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## Über dieses Buch

Landslides and debris flows belong to the most dangerous natural hazards in many parts of the world. Despite intensive research, these events continue to result in human suffering, property losses, and environmental degradation every year. Better understanding of the mechanisms and processes of landslides and debris flows will help make reliable predictions, develop mitigation strategies and reduce vulnerability of infrastructure.

This book presents contributions to the workshop on Recent Developments in the Analysis, Monitoring and Forecast of Landslides and Debris Flow, in Vienna, Austria, September 9, 2013. The contributions cover a broad spectrum of topics from material behavior, physical modelling over numerical simulation to applications and case studies. The workshop is a joint event of three research projects funded by the European Commission within the 7th Framework Program: MUMOLADE (Multiscale modelling of landslides and debris flows, www.mumolade.com), REVENUES (Numerical Analysis of Slopes with Vegetations, http://www.revenues-eu.com) and HYDRODRIL (Integrated Risk Assessment of Hydrologically-Driven Landslides, www.boku.ac.at/igt/).

## Inhaltsverzeichnis

### Some Ideas on Constitutive Modeling of Debris Materials

Abstract
Debris flows are dangerous natural hazards in countries with mountainous terrains. Debris materials show complex behavior depending on material composition and loading condition. Constitutive model plays an important role in better understanding the triggering mechanisms and reliable prediction of runout and deposition behavior. This paper reviews some constitutive models for debris materials in particular the Bingham fluid model. The hypoplastic constitutive model with critical state for granular materials is briefly recapitulated. Some ideas are presented to integrate the Bingham model into the hypoplastic constitutive model to account for both slow and fast flow of debris materials. The structure of this combined model and some specific formulations are discussed.
Xiaogang Guo, Wei Wu

### Theoretical and Experimental Investigation into Unsaturated Soil Constitutive Relationship – Preliminary Studies

Abstract
This article presents the objective of current research to conduct experimental testing on the chosen set of granular materials composed of sand, silt and clay as the reference test materials used in the project, and to develop the advance constitutive model to predict the stress-strain behaviour of materials with diversified properties.
Aleksandra Jakubczyk, Xia Li, Hai-Sui Yu

### Shear Behaviors of Saturated Loess in Naturally Drained Ring-Shear Tests

Abstract
A series of ring-shear tests was conducted on saturated loess to investigate the effects of shear rates and normal stress levels on the shear behaviors. The test results revealed that the effect of shear rate on shear strength of loess is strongly dependent on the normal stress levels. The peak friction coefficient of the samples is positively dependent on the shear rate when relatively low normal stress was imposed, whereas shear rate do not significantly affect the peak or residual friction coefficient for samples under high normal stress levels. But on the contrary, the residual shear strength of samples with pre-exiting failure surface increase slightly with increased shear rate under high normal stress level while kept constant under low normal stress level. Excess pore pressure was estimated to have built up within the shear zone and then can lead to a reduction in the shear strength after failure occur and the degree of reduction in post-failure shear strength was evaluated by the brittleness index.
Shun Wang, Wei Wu, Wei Xiang, Qingbing Liu

### Soil Aggregate Stability in Eco-engineering: Comparison of Field and Laboratory Data with an Outlook on a New Modelling Approach

Abstract
Stabilisation effects of plants are developing as a function of time. Within this scope, soil aggregation processes play a decisive role in re-establishing a protective vegetation cover. From this perspective we compared bare and vegetated soil, on the one hand artificially prepared and, on the other hand, derived from a recently landslide affected slope and an adjacent gully with 25 year old eco-engineering measures, respectively.
In both cases, the planted specimens had a significantly higher soil aggregate stability compared to their respective control samples, with the relative increase from control to planted equal for both the natural and artificial samples.
Aspects of the development and succession processes of plants are compared as well as rooting and the degree of mycorrhization. Additionally, soil development and the methodical approach are discussed as well as a new approach to modelling soil aggregate stability in respect of eco-engineering measures for slope stabilisation presented.
Frank Graf, Lothar te Kamp, Michael Auer, Madhu Sudan Acharya, Wei Wu

