underwent two different creep patterns. An attenuating creep pattern was observed when the soil specimens were subjected to creep stress less than the residual strength, and a creep with increasing strain rate, or a nonattenuating creep pattern, with no evident secondary creep was noted when creep stress was intestinally increased to a level slightly greater than the residual strength. The creep patterns of clayey soil with gravel at the residual state observed in this study were noted to be very consistent with those of the gravel-free clayey soil reported in Bhat et al. (Int J Geomater 1(1):39–43, 2011, Nat Hazards 69(3):2161–2178, 2013) and Di Miao et al. (Eng Geol 162:53–66, 2013). This gives an indication that creep patterns of clayey soils with and without gravel at the residual state are essentially the same, or in other words, that the presence of gravel does not change the creep pattern of clayey soil. However, the test results in this study illustrate that the presence of gravel does have a notable effect on creep behavior of clayey soil. Specifically, the creep stress leading to creep failure of clayey soil and the minimum ratio of the creep stress to residual strength (RCSR) increased with gravel content, and the displacement until the tertiary creep was also larger in samples containing more gravel particles. It is postulated that creep behavior of clayey soil at the residual state in this study and its relationship with gravel content may be related to strength recovery and crushing of gravel-sized particles during creep.     

黄土地基排土场滑坡演化机制研究

由于场地的惟一可选性,尖山铁矿南排土场布置在软弱黄土斜坡地基上。从2008年开始,先后发生过不同程度的失稳破坏。基于工程类比方法,采用现场勘查、监测和跟踪调查等手段,研究黄土软弱斜坡地基排土场的滑坡孕育演化机制,就其滑坡类型及模式（低速缓动、旋转、牵引式）、成因机制（超载和侧向挤压主导、降雨触发）和孕育演化过程（持力层蠕滑－地基底鼓－黄土梁拉裂－旋转滑动－裂缝扩展－上部坐落）进行分析。从排土场边坡与基底的相互作用特征出发,采用理论分析确定最大堆载高度,基于工程类比和统计分析确定破坏范围,评估失稳对周边环境的影响后果。滑坡孕育演化机制指出,合理使用排土空间、调整排土场生成过程的时空关系、控制排土速度是调控系统各子结构过程稳定性的关键。并提出允许适度破坏的设计理念。     

Potential effects of incoming climate changes on the behaviour of slow active landslides in clay

Today, a stimulating debate involves the scientific community about the impact of presumable future climate changes on the human life. One of the main question marks concerns their effects on hydrological hazards. Unfortunately, often such a debate is not based on reliable data. The paper proposes a methodology based on the coupling of climatic scenarios and geotechnical analyses accounting for the potential changes in climate parameters. Some analyses have been carried out to forecast the future behaviour of a slow landslide in clay. According to the adopted model, local climate effects should cause a slow decrease in the displacement rate.     

Effects of soil-engineering properties on the failure mode of shallow landslides

Some landslides mobilize into flows, while others slide and deposit material immediately down slope. An index based on initial dry density and fine-grained content of soil predicted failure mode of 96 landslide initiation sites in Oregon and Colorado with 79% accuracy. These material properties can be used to identify potential sources for debris flows and for slides. Field data suggest that loose soils can evolve from dense soils that dilate upon shearing. The method presented herein to predict failure mode is most applicable for shallow (depth <5?m), well-graded soils (coefficient of uniformity >8), with few to moderate fines (fine-grained content <18%), and with liquid limits <40.     

Effects of soil heterogeneity on susceptibility of shallow landslides

Uncertainties in parameters of landslide susceptibility models often hinder them from providing accurate spatial and temporal predictions of landslide occurrences. Substantial contribution to the uncertainties in landslide assessment originates from spatially variable geotechnical and hydrological parameters. These input parameters may often vary significantly through space, even within the same geological deposit, and there is a need to quantify the effects of the uncertainties in these parameters. This study addresses this issue with a new three-dimensional probabilistic landslide susceptibility model. The spatial variability of the model parameters is modeled with the random field approach and coupled with the Monte Carlo method to propagate uncertainties from the model parameters to landslide predictions (i.e., factor of safety). The resulting uncertainties in landslide predictions allow the effects of spatial variability in the input parameters to be quantified. The performance of the proposed model in capturing the effect of spatial variability and predicting landslide occurrence has been compared with a conventional physical-based landslide susceptibility model that does not account for three-dimensional effects on slope stability. The results indicate that the proposed model has better performance in landslide prediction with higher accuracy and precision than the conventional model. The novelty of this study is illustrating the effects of the soil heterogeneity on the susceptibility of shallow landslides, which was made possible by the development of a three-dimensional slope stability model that was coupled with random field model and the Monte Carlo method.

     

Effects of the attitude of dominant joints on the mobility of translational landslides

Landslides - Geological structures and ground undulation affect the mobility of translational landslides. Combining field investigation, back-analysis, and numerical simulation, we explore the role...     

