Gioacchino (Cino) Viggiani
Laboratoire 3S, INPG - UJF - CNRS, Grenoble, France
Experimental observation of shear banding in stiff clays

Plane strain compression tests allowed for detailed observations of the onset and development of shear banding in undisturbed samples of a French soft marl. The tests were performed from isotropic stress conditions, and the shearing stress was applied under either drained or undrained strain-controlled compression. Together with the measurement of boundary forces and deformations, stereophotogrammetry was used to capture non-homogeneous deformation throughout a test. Systematic analysis of photographs of the deforming specimen allowed for measuring deformations and determining strain fields throughout the test, that is: prior to, at, and after the onset of strain localization. In all tests, the onset of shear banding occurred while the overall response was dilatant, as indicated by a negative rate of excess pore pressure (in an undrained test) or positive rate of the volumetric strain (in a drained test). In no case the onset of a globally dilatant behavior was detected after strain localization.

However, where available, measured pore water pressure were different near the band and away from the band when the non-uniform deformation began. The concentration of straining at the band location gives rise to (negative) excess pore pressure concentrations at these locations (even for the low rate of deformation imposed) which makes it difficult to evaluate the effective stress conditions in the band, and impossible to do so based on standard measurements of pore pressure.

The differences between the observed pore pressures are related to a large extent to the distance of the measuring probe from the shear band. The different pore pressure response close to the band establishes a hydraulic gradient within the specimen resulting in non-uniform pwp during the test. Even the "drained" tests are in reality partially drained tests, where the effects of the generation of excess pwp at the band are masked to a greater extent than in a (globally) undrained test, by the greater amount of pressure equalization which occurs in a drained test due to the drained boundaries. There is a significant difference between a globally drained and a locally drained response. The same concept applies to globally undrained response, where local drainage and volume changes can occur close to the localized zone. The experimental observations presented in this paper emphasize the importance of considering pore water pressures: a coupling between the effective stress and pore water responses must be incorporated in any model to understand strain localization and shear banding in saturated fine-grained geomaterials.

References
  1. Viggiani G., Finno R.J., Harris W.W. (1994) - Experimental observations of strain localisation in plane strain compression of a stiff clay.  In: Localisation and Bifurcation Theory for Soils and Rocks, R. Chambon, J. Desrues and I. Vardoulakis editors, Balkema, 189-198.
  2. Viggiani G., Marello S., Lenti L., Desrues J. (2003) - Shear bands in plane strain  compression  of stiff clay. (in preparation, to be submitted)

First reference can be obtained as a pdf file, just ask to cino.viggiani@hmg.inpg.fr

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