### Centrifuge Model Test of a Bamboo Crib Wall

Abstract
Centrifuge testing is particularly suitable to model geotechnical processes and events because the increase in centrifugal force creates stresses in the model that are equivalent to gravitational force acting on the much larger prototype. The mechanisms of failure and the stresses observed in the model tests are generally the realistic representation of the actual ground movements and accumulation of stresses on prototype. Crib retaining walls made of different materials are used as special form of gravity walls in stabilizing slopes. The centrifuge test results of a model crib retaining wall made of mini bamboo sticks are reported in this paper. The centrifuge model test results are similar to the field test results of concrete crib walls.
Madhu Sudan Acharya, Wei Wu, Michael Auer, Lothar te Kamp

### Viscous Effects on Granular Mixtures in a Rotating Drum

Abstract
In debris flow research, fine particles are considered to be part of the fluid phase of the flow. Nonetheless the particle-size threshold of this phase is not generally clear and could be dependent on the motion-characteristics of the flow itself. In the present study, rotating drum experiments are employed to investigate the effects of kaolin and silt fractions mixed in a fluid phase over a granular mixture of fine and coarse sand. Eight soil mixtures were prepared and tested at variable rotational velocities and water content. The mixtures are subjected to a constant shear that shifts the tested material and due to the action of gravitational forces a recirculating flow is produced inside the rotating drum. Our experiments found evidence of a solid concentration limit value where a shear-dependent behaviour is developed in the mixtures.
Miguel Angel Cabrera, Devis Gollin, Roland Kaitna, Wei Wu

### Centrifuge Model Tests of Rainfall-Induced Landslides

Abstract
Rainfall-induced landslides and debris flows constitute very serious threats to human lives and infrastructure. In many cases, rainfall characteristics which cause the initiation of landslides are not very well determined and this might lead to the misunderstanding of the failure mechanism, the kinematic characteristics and the run-out distance of the failure. In this paper, the design of three series of centrifuge model tests on soil slopes, subjected to rainfall conditions, is presented. The main goal is to investigate rainfall characteristics which cause failure initiation in soil slopes in respect to soil properties and slope geometry. Tests will be performed in a geotechnical centrifuge at the Nottingham Centre for Geomechanics (NCG) under very well defined initial and boundary conditions. For the accomplishment of these tests, a climatic chamber has been developed which accommodates plane-strain slope models and sufficient instrumentation and embodies a rainfall and an evaporation simulation systems. During the centrifuge tests, changes in pore water pressures and soil state as well as deformations of the slopes will be measured, while rainfall intensity and total rainfall will be accurately defined. Three different soil types will be used to create uniform slope models, i.e. fine sand, silty clay and clay, while rainfall intensity will be proportional to the infiltration capacity. The paper describes, also, the saturation and calibration of Druck PDCR-81 miniature pore pressure transducers and SWT5 tensiometers used for pore water pressure measurements.
Vasileios Matziaris, Alec M. Marshall, Hai-Sui Yu

### Shallow Tunnelling in Partially Saturated Soil Numerical Analysis of the Contribution of Dewatering to the Enhancement of Face Stability

Abstract
Partially saturated soils exhibit higher shear strength than saturated soils. Shallow tunnels are often driven closely to or inside the saturated layers. The water table is sometimes lowered to excavate the tunnel in a partially saturated condition. In doing so, the face stability is improved and the necessary support pressure can be reduced. In this paper a numerical study is performed to assess the effect of dewatering on the face stability of shallow tunnels. The analyses are performed with an advanced constitutive model for partially saturated soil. The model is implemented in a commercial software using the Bishop’s effective stress for the equilibrium equation.
Enrico Soranzo, Roberto Tamagnini, Wei Wu

### Modeling the Impact of Granular Flow against an Obstacle

Abstract
This paper presents a numerical model based on Discrete Element Method (DEM) used to reproduce a series of tests of dry granular flow. The flow was composed of poly-dispersed coarse-grained angular particles flowing in an inclined flume and interacting with a divided rigid wall. The normal impact force against the wall has been studied in details considering the force on each part of the wall. The model has been calibrated based on the flow thickness measurements. By quantitative comparison with experimental data, the model showed good agreement in terms of peak force on each part of the wall, the time of the peak and also the residual force values at the end of the tests.
Adel Albaba, Stéphane Lambert, François Nicot, Bruno Chareyre

### One-Dimensional Transient Analysis of Rainfall Infiltration in Unsaturated Volcanic Ash

Abstract
The paper presents a one-dimensional hydro-mechanical analysis of rainfall infiltration in a loose volcanic ash and the utilisation of a factor of safety for the implementation of an early-warning system. Three different rainy seasons with different rainfall patterns were analysed . The analysis aims to understand the influence of the antecedent rainfall on the wetting front, the pore-water pressures and the factor of safety. The analysis was carried out in the context of a Master project of the first author at the Laboratory for Soil Mechanics of EPFL.
James Fern, John Eichenberger, Alessio Ferrari, Lyesse Laloui

