Fluidization in dry landslides

This paper aims at determining the physical properties affecting the distance travelled by landslides with dry particles, and elucidating the mechanism of the properties affecting landslide fluidization. The procedure is as follows: laboratory landslide experiments were conducted to verify the simulation model which was proposed to represent the movement of landslides. Further, sensitivity analysis for some physical properties affecting the travel distance were conducted using this model. The simulation model could identify the coordinates, velocity and angular velocity for every particle in three-dimensional space, and the kinetic energy of particles consumed by inelastic and frictional collision with each other in the model. The travel distances of landslides simulated by the model were verified by laboratory experiments statistically where the physical properties for the particles and slope angle were changed. Then, sensitivity analysis for the physical properties were conducted using the model to clarify the effect of the properties on the travel distance. It was proven that there were no significant differences between the travel distances represented by the simulation model and those found experimentally using statistical analysis (i.e. the simulation model could represent virtual real landslides). From the sensitivity analysis using the simulation model, the travel distances were positively correlated to volume, or number of particles, and negative correlated to the slope angle, kinetic friction and rolling friction of particles when the initial potential energies were the same. The mass of particles of equal size did not affect the distance travelled. As the number of particles increased, the simulated travel distances tended to be longer compared with theoretical travel distances calculated using the friction between particles and the slope. Consequently, we could determine the distance of the lumped mass model as being critical between fluidized and non-fluidized landslides.     

The longdendale landslides

Throughout its geomorphological history the South Pennine valley of Longdendale has been especially prone to landsliding and some ten landslide areas have been identified. These prove on analysis to be bedding plane slides with a strong degree of translation but often exhibiting non-circular rotational elements as well. Instability in the lower slopes has often led to cambering and valley bulging so that the slopes become deformed and slump earth flows developed at some sites. Several different types of landslide are examined in detail and by means of pollen analysis the slides are dated from the early postglacial to at least as recently as two thousand years B.P. although it has not yet been possible to decide when these slides started. The causes of sliding are identified, and comments are made upon.

• a the location of slip surfaces–which are frequently at the base of the Grindslow Shales above a sandstone aquifer;
• b the geomorphic processes which built up high pore-water pressures and steep pressure gradients in the past; and
• c the effects of sliding on the development of hillslopes in the post-glacial period.

     

Earthquake-induced landslides in Central America

Central America is a region of high seismic activity and the impact of destructive earthquakes is often aggravated by the triggering of landslides. Data are presented for earthquake-triggered landslides in the region and their characteristics are compared with global relationships between the area of landsliding and earthquake magnitude. We find that the areas affected by landslides are similar to other parts of the world but in certain parts of Central America, the numbers of slides are disproportionate for the size of the earthquakes. We also find that there are important differences between the characteristics of landslides in different parts of the Central American isthmus, soil falls and slides in steep slopes in volcanic soils predominate in Guatemala and El Salvador, whereas extensive translational slides in lateritic soils on large slopes are the principal hazard in Costa Rica and Panama. Methods for assessing landslide hazards, considering both rainfall and earthquakes as triggering mechanisms, developed in Costa Rica appear not to be suitable for direct application in the northern countries of the isthmus, for which modified approaches are required.     

The Longdendale landslides

Throughout its geomorphological history the South Pennine valley of Longdendale has been especially prone to landsliding and some ten landslide areas have been identified. These prove on analysis to be bedding plane slides with a strong degree of translation but often exhibiting non-circular rotational elements as well. Instability in the lower slopes has often led to cambering and valley bulging so that the slopes become deformed and slump earth flows developed at some sites. Several different types of landslide are examined in detail and by means of pollen analysis the slides are dated from the early postglacial to at least as recently as two thousand years B.P. although it has not yet been possible to decide when these slides started. The causes of sliding are identified, and comments are made upon.

• (a) The location of slip surfaces–which are frequently at the base of the Grindslow Shales above a sandstone aquifer.
• (b) The geomorphic processes which built up high pore-water pressures and steep pressure gradients in the past.
• (c) The effects of sliding on the development of hillslopes in the post-glacial period.

     

Very large landslides in the Himalayas

Landslides are a major component of geomorphological processes on the steep slopes of the Himalayan mountains. Three slides in the Annapurna region demonstrate a sequence from large to very large failures.     

