Francesco Froiio1, Gioacchino Viggiani2, Farid Laouafa 3
1 DIC, Universitá di Tor Vergata, Rome, Italy (froiio@ing.uniroma2.it)
2 Laboratoire 3S, CNRS - UJF - INPG, Grenoble, France (cino.viggiani@inpg.fr)
3 Direction Risques du Sol et du Sous-sol, INERIS, Verneuil en Halatte, France (Farid.Laouafa@ineris.fr)
Numerical photogrammetry in laboratory experiences on 2D slopes

Whole-field and non-intrusive measuring was performed, with a numerical photogrammetry technique (Particle Image Velocimetry), on the incremental displacement and velocity fields of 2D slope models. Different kinds of boundary conditions on the displacements where applied quasi-statically; "regular" deformation sceneries as well as shear-band formation and sudden diffuse collapse were observed.

This work was motivated by the need of a better understanding of phenomena like shear banding and material instability in granular materials, that are not satisfactorily interpreted by current modelling (Darve and Laouafa, 2000). Digital PIV having been originally developed to measure plane velocity fields in fluids, gases and flame (Grant, 1997), this work is also an attempt to optimise this now-available technique on laboratory slope-like specimens realised with Schneebeli analogue material.

Two tests, with different boundary conditions, were run for two respective specimens, using a specific loading device (Joer, 1992; Bilotta et al., 2002) In a first tests a phase of homogeneous controlled deformation was followed by a sudden collapse turning into a quick surface flow. In the second test, shear-banding was induced in the specimen without loosing control on the deformation.

The measures relative to the phases of controlled deformation, in the both tests, were found to be in very good agreement with the visual observation of the motion. They were confirmed by direct "manual" measures of rod displacements on the images. At the velocity rise at collapse in the first test, the obtained vector fields were in reasonable agreement with the visual observation, but less clear results - and only a qualitative picture of the motion - were obtained as the granular flow became more rapid and turbulent: it was found that this kind of fast processes require a specific design of tracer-particle seeding, and probably higher image-acquisition frequencies than those allowed by standard professional video-cameras.

References
  1. Bilotta E., Flora A., Lanier J., Viggiani G. (2002), Experimental observation of the behaviour of a 2D granular material with inclusions. Rivista Italiana di Geotecnica, 2 (2002): 9-22.
  2. Darve F., Laouafa F. (2000). Instabilities in granular materials and application to landslides. Mech. Choes. Frict. Mater. 5 (8): 627-652.
  3. Grant I., Particle Image Velocimetry: a review Proc. Institution of Mechanical Engineers, (211): 55-76.
  4. Joer H. A. (1992). 1gamma2epsilon: une nouvelle machine de cisaillement pour l'étude du comportement des milieux granulaires. PhD. Thesis. UJF, Grenoble.
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