Stick-slip behaviours of dry glass beads in triaxial compression

Consolidated undrained triaxial compression tests are conducted on standard cylindrical samples of nearly monodisperse glass beads, initially assembled in a dense state by a funnel pouring technique. Deviator stress, volumetric strain and induced pore air pressure are measured as functions of vertical strain under different confining stress and vertical strain rates. We find that in stick phase the glass beads accumulate energy and regain strength, which can lead to slow sliding in volumetric strain curve without stress drop in deviatoric stress curve for confining stress of 1000 kPa. Particularly after the previous slip event, as the deviatoric stress increases, the volume of sample repetitively compacts and then dilates before reaching the next slip events under high confining stress, which never happens at the lower. In slip phase, a stress drop coincides with a jump of volumetric strain and vertical strain, which make the pore air pressure grow sharply. The effects of confining stress and vertical strain rate on stick-slip instabilities are discussed in dry dense glass beads samples.