Microgrid reinforcement of scaled railway ballast

Ballasted tracks are predominant on railways worldwide. Previous research on random fibre reinforcement of scaled ballast has shown that flexible polymeric fibres can increase the shear resistance and ductility of the ballast. Owing to the difficulty of testing very coarse materials in standard laboratory apparatus, scaled-down materials have been introduced. A coarse aggregate with a particle size distribution curve parallel to that of full-scaled ballast can be used to investigate relevant shear stress-strain properties. Geogrids, usually as continuous sheets, interlock with aggregate particles, enhancing the overall shear resistance and stiffness. This paper investigates the reinforcement effectiveness of individual microgrids. obtained from a commercially available geogrid, mixed with a granular material corresponding to ⅓-scaled railway ballast. Packing tests were used to inform the fibre mix design. The effect of microgrid shape and content on the mechanical behaviour of ⅓-scaled ballast was investigated by means of a series of monotonic triaxial tests. Microgrids behave similarly to strip fibre random reinforcement, in reducing dilatancy while increasing the deviator stress. However, a benefit compared with strip fibre reinforcement is that the reduction in dilatation arising from the disruption to the packing caused by the microgrids does not result in a loss of stiffness.