### Investigations of Gravity-Driven Two-Phase Debris Flows

Abstract
A depth-integrated theory is derived for the gravity-driven two-phase debris flows over complex shallow topography. The mixture theory is adopted to describe the mass and momentum conservation of each phase. The model employs the Mohr-Coulomb plasticity for the solid rheology, and assumes the Newtonian fluid for the fluid phase. The interactive forces assumed here consist of viscous drag force linear to velocity difference between the both phases, and buoyancy force. The well-established governing equations are built in 3D topography; as a result, they are expressed in the curvilinear coordinate system. Considering the characteristics of flows, a shallow layer assumption is made to simplify the depth-integrated equations. The final resulting equations are solved numerically by a high-resolution TVD scheme. The dynamic behaviors of the mixture are investigated. Numerical results indicate that the model can adequately describe the flows of dry granular material, the pure water and general two-phase debris flows.
Xiannan Meng, Yongqi Wang

### Lattice-Boltzmann Method for Geophysical Plastic Flows

Abstract
We explore possible applications of the Lattice-Boltzmann Method for the simulation of geophysical flows. This fluid solver, while successful in other fields, is still rarely used for geotechnical applications. We show how the standard method can be modified to represent free-surface realization of mudflows, debris flows, and in general any plastic flow, through the implementation of a Bingham constitutive model. The chapter is completed by an example of a full-scale simulation of a plastic fluid flowing down an inclined channel and depositing on a flat surface. An application is given, where the fluid interacts with a vertical obstacle in the channel.
Alessandro Leonardi, Falk K. Wittel, Miller Mendoza, Hans J. Herrmann

### Modelling of a Free Surface Flow at Variable Gravity Conditions with SPH

Abstract
Smoothed Particle Hydrodynamics (SPH) are adapted to model a free surface flow at variable gravity conditions. Implementation of SPH related to high gravity fields are discussed. The analysis shows that the original formulation of SPH needs no modification for the variation of gravity, though a smaller time step must be used. Numerical simulations of a water flow problem show that SPH is consistent at different gravity fields, and produce reasonable results. The scaling principle of the velocity and time of a flowing mass down an incline is discussed.
Chong Peng, Miguel Angel Cabrera, Wei Wu

### The Role of Phase Transition in Slope Stability Analyses

Abstract
The paper presents a thermodynamically consistent analysis of rainfall induced landslides in partly saturated porous materials. The thermodynamic framework is based on non-equilibrium thermodynamics and the theorem of minimum entropy production. The model is implemented in the FE software Abaqus and it is applied in the analysis of an ideal problem in which the role of water condensation is modeled. The analysis is performed in plane strain conditions and it shows as associative plasticity is able to capture the initialization of the mud flow. The numerical algorithm for the constitutive law is derived by the thermodynamics and it is able to describe the collapse induced by wetting. The presented mathematical framework is able to explain the mechanical instability of hill slopes during a coupled strain-diffusion analysis.
Roberto Tamagnini, Wei Wu

### Effect of Vegetation on Stability of Soil Slopes: Numerical Aspect

Abstract
Soil bioengineering makes use of living plants to enhance soil stability against erosion and failure. Its practice is strongly dominated by empiricism. Recently much effort has been made towards quantifying soil bioengineering measures. This paper provides a critical review of the numerical modelling of some soil bioengineering measures. We discuss the application of the numerical methods including the finite element method and the limit equilibrium method for the composite of soil-plant root. A detailed review of the mechanical and hydrological models for the complex interaction between soil, plant, water and atmosphere is provided.
Wei Wu, Barbara Maria Switala, Madhu Sudan Acharya, Roberto Tamagnini, Michael Auer, Frank Graf, Lothar te Kamp, Wei Xiang

