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Triggering Mechanisms of Snow Slab Avalanches

Final Report Summary - TRIME (Triggering Mechanisms of Snow Slab Avalanches)

{a summary description of the project objectives}

Snow avalanche is the most frequent type of slope failure in mountainous areas of the world. In total, approximately 15% of Europe’s and 6% of Earth’s land may be considered as avalanche prone-areas. Avalanches claim hundreds of lives annually worldwide and there have been historical cases with death tolls between hundreds and thousands of people during one event or winter season.

One of the main goals of snow avalanche research is to forecast natural or induced mechanical failure of a sloping layered snowpack in order to estimate potential damage to human activities in mountain regions and to be able to conduct risk-oriented planning. The deadliest type of snow avalanches are slab avalanches, which start from a failure of a cohesive horizon of snow, or “slab”, underlaid by a so called “weak layer”. The nature of weak layer deformation is considered the crux of slab stability. At present, the origin of initial failure and the failure process in the weak layer remain poorly understood.

In order to contribute to development of a mechanical model of snow fracture, and to provide tools to estimate initial conditions for avalanche dynamic models for hazard mapping and risk management, we proposed to investigate the mechanical behavior of weak layers and their relationship to slope-scale instability. Since forecasting of natural or induced snow avalanches requires a holistic approach, which couples experiments, in situ measurements and numerical modeling, we applied such integrative approach focused on new instrumentation, indoor and outdoor experiments and computer modeling investigating the mechanical behavior of snow as related to slope instability.

{a description of the work performed since the beginning of the project}

- A strong collaboration was built with a group of Italian engineers (Politecnico di Torino) and cold laboratory facilities of the Centre Etude de la Neige (Meteo-France) for developing, improving and testing a new instrumental apparatus for measuring snow failure properties.

- Through intense collaborative cold laboratory and mountain field work we obtained a vast amount of records about strength properties of different types of failure plains within the snowpack. Key observations were based on shear-box tests supported with high-frequency pressure sensors and high-speed video records of failure process. Measurements also included other instrumental techniques like Specific Surface Area measuring system, microphotography, and etc.

- Numerical simulations of snow samples under stress were performed at the super-cluster of computers and validated against previous experimental results. Modeling was based on Finite Element Method (FEM), which was carefully reviewed for creating a key literature reference for other scholars.

- Supplementary statistical analysis of snow avalanche disasters in Russia was conducted in collaboration with IRSTEA experts, Russian and Japanese scholars.

{a description of the main results}

- Results of analysis exploring complex influence of sintering process on shear strength of snow interfaces, which were possible due to the new instrument and which are important for avalanche release forecasting, were already published (Podolskiy et al., ISSW-2013, 2013a; Podolskiy et al., TCD, 2014a; for bibliography, please, refer to the web-site as indicated at the end of the document).

- A comprehensive review of existing FEM numerical approaches to snow slab avalanches and snow mechanics was conducted and published (Podolskiy et al., JoG, 2013). Hopefully, such overview of the state-of-the-art will become a valuable reference and a stepping stone for researchers and engineers in order to couple best existing methods with experimentally obtained results.

- As a step towards development of numerical model describing the mechanical behavior of a snow slab on a slope we attempted to construct and test a new FEM-based model allowing to study meso-scale weak-layer behavior under complex stresses (Podolskiy et al., ISSW-2012, 2012; Podolskiy et al., NHESSD, 2014).

- Moreover, we used this opportunity to investigate factors driving high snow avalanche disaster rates in cold regions on example of Sakhalin (Russia) through statistical analysis of unique archive data, which showed that intense colonization of mountain terrains (like the one ongoing rapidly today in many developing regions) can be the main cause of an increased number of incidents (Podolskiy et al., ISSW-2013, 2013b; Podolskiy et al., JoG, 2014).

{conclusions about final results and their potential impact and use (including the socio-economic impact and the wider societal implications)}.

