ABSTRACT
It is well known that the response of granular materials to mechanical loading is strongly dependent on their initial density. Loose materials are generally contractive, whereas dense materials are dilative. These features have been widely studied in classical drained and undrained triaxial compression or extension. The undrained behaviour is directly linked to the volume change tendency in drained condition. Indeed, loose sand contractancy may lead to the quasi-static liquefaction of a sample when subjected to a monotonic isochoric compression. This particular case of a loose sand specimen under an isochoric loading path can be generalized to other loading paths. Dense and dilative sand samples can also liquefy as can loose ones, if the volumetric strain rate imposed is more dilative than the volumetric strain rate measured at failure. Similarly, a loose material may liquefy not only along the isochoric path but also along some imposed contractive volumetric strain rate (Jrad et al., 2012, Daouadji et al., 2013, Nicot et al, 2014), as long as the latter is less contractive than the contractancy developed by the sample during a drained compression. This study attempts to extend the discussion on the dependency of the mechanical response to the imposed volumetric strain rate (or the imposed strain direction) by presenting numerical simulations with a micromechanical approach.The constitutive response along proportional strain loading is discussed with regard to the natural dilatancy simulated along classical drained triaxial compression. This discussion is then extended to the question of material instability along proportional strain paths.