Tsunamis from submarine landslides

Most tsunamis are generated by earthquakes, with secondary, less frequent, mechanisms including subaerial and submarine landslides, volcanic eruptions and (extra‐terrestrial) bolide impacts. Different mechanisms generate tsunamis with different magnitudes, travel distances and impacts. Submarine landslides had been mapped and studied for decades but records suggested that only a few had generated tsunamis, and that these were minor. It was not until 1998, when a slump on the seabed offshore of northern Papua New Guinea caused a tsunami wave up to 15 m high that killed over 2200 people, was the significance of submarine landslides in tsunami generation realised. A combination of new (multibeam) seabed mapping technology and the development of improved numerical tsunami models for tsunami generation led to the recognition of the landslide tsunami mechanism of the PNG event. As a result the hazard from submarine landslides in tsunami generation is now recognized and better understood. Extensive mapping of ocean margins reveals that submarine landslides are common. Although many of these probably generated tsunamis, few have been identified, so their hazard remains uncertain. This article describes how the hazard from submarine landslide tsunamis was first recognized, how submarine landslides generate tsunamis, why they were previously discounted as a major hazard, and their potential hazards. An important aspect of the recognition of the tsunami hazard from submarine landslides has been the significance of geology, which has contributed to a subject previously dominated by seismologists.     

Mechanisms of large landslides

SummaryMechanisms of Large Landslides The moving components of many large landslides show a remarkable tendency to remain in more or less undisturbed sequential order. In the present study this lack of laminar or turbulent flow is mathematically analyzed and explained. The high energy concentration thus resulting for the zone near the gliding surfaces points to self-lubrication by transformed (fused or dissociated) rock as a fundamental tribological mechanism. The idea is backed by two important facts: the impossibility to explain the observed economy of locomotion (and especially its increase with size) by any of the mechanisms considered hitherto; and a calculation yielding an unexpectedly low amount of energy susceptible to be dissipated far away from the gliding surfaces. Further analysis demonstrates numerically the plausibility of the self-lubrication concept both for primitive and carbonate rock. The development of useful prediction algorithms is not impossible in the future.

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ZusammenfassungMechanismen groer Bergstürze Die Trümmerströme vieler großer Bergstürze zeigen eine bemerkenswerte Tendenz zum Verbleiben in mehr oder weniger undurchmischtem Verband. In der vorliegenden Arbeit wird dieses Fehlen laminarer oder turbulenter Strömung auf Grund einer mathematischen Untersuchung erklärt. Die dabei resultierende hohe Energiedichte in der Nähe der Gleitflächen deutet auf Selbstschmierung durch transformiertes (geschmolzenes oder dissoziiertes) Gestein als entscheidenden tribologischen Mechanismus. Zwei wesentliche Tatsachen sprechen für diese Idee: das Versagen aller bisher vorgeschlagenen Mechanismen bei der Erklärung der beobachteten Bewegungsökonomie, speziell im Hinblick auf deren Steigerung in Funktion der Größe; und der rechnerisch ermittelte überraschend niedrige Energieanteil, der in größerer Entfernung von den Gleitflächen umgesetzt werden kann. Eine weitere Rechnung demonstriert zahlenmäßig die Plausibilität der Selbstschmierung sowohl für Urgesteine wie für Kalke. Die Entwicklung brauchbarer Vorhersage-Algorithmen rückt damit in den Bereich des Möglichen.

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RésuméLes mécanismes de gros éboulements Les composantes de gros éboulements montrent une tendance remarquable à se déplacer dans un ordre plus ou moins invariable. L'étude explique mathématiquement cette absence d'écoulements laminaire ou turbulant. La haute concentration d'énergie aux environs immédiats des surfaces de glissement implique la possibilité d'une auto-lubrification par le rocher métamorphosé (soit fondu, soit dissocié) en tant que mécanisme tribologique fondamental. Deux faits importants appuient cette hypothèse: d'une part l'impossibilité jusqu'à maintenant d'expliquer l'économie d'énergie observée (et spécialement en corrélation avec la grandeur de l'évènement) par un mécanisme quelconque; d'autre part la fraction d'énergie déterminée par calcul, étonnament basse, susceptible d'être métamorphosée à grande distance des surfaces de glissement. En outre le calcul démontre la possibilité du mécanisme d'auto-lubrification pour les plus importants types de rochers (primitifs et sédimentaires). Le développement futur d'algorithmes de prédiction utilisables entre de ce fait dans le domaine du possible.

