The Importance of Laboratory Experiments in Landslide Investigation

This study focuses on a better understanding of mass movements and on the influences of different boundary conditions on velocities of creeping slopes. A well monitored example of a slowly creeping landslide is the mass movement Hochmais - Atemkopf, situated in the Kaunertal, Tyrol, Austria (Fig. 1). The long term monitoring program for more than 40 years of this landslide gives a good impression of its time dependent behaviour. A large amount of additional data, as geological mapping, boreholes, geophysical investigation and so on provides a funded base for the model’s geometry. The most influencing factor for finite element calculations is besides the model’s geometry the rheological model and the therefor adapted material properties. Creep laboratory experiments have been performed and evaluated for the most active sliding zone. Long term shear tests from 1964 have been reevaluated and compared with current long term triaxial tests. The experiments reveal a non linear dependence between equivalent stress and displacement rate. An elasto, visco - plastic rheological model with a non-linear viscose deformation has been fitted to those results

Beiträge zum 28. Darmstädter Geotechnik-Kolloquium am 09. März 2022

Themenschwerpunkte: 1. Modell- und Feldversuche 2. Digitalisierung und künstliche Intelligenz in der Geotechnik 3. Nationale und internationale Großprojekte 4. Normung und Rechtliche

Beiträge zum 28. Darmstädter Geotechnik-Kolloquium am 09. März 2022

Themenschwerpunkte: 1. Modell- und Feldversuche 2. Digitalisierung und künstliche Intelligenz in der Geotechnik 3. Nationale und internationale Großprojekte 4. Normung und Rechtliche

Simulations of Fine-Meshed Biaxial Tests with Barodesy

Recent experimental studies showed that shear band development starts at the beginning of triaxial tests. In experimental testing, it is impossible to obtain a soil sample with a homogeneous void ratio. Therefore, a homogeneous deformation, i.e., an element test, is questionable well before the peak. In this article we carry out finite element simulations of fine-meshed biaxial tests with the constitutive model barodesy, where the stress rate is formulated as a function of stress, stretching and void ratio. The initial void ratio in the simulations is normally distributed over all elements in a narrow range. In this article, we evaluate the pre-peak shear band development. We further compare stress paths and stress-strain curves of the biaxial test of relevant elements (e.g., in- and outside the shear band) with the results of the average response of all elements. We show how the response in an element test differs from the average response of the fine-meshed test. We present the resulting potential for understanding (early) shear band development and stress-strain behaviour in a biaxial test: The inhomogeneous void ratio distribution in a sample favours early shear band development. This effect is modelled with barodesy. The obtained stress paths and stress-strain curves show that the maximum deviatoric stress is higher in the element test than it is in the average response of the fine-meshed test

Slope stability analysis: Barodesy vs linear elastic – perfectly plastic models

The results of slope stability analysis are not unique. Different factors of safety are obtained investigating the same slope. The differences result from different constitutive models including different failure surfaces. In this contribution, different strength reduction techniques for two different constitutive models (linear elastic - perfectly plastic model using a Mohr-Coulomb failure criterion and barodesy) have been investigated on slope stability calculations for two different slope inclinations. The parameters for Mohr – Coulomb are calibrated on peak states of element tests simulated with barodesy for different void ratios. For both slopes the predictions of the factors of safety are higher with barodesy than with Mohr-Coulomb. The difference is to some extend explained by the different shapes of failure surfaces and thus different values for peak strength under plane strain conditions. The plane strain predictions of Mohr-Coulomb are conservative compared to barodesy, where the failure surface coincides with Matsuoka-Nakai

Are real-world shallow landslides reproducible by physically-based models? Four test cases in the Laternser valley, Vorarlberg (Austria)

In contrast to the complex nature of slope failures, physically-based slope stability models rely on simplified representations of landslide geometry. Depending on the modelling approach, landslide geometry is reduced to a slope-parallel layer of infinite length and width (e.g., the infinite slope stability model), a concatenation of rigid bodies (e.g., Janbus model), or a 3D representation of the slope failure (e.g., Hovlands model). In this paper, the applicability of four slope stability models is tested at four shallow landslide sites where information on soil material and landslide geometry is available. Soil samples were collected in the field for conducting respective laboratory tests. Landslide geometry was extracted from pre- and post-event digital terrain models derived from airborne laser scanning. Results for fully saturated conditions suggest that a more complex representation of landslide geometry leads to increasingly stable conditions as predicted by the respective models. Using the maximum landslide depth and the median slope angle of the sliding surfaces, the infinite slope stability model correctly predicts slope failures for all test sites. Applying a 2D model for the slope failures, only two test sites are predicted to fail while the two other remain stable. Based on 3D models, none of the slope failures are predicted correctly. The differing results may be explained by the stabilizing effects of cohesion in shallower parts of the landslides. These parts are better represented in models which include a more detailed landslide geometry. Hence, comparing the results of the applied models, the infinite slope stability model generally yields a lower factor of safety due to the overestimation of landslide depth and volume. This simple approach is considered feasible for computing a regional overview of slope stability. For the local scale, more detailed studies including comprehensive material sampling and testing as well as regolith depth measurements are necessary.(VLID)459071

