Effect of rubber content on the unstable behaviour of sand-rubber mixtures under static loading: a micro-mechanical study

While it has been acknowledged from cyclic laboratory tests that the inclusion of rubber particles increases liquefaction resistance, little attention has been paid to the influence of rubber content on stress paths that could lead to static liquefaction or collapse. Although laboratory experiments of sand-rubber mixtures under undrained loading have offered macro-scale insights into their stress-strain response, additional understanding at a particle scale under such conditions still remains unexplored. This study presents a series of conventional undrained and constant shear drained monotonic triaxial tests consisting of mixtures of sand (stiff) and rubber (soft) particles simulated by the discrete-element method. Sets of loose mixtures were prepared with rubber contents ranging from 0 to 30% by weight, having the same void ratio. The instability state was found to be dependent on rubber content; however, this was not the case for the angle of the instability line. Besides, instability states reached at higher strengths did not indicate increase in resistance against liquefaction. As rubber content increased, less susceptibility to liquefaction was found, with the sample having 30% of rubber content only experiencing limited reduction of strength. The micro-scale information was obtained for all tests and particular attention was given to the micro-mechanical response at the onset of instability and at liquefaction or quasi-steady state. The trends of micro-mechanical parameters with rubber contents changed depending on test stage, which was related to the different macro-scale response at instability state and at liquefaction or quasi-steady state.