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Notation a acceleration (m/s²) - A area (m²) - c specific heat (J/kg °C) - C transformation heat (J/kg) - d distance between two narrow passages (m) - e base of natural logarithms = 2.72 - e thickness of layer (m) - g gravitational acceleration = 9.81 m/s² - h thickness of mass (m) - k resistance coefficient - L global coefficient describing tribological quality (m/s⁴) - m mass (kg) - M lubrication quality factor - p pressure (Pa) - P power (W) - r wave amplitude (m) - s air cushion or gap thickness (m) - t time (s) - v velocity, speed (m/s) - V volume (m³) - W specific energy (J/kg) - x distance travelled (m) - z height (m) - Z height of center of gravity (m) - slope angle - relative portion of volume occupied by voids - dissipation coefficient - difference - relative fraction of A acting as narrow passage - depth (m) - viscosity (kg/ms) - temperature (°C) - adiabatic constant of air = 1.4 - wavelength (m) - µ coefficient of friction (solid state) - average coefficient of friction - Ludolf's number = 3.14 - density (kg/m³) - fidelity coefficient - jet contraction factor Indices a primitive rock - A air cushion - b feldspar - c quartz - d lubricant generated from primitive rock - d dissipation - e sedimentary rock - E air escape gap - f CaO - g CO₂ - h lubricant generated from sedimentary rock - i ordinal number - L lubricant - M total area - R rock - o initial, basic, 30 = after 30 s With 12 Figures     

Slope inclinometers for landslides

Slope inclinometers/indicators are used to determine the magnitude, rate, direction, depth, and type of landslide movement. This information is usually vitally important for understanding the cause, behavior, and remediation of a landslide. However, many inclinometer measurements fail to achieve these intended aims because of lack of appreciation of the many factors that need to be correctly implemented during installation, monitoring, and data reduction to yield useful data. This paper presents some guidelines for understanding, installing, and interpreting slope inclinometers and presents three case histories that illustrate some of the pitfalls that can develop if these guidelines are not followed.     

Rainfall thresholds for rainfall-induced landslides in Slovenia

In Slovenia, rainfall-induced landslides lead to considerable damages, even causing human losses. In order to reduce the impact of this kind of landslide, several researchers analyzed rainfall-induced landslides in this country, but to date, no rainfall thresholds have been developed for a landslide warning system at national scale. In this paper, the definition of rainfall thresholds for rainfall-induced landslides in Slovenia is presented. The thresholds have been calculated by collecting approximately 900 landslide data and the relative rainfall amounts, which have been collected from 41 rain gauges. To define the thresholds, an existing procedure characterized by a high degree of objectiveness has been used. This approach is based on a software that was developed for a test site with very different characteristics (Tuscany, central Italy). At first, a single national threshold has been defined; subsequently, the country was divided into four zones, on the basis of the major river basins. The effectiveness of the thresholds has been verified by the use of several statistical parameters and it resulted in quite good performances, even if with some uncertainties, probably due to the quality of the available data. Besides the setting of a threshold system, usable for civil protection purposes at national scale, an additional outcome of this work was the possibility of applying, with good results, a methodology defined for another region, therefore testing its degree of exportability in different settings.     

Distribution of landslides in southwest New Zealand

This study examines the size distribution of a regional medium-scale inventory of 778 landslides in the mountainous southwest of New Zealand. The spatial density of mapped landslides per unit area can be expressed as a negative power–law function of Landslide area A_L spanning three orders of magnitude (10^–2–10¹ km²). Although observed in other studies on landslide inventories, this relationship is surprising, given the lack of absolute ages, and thus uncertainty about the temporal observation window encompassed by the data. Large slope failures (arbitrarily defined here as having a total affected area A_L>1 km²) constitute 83% of the total affected landslide area A_LT. This dominance by area affects slope morphology, where large-scale landsliding reduces slope angles below the regional modal value of hillslopes, _mod39°. More numerous smaller and shallower failures tend to be superimposed on the pre-existing relief. Empirical scaling relationships show that large landslides involve >10⁶ m³ of material. The volumes V_L of individual preserved and presumably prehistoric (i.e. pre-1840) landslide deposits equate to 10⁰–10² years of total sediment production from shallow landsliding in the respective catchments, and up to 10³ years of contemporary regional sediment yield from the mountain ranges. Their presence in an erosional landscape indicates the geomorphic importance of landslides as temporary local sediment storage.     