Activity and kinematic behaviour of deep-seated landslides from PS-InSAR displacement rate measurements

Large landslides and deep-seated gravitational slope deformations (DSGSD) represent an important geo-hazard in relation to the deformation of large structures and infrastructures and to the associated secondary landslides. DSGSD movements, although slow (from a few millimetres to several centimetres per year), can continue for very long periods, producing large cumulative displacements and undergoing partial or complete reactivation. Therefore, it is important to map the activity of such phenomena at a regional scale. Ground surface displacements at DSGSD typically range close to the detection limit of monitoring equipment but are suitable for synthetic aperture radar (SAR) interferometry. In this paper, permanent scatterers (PSInSAR?) and SqueeSAR? techniques are used to analyse the activity of 133 DSGSD, in the Central Italian Alps. Statistical indicators for assigning a degree of activity to slope movements from displacement rates are discussed together with methods for analysing the movement and activity distribution within each landslide. In order to assess if a landslide is active or not, with a certain degree of reliability, three indicators are considered as optimal: the mean displacement rate, the activity index (ratio of active PS, displacement rate larger than standard deviation, overall PS) and the nearest neighbor ratio, which allows to describe the degree of clustering of the PS data. According to these criteria, 66% of the phenomena are classified as active in the monitored period 1992–2009. Finally, a new methodology for the use of SAR interferometry data to attain a classification of landslide kinematic behaviour is presented. This methodology is based on the interpretation of longitudinal ground surface displacement rate profiles in the light of numerical simulations of simplified failure geometries. The most common kinematic behaviour is rotational, amounting to 41 DSGSDs, corresponding to the 62.1% of the active phenomena.     

Effects of degree of saturation on shallow landslides triggered by rainfall

The empirical rainfall threshold concept and the physical-based model are two commonly used approaches for the assessment of shallow landslides triggered by rainfall. To investigate in detail the rainfall-triggered shallow landslides, many physical-based models coupling the infinite slope stability analysis with the rainfall infiltration modeling in variably saturated soil were developed. However, in those physical-based shallow landslide models, the unit weight and the unsaturated shear strength were assumed constant rather than depending on the degree of saturation. In this study, the effects of the unit weight and the unsaturated shear strength as function of degree of saturation on rainfall-triggered shallow landslides are examined. Several designed scenarios and a real case scenario are used to conduct the examinations. The results show that not only the occurrence of shallow landslides but also the failure depth and the time to failure could be misassessed if the influences of degree of saturation on the unit weight and the unsaturated shear strength are neglected.     

Shallow landslides as drivers for slope ecosystem evolution and biophysical diversity

Shallow landslides may be seen as local disturbances that foster the evolution of slope landscapes as part of their self-regulating capacity. Gaining insight into how slope ecosystems function and evolve could make eco-engineering interventions on slopes more successful. The objective of the present study is to detect traits of shallow landslide-triggered ecosystem evolution, self-regulation and biophysical diversity in a small-scale landslide-prone slope in Northeast Scotland. A protocol was defined to explore the emergence of landslide-driven slope habitats. This protocol studied plant diversity, species richness and plant biomass differences and their interactions with certain soil and topographic attributes at three slope strata during two consecutive growing seasons following an assemblage of shallow landslide events. Plant species and soil properties with potential as indicators of the different landslide-driven slope habitats and landscape evolution were also considered. Shallow landslides contributed to biophysical diversity and created distinct slope habitats within the landscape. Habitat differences in terms of species richness and composition were a direct consequence of the slope self-regulation. Certain plant species were found to be valid indicators of landslide-driven biophysical diversity. Soil total nitrogen and resistance to penetration were related to slope habitat and landscape evolution. As expected, plant establishment relied upon light and nitrogen trade-offs, which in turn were influenced by landscape topography. The insights derived from this study will be useful in slope restoration, particularly in harmonising effective actions with the functioning of landslide-prone ecosystems. Further research directions to clarify the observed variability and interactions are highlighted.     

基于物理模型试验的多层滑带滑坡变形演化特征

滑坡变形演化特征一直是滑坡灾害预测与防治领域急需解决的关键问题,但对于多层滑带滑坡的变形演化特征却少有研究.以物理模型试验为手段建立了三层滑带滑坡物理试验模型,完成了多层滑带滑坡变形演化全过程的模拟.基于PPIV技术获取坡表位移数据,通过柔性测斜仪监测滑坡深部位移,同时布设土压力盒获取滑坡内部土压力的变化情况,实现了多...     

Automatic identification of continuous or non-continuous evolution of landslides and quantification of deformations

Landslides - Nowadays, the 3D modelling of surfaces is widely used to provide point clouds in geosciences. To study the evolution of landslides, many point clouds are available but post-processing...     

Expected damage from displacement of slow-moving slides

Facilities such as buildings, highways, railways, bridges, dams and pipelines often are built on natural slopes where the risk of landslides is not low. The vulnerability of these facilities to slow-moving slides has sometimes been underestimated, although the velocity of some classes of slow slides is uncontrollable. More than 50 cases of slow slides were compiled from the literature for this study. Some statistics about the movement trigger(s), the methods used to measure displacement, the material forming the rupture surface and the type of the vulnerable facilities are presented. It is shown that the expected degree of damage to urban settlements, highways, bridges and dams can be related to the slide velocity or accumulating displacement. Buildings and residential houses may tolerate higher slide velocities and total displacements than other facilities before experiencing serious damage. Movements as low as 100 mm may severely damage bridges, but such low rates may cause only moderate damage to urban communities. The relationship between movement and the expected extent of damage should be useful to geotechnical engineers who deal with different classes of slow slides and will help in the choice of appropriate mitigation measures based on preliminary estimates of movement rates.