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Polydisperse granular avalanche impact on civil engineering structures

Final Report Summary - GRAINPACT (Polydisperse granular avalanche impact on civil engineering structures)

{A summary description of the project objectives}

Avalanches of granular geomaterials (rock, soil, snow, ice, etc.) are frequent and pose varying degrees of risk to land use, infrastructure, and personal safety in mountainous areas of the world.
One of the main goals of avalanche research is to forecast the flow-obstacle interaction in order to (i) design civil engineering structures able to withstand the impact forces and (ii) assess the physical vulnerability of existing structures in avalanche prone-areas. The granular nature of sliding geomaterials is considered the crux of flow-obstacle interaction. The granular mechanisms involved in the flow-obstacle interaction and the induced forces remain poorly understood. In order to provide efficient tools to calculate impact forces exerted by full-scale granular avalanches and to contribute to hazard mapping, vulnerability assessment and risk management, the GRAINPACT project investigated the mechanical behavior of granular flows around obstacles, accounting for grain-size dynamics.
Key objectives of the study were: (1) to improve our fundamental understanding of the dynamics of avalanche-flows around obstacles and the induced forces by paying attention to the role of grain-size dynamics with the help of innovative discrete numerical simulations, laboratory tests and field-based observations backed-up with continuum mechanics and granular physics, (2) to develop universally recognized reliable methods for the design of efficient protection structures built to brake, divert and stop the avalanche-flow in run-out zones, and of safe civil engineering structures, (3) to introduce overall final project deliverables to wider European and international communities (researchers, practitioners, decision makers, general public) through publications, presentations and outreach activities.

{A description of the main results and conclusions of the project}

-in collaboration with Prof. Itai Einav, head of the Sydney Centre in Geomechanics and Mining Materials, at the School of Civil Engineering of the University of Sydney, Dr. Thierry Faug (the fellow) designed a new granular chute which enables to produce standing jumps formed in flows of dry granular materials down an incline, and to study for the first time the shape of standing granular jumps. New equations were developed to describe the granular jumps over a wide range of incoming flow conditions. The main results are published (Faug et al., ACMSM-2014; Faug et al., Phys. Fluids 2015). The newly-established theory for compressible and frictional shocks contributes to basic knowledge on the rheology of granular flows and has a number of applications not only for granular flows and the functional design of avalanche protection dams, but also for water flows and the design of dissipative structures in hydraulics (Méjean et al., article under preparation; PhD thesis of Ségolène Méjean, co-supervised by Dr. T. Faug and Prof. I. Einav, funded by the ANR Laboratory of Excellence Tec21).

-Dr. T. Faug, conducted a comprehensive review of the force experienced by slender objects immersed in granular flows, and developed a depth-averaged hydrodynamical description, which can explain the force amplification observed in the depth-dependent granular force regime, as well as the transition toward the velocity-squared dependent granular force regime. This work was published (Faug, Eur. Phys. J. E 2015). In main collaboration with Dr. Betty Sovilla (SLF/WSL, Switzerland), the model was successfully applied to a well-documented snow avalanche event that impacted two pylon-like structures at Vallée de la Sionne test-site operated by SLF. The results were published (Sovilla et al., CRST 2016).

-the fellow, Dr. T Faug, revisited and extended the depth-averaged analytical solutions for free-surface gravity-driven flows of dry granular materials impacting a wall that spans the entire width of the incoming flow. This theoretical work was published (Faug, Phys. Rev. E 2015).

-advanced and original discrete numerical simulations on the force experienced by (i) walls in lid-driven cavity granular flows and (ii) walls overflowed by granular flows allowed François Kneib (PhD student), Dr. T. Faug (the fellow, main PhD thesis supervisor), Prof. F. Dufour (3SR/CNRS, PhD thesis co-supervisor), and Prof. M. Naaim (Irstea) to evidence a depth-dependent granular force regime, which is strongly controlled by the \mu(I) rheology of dense granular flows. This work was published (Kneib et al., Comp. Part. Mech. 2015). The force fluctuations are also controlled by the \mu(I) rheology (Kneib et al., article in preparation).

-a number of additional results and deliverables are available through other publications and presentations with other collaborators: please refer to the web-site page as indicated at the end of the document.

{Potential impact and use and any socio-economic impact of the project}

The Marie Curie IOF GRAINPACT project has combined theoretical depth-averaged hydrodynamics, small-scale laboratory tests, advanced discrete numerical simulations and field-based observations. Basic knowledge on granular flows and their interaction with different kinds of obstacle was developed, and published in top international journals in physics and mechanics, which are important scientific outcomes of the GRAINPACT project. Furthermore, as one of the international experts in snow avalanche dynamics and the design of protection structures against avalanche-flows, the fellow paid particular attention to the transfer of basic knowledge developed within the GRAINPACT project:

-in narrow collaboration with Dr. Sovilla (SLF/WSL, Switzerland), he demonstrated that the newly-established granular model which he proposed for the force on slender objects (Faug, Eur. Phys. J E 2015) is able to explain the force signal measured on pylon-like structures impacted by granular snow avalanches at Vallée de la Sionne (Sovilla et al., CRST 2016). This important outcome of the GRAINPACT project has a direct impact on the method used in avalanche engineering for the calculation of impact force on structures.

-the new models developed for compressible and frictional granular shocks (Faug et al., Phys. Fluids 2015; Méjean et al., article in preparation) have a direct impact on the methods used in avalanche engineering for the calculation of the height of protection dams, as well as the calculation of avalanche impact forces on walls (part of the Post-doc work of Dr. Adel Albaba, supervised by Dr. T. Faug).

The main outcomes of the GRAINPACT project (new theory for compressible and frictional granular steady jumps and bores, new knowledge about the granular force on slender obstacles and wall-like obstacles, granular force fluctuations, etc.) contributes to advance our understanding of avalanche interaction with civil engineering structures. This will strengthen the existing guidelines regarding avalanche protection dam design (Johannesson et al., 2009, Handbook edited by the European Commission), as well as the quality of transfer of training toward engineers in charge of the design of efficient protection structures built to brake, divert and stop the avalanche-flow in run-out zones, and of safe civil engineering structures. Ultimately, the GRAINPACT project is an essential contribution to the vulnerability and risk assessment related to geohazards.

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

Please contact Dr. Thierry FAUG ( ) for any further information about the Marie Curie IOF GRAINPACT project.