Effects of various loading stress paths on the stress-strain properties and on crushability of an industrial soft granular material

International audienceMechanical characteristics (i.e., stiffness, internal friction angle, peak strength) and crushability of a soft granular material were evaluated by performing a comprehensive series of laboratory tests using the following devices: standard and non-standard triaxial apparatus, direct and annular shear box, oedometer and hydrostatic devices. The initial tested specimens differ by initial void ratio, grading characteristics and particle hardness. The air-dried specimen of soft particles were then subjected to monotonic loadings for various stress paths (direct and annular shear stress paths, oedometer stress paths until different upper normal pressures, triaxial stress paths including different confining pressures). After each homogeneous test, sieving has been performed in order to characterize the evolution of grading characteristics of the granular packing. Experimental results on mechanical properties show that maximum internal friction angle is rather independent of the particle stiffness even though small differences may exist before peak stress-state. As highlighted by recent studies (Arslan in Granul Matter 11(2): 87-97, 2009), the volumetric response of the specimen indicates that classical critical state is no more a relevant framework when particle crushability is too high compared with the applied stress-state. Crushability related to loading paths has been evaluated through the relative breakage ratio (Br). The first results pointed out the effects of initial geometrical configuration (i.e., void ratio, grading) and particle stiffness. Analysis of the stress paths effects on the amount of breakage revealed that stress-state is not sufficient to describe properly breakage undergone by the material which is confirmed by an obvious link between volumetric strain and total breakage. Finally, the present study showed that the percentage of fine particles content during breakage may be seen as a function of the "level" of deviatoric loading paths

Size effects on the strength properties of low cement-mixed granular soils

Cement-mixed gravelly soil (CMG) compacted at an energy level lower than the one for RCC has recently been used to construct bridge abutments for high-speed trains in Japan. A series of studies on the stress-strain properties of CMG were performed to develop the design and construction procedures of such structures (Watanabe et al. 2003; Kongsuprasert et al. 2007, Tatsuoka et al. 2008, Ezaoui et al. 2010). As laboratory stress-strain tests on large specimens (i.e., 30 cm in diameter) on in-situ material are usually very difficult to perform because of large test scale and time limitation (i.e., curing period), it is usual practice to perform tests on small young specimens of materials removing large particles. Among these issues, we discuss scale effect in terms of specimen size in this paper based on results from a series of non-standard drained triaxial compression (TC) tests using three different apparatuses on specimens with volumes different by a factor up to 50 times of low cement-mixed granular material (CMG) compacted at low and high energy levels.Ezaoui A., Tatsuoka, F. Sasaki Y., Furusawa, S., Arakawa, K. Taheri,

Elasticity and Mechanical Behaviour of Granular Materials: Some Insights from Numerical Studies of Simple Systems

International audienceWe briefly review the elastic properties of granular materials, as explored in numerical studies of simple model materials by the "discrete element method " (DEM). Elastic or quasielastic responses are obtained as stable contact networks are probed with negligible friction effects. Elastic moduli, at the macroscopic scale, or contact stiffnesses at the contact network scale, have very limited influence on macroscopic constitutive laws ruling quasistatic deformation and inertial flow. The elastic moduli nevertheless provide useful indirect information on internal variables such as coordination and fabric. Singularities in the tensor of elastic moduli are related to the proximity of failure in the microscopic sense (contact network) but not in the macroscopic sense (yield condition). Elastic properties are also useful in the characterization of the directional dependence of incremental stress-strain response, a key ingredient in the identification of instabilities causing localization phenomena