Mechanics of snow avalanches and interaction with structures

The interaction between snow avalanches and structures represents a topic of interest both from a scientific point of view, since different study domains and knowledge are involved (structural mechanics, fluid dynamics…), and due to its applicability in practice for a correct design of structures located in avalanche risk areas. In this thesis the interaction between the snow avalanches and structures is investigated together with the avalanche dynamics. Chapter 1 deals with the state of the art of the avalanche dynamics and interaction between snow in movement and structures. The snow avalanches are classified, giving the basics concepts. Secondly the different approaches to study the interaction between avalanches and structures are analysed. The observations of the damages caused on structures by real events are not sufficient to understand all the complex processes inner the dynamics itself and the impact strictly. Furthermore experiments are carried in order to analyse deeper velocity profiles, to which pressure ones are linked, entrainment of snow, from which the volumes involved depended as well as the pressure behaviour. In fact pressure values evolve in time and in space and change with the obstacle shape. Experimental studies are made at real scale avalanches, in the test sites, or at reduced scale, in laboratory chutes. To translate the results from the small scale to the real one similitude criteria have to be satisfied. Hence the dimensional analysis is proposed. Another approach to study the problem in object is to use analytical and numerical models. For this reason a summary of the state of art of dynamics models is proposed, focusing the attention on those taking into account the erosion and the interaction with obstacles. From both experimental and theoretical analysis recommendations are born in order to help the expert to correctly design the structures in avalanche areas. In Chapter 2 a new model is described, able to provide the pressure and the velocity in all the points of the avalanche, without impose a proportional relationship between them. The model describe the evolution of the avalanche shape thanks to the level set method, suitable for free-boundary problems, and the Navier-Stokes equations, since the avalanche is considered a fluid. A first validation on experimental data of a laboratory chute is given. Afterwards the attention is set at the avalanche bottom. In particular the boundary condition of the slip velocity is analysed, giving an analytical justification. The slip condition, coupled with a non-newtonian fluid, is able to correctly describe the velocity profile. Finally a new model for the erosion is proposed, starting from general continuum mechanics hypothesis. In particular both the avalanche and the snow at rest are considered as the same fluid having a viscosity depending from the shear rate. It is shown as the model is in agreement with other theories in the literature and takes into account the influence of snow and avalanche properties, the avalanche depth, the slope angle, and the position in the avalanche (front or tail). Chapter 3 focuses the attention on the definition of a model to describe the impact of an avalanche with obstacles. Different approaches can be pursued: a stationary and a transient ones, as well as a two-dimensional analysis in the avalanche depth plane, in the slope plane and a three-dimensional one. Some preliminary simulations are shown and qualitatively compared with the state of the art concerning the impact pressure. For instance the pressure profile along the avalanche depth, the influence on the obstacle shape and dimension, and the dependence on the relative position obstacle-avalanche (directly or not directly exposed) are investigated. In Chapter 4 the new Italian P.ta Seehore test site is described. Its peculiarity is to study the small-medium avalanches that occurred with high frequency, since artificially triggered for safe reasons. The attention is focused on the design of an obstacle, located in the avalanche track, to study the interaction between snow in movement and structure. The static and dynamic test carried to characterise it are shown as well as its instrumentation. Finally an overview of the surveys is proposed focusing the attention on the measurements carried in some events. Chapter 5 deals with the analysis of the measurement data concerning experiments in the P.ta Seehore test site from different point of view. Firstly, the erosion and deposition processes are analysed, using laser scan data, analytical and numerical methods and presenting a new cheap test to detect the net erosion and deposition. Secondly, a commercial dynamics model is applied to obtain the flow density and velocity at the obstacle, data not experimentally recorded. Thirdly, our dynamics model is used for instance to simulate the creation of a dihedral shape upward the obstacle, experimentally measured and to give information on the pressure. Finally analytical approaches are used to describe the pressure, applying for instance the Mohr-Coulomb criterion, to simulate the pressure in the avalanche tail. Concepts reported in the available recommendations, as for instance the compressibility of the snow during the impact are used too. In Chapter 6 applications concerning the impact against houses destroyed in 15th of December 2008 are reported. In particular both the transient and stationary models (in their two and three-dimensional versions) are applied and compared with a back-analysis of damages. General laws for the influence of the impact angle on the pressure are respected as well as the areas of positive and negative pressure. In addition, the protection role played by a house on the structures downstream, especially in term of reduced pressures, is analysed. The Conclusions and outlooks finalize the work.