### Landslide Susceptibility Analysis and Mapping Using Statistical Multivariate Techniques: Pahuatlán, Puebla, Mexico

Abstract
Susceptibility analyses are frequently based on the idea that landslides occur in the same areas where they have taken place previously, and also in areas under similar conditions. Based on that assumption, four different statistical techniques—Linear Discriminant Analysis (LDA), Quadratic Discriminant Analysis (QDA), Logistic Regression (LRA), and Neural Networks (NN) —have been applied for the municipality of Pahuatlán, Puebla, México. The base for the analysis was a geomorphological landslide inventory derived from the stereo-interpretation of Very High Resolution (VHR) satellite images.
The quality of each model was controlled by using ROC curves and Cohen’s Kappa coefficient. Also, a temporal validation with a data set of landslides occurred on 2012 was carried out for each model. The resulting analysis showed that the aspect, the slope angle and the lithological unit were the variables with the highest weight associated with the occurrence of landslides in the study area.
Franny Giselle Murillo-García, Irasema Alcántara-Ayala

### Application of the Program PCSiWaPro® for the Stability Analysis in Earth Dams and Dikes Considering the Influence from Vegetation and Precipitation— A Case Study in China

Abstract
Earth dams and dikes are always a safety issue, as it can experience catastrophic destruction due to the slope failure caused by various factors, such as construction materials, vegetation, atmospheric conditions and so on. The preliminary tests on a physical model have shown that the security and stability has been already severely compromised in the partially saturated region, i.e. the area above the seepage line is in great danger and it comes quickly to landslides on the air side. Before the stability analysis can be done for those unsaturated zones, the water content and geohydraulic processes in the saturated and partially saturated soil area were simulated using the simulation program PCSiWaPro®. The simulation results of several scenarios both in the laboratory and in the field in China clearly demonstrated the impact of building materials and construction on the behavior of water saturation and thus the stability of the dam. The accordance between measured and calculated values for water content using the program PCSiWaPro® was very good.
Jinxing Guo, Issa Hasan, Peter-Wolfgang Graeber

### A Pile-Soil Separation Concerned Model for Laterally Loaded Piles in Layered Soils

Abstract
A pile-soil separation concerned continuum method is proposed to analyze the response of laterally loaded piles in a homogeneous multilayer elastic medium. The displacement fields of soil vary in the vertical direction and decrease in the radical direction. Variational principles are used to obtain the governing differential equations for the pile and soil displacements. Closed-form solutions are derived and the pile deflection, pile rotation, bending moment and shear force for the entire length of the pile can be obtained. The validity of proposed approach is demonstrated through comparisons with previous methods, 3D FD analysis and data obtained from a published pile test. The influence of pile-soil separation is explicated by the results of a comprehensive parametric study involving the slenderness ratio and flexibility factor of the pile. It can be concluded that the deflection of pile is larger when pile-soil separation happens, and the effect can be reduced for piles with larger slenderness ratio and medium flexibility.
Guoping Lei, Huiming Tang, Wei Wu

### The Effect of Overconsolidation and Particle Shape on the CPT End Resistance of Granular Soils

Abstract
This paper examines the effect of overconsolidation and particle shape on the end resistance (qc) measured in a series of centrifuge Cone Penetration Tests (CPTs) conducted in three uniformly graded silica materials with distinct particle shape. For each soil type, the end resistances were measured for both normally and overconsolidated soil samples at centrifuge g-level of 100g. All samples were prepared and tested at two different relative density, and the over-consolidated samples were achieved by reducing the centrifuge g-level from 200g to 100g (with over-consolidation ratio, OCR=2). At a given relative density and stress level, the striking dependency of the CPTs end resistance (qc) on the particle shape can be observed. For a particular material, a tendency for qc value to increase with the OCR was in evidence. An approach based on the spherical cavity expansion method was proposed to predict the qc value of each soil, and particularly to investigate how the OCR and particle shape influence on the end bearing resistance. It was found that the predicted qc are shown to match the measured data well, and the end bearing resistances were significantly affected by the critical friction angle and horizontal stress, which were closely related to the particle shape and overconsolidation ratio.
Qingbing Liu, Wei Xiang, Barry Michael Lehane

### Depositional Regularities Analysis of Incompetent Beds in Dam Foundation of Xiaonanhai Reservoir Using Markov Chain

Abstract
Incompetent beds, which are composed of clastic rocks, are widely distributed in Upper Jurassic red strata in Sichuan Basin. These incompetent beds are prone to lead to the instability of Xiaonanhai Reservoir. The Markovian simulation results show that the semi-rhythmic structures of J$$_{\rm 3S}^{\rm 4-1}$$ section are mainly of two types: one is siltstone and argillaceous siltstone alternation with great rhythmic thickness, the other is thin bedded argillaceous siltstone with interbedded mudstone. The incompetent beds are divided into three types: (I) cracked rock; (II) cracked rock with mud interbedded; (III) mudstone with cracked rock interbedded, or mudstone. Type I mainly exists in the thick mudstone or at the junction of thick mudstone and siltstone. Type II generally lies in the soft rock. Type III with more than 30% of clay, frequently appears in the second semi-rhythmic structure mentioned above.
Nie Qiong, Xiang Wei, Du Shuixiang

