Drained triaxial axisymmetric compression tests are performed on water-saturated short cylindrical samples of nearly monodisperse glass beads, initially assembled in a loose state by a moist tamping technique. Both deviator stress and volumetric strain epsilon, measured as functions of axial strain epsilon, for different strain rates, are affected by stick-slip events of very large amplitude, while the classical behavior of loose, contractant granular assemblies, approaching the critical state for large epsilon, corresponds to the upper envelop of the stress-strain behaviour. Those events consist in (i) a very fast (slip) part in which a drop of coincides with a jump of epsilon (contraction), while loss of control of epsilon and generation of pore pressure signal a dynamic collapse of the material structure triggered by an instability; and then (ii) a quasi-static (stick) part in which the sample regains its strength and, over a short strain interval, behaves similarly to a denser system that dilates before reaching its critical state. A unique stress-dilatancy relation applies to all stick-slip events. Apparent internal friction angles and effects of strain rate and confining pressure are discussed, and it is argued that stick-slip instabilities originate in physico-chemical aging phenomena coupled to contact mechanics. © 2012 Springer-Verlag Berlin Heidelberg.