Characteristics of rapid giant landslides in China

Factual data for 70 rapid, giant landslides since 1900 show that the occurrence of these landslides was largely predisposed by tectonics, geological structures, lithology and topography, and often triggered by rainfall and earthquakes. In terms of mobile behavior, the giant landslides can be classified into three types: slides, slide-flows and flows. It is found that each type of landslide was constrained to certain geologic and topographic regimes. There are good correlations between kinematic parameters of landslides and slope geometries, which confirm the important role played by topographical condition in the mobile behavior of landslides. Moreover, it is also found that each type of landslide presents distinct geotechnical characteristics in terms of nature of the slip zone and properties of sliding mass. Brief analyses of five typical cases illustrate that landslide mechanisms can be conceptually depicted by failure mechanisms of their slip zones prior to onset of movement and following energy conversion during movement. Problems and questions related to experience in China suggest that comprehensive and systematic investigation and study on rapid giant landslides are urgently needed.     

云南省新平县滑坡预警区划研究

以新平县为研究区,采用确定性系数(CF)方法分析滑坡与地质环境因子各区段(类型)之间的敏感性关系,得出各因子不同区段(类型)的敏感性大小,并运用Logistic回归分析方法建立预警区划模型,认为坡度、岩组、年均降雨量、高程、构造等五个因子是影响研究区滑坡发生的敏感性因子,建立Logistic概率预测模型,并对研究区进行五级预警区划,区划结果对该区预警工作有积极的指导意义。     

Boundary effects of rainfall-induced landslides

In the study of landslides, it is generally assumed that an impermeable boundary exists at a certain depth and failure occurs at this boundary. In reality this is not always the case and failures can occur at any depth. This paper aims to study the effect of boundary conditions on landslides, using a series of seepage and stability analyses performed over a range of rainfall intensities, and for different failure mechanisms, by studying the failure time and depths corresponding to fully drained, partially drained, and impermeable boundaries. It is shown that these conditions can significantly affect the occurrence and depth of rainfall-induced landslides.     

入渗诱发黄土滑坡的力学机制

在农田水利建设蓬勃发展的同时,灌溉入渗在黄土塬边诱发了严重的滑坡灾害。作为黄土滑坡的典型区域,甘肃黑方台地区近年来受到了密切关注。为了研究入渗条件下黄土边坡的变形破坏机制,在黑方台典型滑坡后壁削取原状土样,开展了3组不同应力路径的三轴试验以模拟边坡的失稳过程。通过开展偏压固结不排水（ACU）试验和饱和恒载（SDL）增孔压试验研究了饱和黄土启动和变形破坏过程;通过开展非饱和恒载（UDL）增湿试验,研究了非饱和黄土启动和变形破坏过程。在此基础上,从应力路径和黄土的微观结构角度阐述了入渗诱发黄土滑坡的力学机制。此外,利用试验结果分析了黄土的典型物理力学特性,结果表明：在埋深应力范围内,该黄土的饱和临界状态线可简化为单一的直线;该黄土的状态边界线可近似为其临界状态线;由UDL试验预测的土-水特征曲线受应力状态影响,较高的应力水平对应较低含水率。     

The mobility of long-runout landslides

Several issues relevant to the mobility of long-runout landslides are examined. A central idea developed in this paper is that the apparent coefficient of friction (ratio of the fall height to the runout distance) commonly used to describe landslide mobility is physically meaningless. It is proposed that the runout distance depends primarily on the volume and not on the fall height, which just adds scatter to the correlation. The negative correlation observed between the apparent coefficient of friction and the volume is just due to the fact that, on the gentle slopes on which landslides travel and come to rest, a large increase in runout distance due to a large volume corresponds to a small increase in the total fall height, hence to a decrease in the apparent coefficient of friction.