The final conclusions can be described together with corresponding context and potential use of results as follows. (We also note that one more full-length original research paper, based on Particle Image Velocimetry analysis of high-speed video records of snow-weak layer fracture, is in preparation and will be published elsewhere [Podolskiy et al., manuscript in preparation]).

- The new shear instrument is operational for both cold laboratory and in situ field work to study the mechanical behavior of snow failure plains. Therefore, in coming winter seasons it will serve as an engine for producing valuable operational mechanical laws of different types of alpine snow (Podolskiy et al., ISSW-2013, 2013a; Podolskiy et al., TCD, 2014). Up to now several academic and industrial representatives have already showed their interest in joint application of the technique (in France and Japan).

- Natural sintering in ice is a fundamental process determining mechanical properties of various ice forms. According to the literature, limited data are available about the complex subjects of snow sintering and bond formation. In this project, through cold laboratory mechanical tests with a new shear apparatus we demonstrate time-dependent effects of isothermal sintering on interface strengthening at various normal pressures. Measurements showed that interfacial strength evolved rapidly, conforming to a power law (mean exponent = 0.21); higher pressure corresponded to higher initial strength and sintering rates. Our findings are consistent with observations on homogeneous snow, provide unique records essential for slope stability models and indicate the significant importance of normal load on data interpretation.

- More than forty years ago the Finite Element method, or FEM, became one of the most common tools in a structural engineer's tool kit and also emerged as a common instrument of analysis in snow science. A significant number of studies focused on a wide spectrum of questions, ranging from microstructure modeling based on X-ray derived geometry to studies on snow avalanches, skier and explosive loads, optimal snow fence design or tire-snow interaction. Various mechanical problems, even if sometimes comparable in principal, were addressed from different angles due to the unique combination of constitutive, numerical and parametric approaches chosen in each study. In the present project we summarized the evolution and growing sophistication of multiple approaches by providing the most essential details of all previous publications in the form of a tabulated review of major FEM studies on snow mechanics (performed between 1971 and 2012). We hope that this tabulated summary of 40 papers will serve for facilitating comparison / merging between different mechanical approaches or numerical recipes and for future reference, which may be useful for solving the many remaining problems.

- Snowpack weak layers may fail due to excess stresses of various natures, caused by snowfall, skiers, explosions or strong ground motion due to earthquakes, and lead to snow avalanches. Numerical part of the research presents a model describing the behavior of “sandwich” snow samples subjected to shaking. The Finite Element model treats weak layers as interfaces with variable constitutive behavior parameters. This approach is validated by reproducing cyclic loading snow fracture experiments. The model evaluation revealed that the Mohr–Coulomb failure criterion, governed by cohesion and friction angle, was adequate to describe the experiments. The “best fit” cohesion and friction angle were 1.6 kPa and 22.5–60, indicating that the cohesion mainly determines the outcome of tests. The model showed complex, non-homogeneous stress evolution within snow samples and especially the significance of tension for fracture initiation at the edges of the weak layer, caused by dynamic stresses due to shaking. Accordingly, the previously used analytical solution, ignoring the inhomogeneity of tangential and normal stresses along the failure plane, may incorrectly estimate the shear strength of weak layers. The obtained parameters may constitute valuable elements in mechanical models used for avalanche forecasting.

- Finally, in the course of the study our team discovered through unique statistical records that the Russian islands of Sakhalin and the Kuril make up the world’s deadliest avalanche-prone region. This highlights that, indeed, snow avalanche research in the country of the fellow has a lot of challenging and stimulating questions for scientists, engineers and policy-makers, which will have to be considered for a sustainable development of the region.

We believe that ultimately our instrumental, experimental and numerical contributions will allow to advance understanding of snow avalanche formation and impacts and therefore to reduce their destructive potential on mountain residents and infrastructure.

More information about the project and its deliverables may be found at the following web-page:

http://trime.irstea.fr