Abstract: Geogrid is used to reinforce the trackbed by increasing the confining pressure and reducing the vertical cumulative settlement and lateral displacement of ballast. In order to further study the reinforcement mechanism of geogrid reinforced ballast, pull-out tests of single and double-layer geogrids with large and small apertures under static load were carried out by using self-designed equipment, and the influences of grid aperture, reinforcement depth and number of layers on pull-out resistance were analyzed. The standard values of pull-out resistance at different depths were obtained by single-layer tests, and the coefficient of double-layer reinforcement was proposed to quantify the reinforcement capacity of double-layer geogrids, and the best laying way was also discussed. Double-layer geogrids with aperture size of 65 mm placed in 200-300 mm were simulated by DEM. The distribution of internal stress chain and contact force vector of geogrid reinforced ballast during pull-out process were analyzed from the micro view. The results show that, for the graded ballast, the geogrid with 65 mm aperture performed better than that with 32 mm aperture; the ultimate pullout resistance of double-layer laying was greater than the sum of the standard values of the two-layer laying. The double-layer reinforcement effect is related to the geogrid aperture and the layer spacing. The simulation results also verified the superposition of reinforcement effect between the two-layer geogrids during pull-out process, and the distribution of normal contact force vector showed that the double-layer geogrid had interlocking reinforcement effect.

Key words: ballast, geogrid, pull-out test, double-layer reinforcement, reinforcement effect coefficient, DEM