It is shown that the spreading of a fluid-absent, granular flow is not able to explain the large runout distances of landslides, and in particular does not allow the centre of mass to travel further than expected for a sliding block. This contrasts with the behaviour of natural landslides, for which the centre of mass is shown to travel much further than expected from a simple Coulomb model. The presence of an interstitial fluid which can partly or entirely support the load of particles allows the effective coefficient of solid friction to be reduced or even suppressed. Air is not efficient for fluidising large landslides and a loose debris cannot slide over a basal layer of entrapped and compressed air, as air would rapidly pass through the debris in the form of bubbles during batch sedimentation. Water is much more efficient as a fluidising medium due to its higher density and viscosity, and its incompressibility. As water is known to enhance the mobility of the saturated debris flows, it is proposed that water is also responsible for the long runout of landslides. This is consistent with the fact that the increase in runout with volume is similar for debris flows and landslides. Field evidence suggests that most landslides are unsaturated with water but not dry, even on Mars.

Comparison of the velocity of well-documented landslides with that predicted by fluid-absent, granular models shows that these models predict landslides that are much faster and less responsive to topography than natural ones. The relatively low velocities of landslides suggest that energy dissipation is dominated by a velocity-dependent stress and that the coefficient of solid friction is very low. This is consistent with the physics of fluidised or partly fluidised debris and suggests that landslide velocity may be controlled by local slope and flow thickness rather than by the initial fall height. In the absence of a supply of fluid at the base, fluidisation requires a net downward flux of sediment, implying some deposition at the base of landslides, which may thus progressively run out of material. In such a model, the spreading of the portion of a landslide beyond a certain distance would primarily depend on the volume passing this distance and not on the total volume of the landslide. Landslide deposits may therefore have self-similar shapes, in which the area covered beyond a certain distance is a constant function of the volume beyond that distance. It is shown that the shape of some well-documented landslide deposits is in reasonable agreement with this prediction. One consequence is that, as recently proposed for debris flows, assessment of hazards related to landslides should be based on the correlation between the volume and the area covered by the deposit, rather than on the apparent coefficient of friction.     

Multiphysics hillslope processes triggering landslides

In 1996, a portion of a highly instrumented experimental catchment in the Oregon coast range failed as a large debris flow from heavy rain. For the first time, we quantify the 3-D multiphysical aspects that triggered this event, including the coupled sediment deformation-fluid flow processes responsible for mobilizing the slope failure. Our analysis is based on a hydromechanical continuum model that accounts for the loss of sediment strength due to increased saturation as well as the frictional drag exerted by the moving fluid. Our studies highlight the dominant role that bedrock topography and rainfall history played in defining the failure mechanism, as indicated by the location of the scarp zone that was accurately predicted by our 3-D continuum model.     

Controlling factors of loess landslides in western China

Based on detailed field data from recent geo-hazards surveys in the Loess Plateau region of western China, this study analyzes the key factors that control loess landslides. The evolution phase of river valleys, the geological structure of slopes and the geometry of slopes are all found to play a role in determining the occurrence, distribution and other characteristics of loess landslides. Groundwater and vegetation also contribute to their formation. A combination of human engineering activities and precipitation events are the principal triggering factors for the instability of loess slopes.     

Mechanisms of landslides in over-consolidated clays and flysch

In this report, we present the situation in Slovenia, Europe with regard to natural hazards, emphasizing land-sliding problems. Furthermore, we shortly present the University of Ljubljana and the Faculty of Civil and Geodetic Engineering as the new member of the ICL that was selected to be the World Center of Excellence on Landslide Risk Reduction for the period of 2008–2013 with the project entitled “Mechanisms of landslides in over-consolidated clays and flysch” in the Activity scale and targeted region as “National.” Some preliminary results of this project are shortly presented at the end.     

Structurally controlled landslides in saprolitic soils in Hong Kong

A number of large slope failures which have occurred in Hong Kong have highlighted the important influence that discontinuities in saprolitic soils can have on slope stability. Conventional stability analyses normally adopted for such soils, which assume failure through intact materials of uniform shear strength, are not appropriate if discontinuity-controlled failures can be postulated. Stress history and present-day weathering effects should also be taken into consideration in the assessment of the long-term stability of slopes in weathered rocks in tropical and subtropical regions. © Rapid Science Ltd. 1998     

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.