Engineering-Geological Analysis of a Subaerial Landslide in Taan Fiord, Alaska

On 17 October 2015, a large-scale subaerial landslide occurred in Taan Fiord, Alaska, which released about 50 Mm3 of rock. This entered the water body and triggered a tsunami with a runup of up to 193 m. This paper aims to simulate the possible formation of a weak layer in this mountainous slope until collapse, and to analyze the possible triggering factors of this landslide event from a geotechnical engineering perspective so that a deeper understanding of this large landslide event can be gained. We analyzed different remote-sensing datasets to characterize the evolution of the coastal landslide process. Based on the acquired remote-sensing data, Digital Elevation Models were derived, on which we employed a 2D limit equilibrium method in this study to calculate the safety factor and compare the location of the associated sliding surface with the most probable actual location at which this landslide occurred. The calculation results reflect the development process of this slope collapse. In this case study, past earthquakes, rainfall before this landslide event, and glacial melting at the toe may have influenced the stability of this slope. The glacial retreat is likely to be the most significant direct triggering factor for this slope failure. This research work illustrates the applicability of multi-temporal remote sensing data of slope morphology to constrain preliminary slope stability analyses, aiming to investigate large-scale landslide processes. This interdisciplinary approach confirms the effectiveness of the combination of aerial data acquisition and traditional slope stability analyses. This case study also demonstrates the significance of a climate change for landslide hazard assessment, and that the interaction of natural hazards in terms of multi-hazards cannot be ignored

Geometry-Based Preliminary Quantification of Landslide-Induced Impulse Wave Attenuation in Mountain Lakes

In this work, a simple methodology for preliminarily assessing the magnitude of potential landslide-induced impulse waves’ attenuation in mountain lakes is presented. A set of metrics is used to define the geometries of theoretical mountain lakes of different sizes and shapes and to simulate impulse waves in them using the hydrodynamic software Flow-3D. The modeling results provide the ‘wave decay potential’, a ratio between the maximum wave amplitude and the flow depth at the shoreline. Wave decay potential is highly correlated with what is defined as the ‘shape product’, a metric that represents lake geometry. The relation between these two parameters can be used to evaluate wave dissipation in a natural lake given its geometric properties, and thus estimate expected flow depth at the shoreline. This novel approach is tested by applying it to a real-world event, the 2007 landslide-generated wave in Chehalis Lake (Canada), where the results match well with those obtained using the empirical equation provided by ETH Zurich (2019 Edition). This work represents the initial stage in the development of this method, and it encourages additional research and modeling in which the influence of the impacting characteristics on the resulting waves and flow depths is investigated

Improving the performance of a dynamic slope stability model (TRIGRS) with integrated spatio-temporal precipitation data

Most shallow landslides are triggered by prolonged or short intense precipitation events. In dynamic physically-based model approaches for landslide susceptibility assessment, the input precipitation data is often derived from a single or a small number of rain gauges. However, precipitation patterns show a high variance in their spatial distribution that is insufficiently captured by standard rain gauge networks, particularly if inter-station distances are large. Spatially distributed weather radar-derived rainfall products have been used as input for physically-based landslide models to overcome the shortcomings of interpolated station measurements. However, the use of weather radar precipitation in physically-based modelling is not straightforward, since it represents an indirect measurement and thus requires pre-processing steps. With this in regard, the Integrated Nowcasting through Comprehensive Analysis (INCA) system (publicly released by GeoSphere Austria) provides historical (from 2011) hourly precipitation data at a 1 x 1 km resolution that combines weather radar data, station data and elevation data for the inclusion of elevation effects. The result is a pre-processed dataset that integrates the quantitative accuracy of station data with the spatial information provided by the radar data. In this study, we investigate whether the use of INCA precipitation data leads to improved model performance of TRIGRS compared to a conventional set-up using station data. We model slope stability in a 53 km2 sub-catchment located in South Tyrol (Italy) for an event that occurred in August 2016 with the INCA data and with precipitation data derived from a single station. The study compares the performances of the two model set-ups and their required parameter calibrations. First tests indicate that the model set-up using INCA data outperforms the station data set-up, as the spatial trend present in the INCA dataset of the modelled storm event follows the spatial trend present in the landslide inventory. In earlier studies and in a preliminary comparison with station data from South Tyrol, the historical INCA data was also shown to underestimate higher precipitation intensities, indicating that the two model set-ups require separate parameter calibrations. In future research, the calibrated model using the historical INCA dataset could be used with the nowcasting datasets from INCA to investigate if and how the INCA dataset can be used for landslide early warning systems. This study is related to the PROSLIDE project that received funding from the research program Research Südtirol/Alto Adige 2019 of the Autonomous Province of Bozen/Bolzano (Südtirol/Alto Adige). In addition, the study also made use of the High-Performance Computing systems at the University of Innsbruck