### Physically – Based Critical Rainfall Thresholds for Unsaturated Soil Slopes

Abstract
In this work, a physically based model for the definition of the critical rainfall thresholds for shallow landslide initiation at regional scale is presented. The model is capable of considering unsaturated conditions in the soil volume, by taking into account the effect of partial saturation in: a) the balance of mass for the pore water; b) the deformability of the solid skeleton; and, c) the soil shear strength. Starting from the simplified hypothesis of infinite slope, a series of numerical simulations was conducted in parametric form to determine the functional relationship between the critical rainfall intensity leading the slope to failure to rainfall duration, in terms of dimensionless variables. This has allowed to identify the functional dependence of the critical rainfall intensity on: a) event duration; b) slope geometry; c) mechanical properties of the soil cover; and, d) initial conditions in terms of pore water pressure distribution. The function thus obtained can be easily and efficiently implemented in GIS–based codes for the evaluation of physically–based, spatially–distributed critical rainfall thresholds.
Diana Salciarini, Claudio Tamagnini

### Numerical Simulations of the Mechanical Contribution of the Plant Roots to Slope Stability

Abstract
Soil bioengineering methods in slope stabilisation are becoming more and more popular, when ecological solutions are desirable. Simple, numerical models were created in order to assess the mechanical contribution of the roots reinforcement to slope stability. Two- and three-dimensional analyses were conducted for different geometries of root architectures and different soil conditions. Obtained values of a factor of safety (FOS) are compared and discussed. From these data, it is possible to determine, which root reinforcement cases have the greatest impact on the stability of the slope.
Barbara Maria Switala, Wei Wu

### Study on Morphological Characteristics of Coarse Particles in Sliding Zones of Huangtupo Landslide in Three Gorges Reservoir Area, China

Abstract
The coarse particles (defined as particle size between 0.25 mm and 2 mm here) in sliding zones of Huangtupo landslide in Three Gorges Reservoir Area are taken as study objects. Particle size distribution and particle profiles of samples are firstly obtained through sieving tests and digital imaging respectively. Combined with fractal theory, the fractal dimensions of particle size distributions and particle profiles are calculated quantitatively. Scanning electron microscopy and energy disperse spectroscopy are employed to capture the micrograph and test the mineral composition of samples. Tests and analysis results indicate that the particle size distributions and profiles of samples have fractal characteristics, and the fractal dimensions can be used as quantitative analytical index for the evolution of sliding zones. As the continuous development of sliding zones, the particle mass – size fractal dimension increase and the profile fractal dimension decrease. The special surface characteristics of particles can also indirectly reflect the stress and deformation conditions during the failure and evolution of the sliding zones.
Jinge Wang, Wei Xiang, Shun Wang

### Study of a Model Slope Reinforced with Jute

Abstract
This paper presents the results of laboratory model tests and corresponding numerical analyses carried out on a model slope representing an actual embankment, which failed on several occasions after overnight heavy rainfall. A study is performed by reinforcing the slope with thin jute strips of 1 mm diameter. The numerical simulation of the model tests is performed by a commercial program called FLAC. The responses of the model slope with the jute strips are observed under different intensities and duration of rainfall. Significant improvement in the deformation and the factor of safety of the slope is observed both in the numerical analyses and the laboratory experiments when jute strips are utilized as a slope protection measure. The study indicates that the stability of the embankment can be improved significantly at a low cost by reinforcing it with jute strips.
Aniruddha Sengupta, Saurabh Kumar

### Soil Nailing, the Variable Static System of the Future

Abstract
Every time we build new constructions, changes occur in the natural environment around the project area. The importance of geotechnics in the design and realisation of several protection measures has increased. Many new construction techniques have been developed to realise big projects in and with the construction material named soil. The development of stone columns, the manufacture of big retaining walls and dams of reinforced soil or the so-called Mixed-in-place method can be cited as examples.
Michael Auer

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