Behavior Evaluation of Geogrid–Polyurethane Composite-Stabilized Ballast

A new method of ballast stabilization wherein a layer of geogrid is placed toward the bottom of the ballast layer, and the top zone of ballast beyond the geogrid influence zone is treated with polyurethane, was investigated in this study. The deformation and degradation behavior of geogrid–polyurethane composite-stabilized ballast (CSB) was assessed based on large-scale cyclic tests conducted at a frequency ($f$) of 32 Hz. The test results indicated that the CSB had significantly reduced vertical and lateral deformations, by about 63% and 65.9%, respectively, compared with those of unstabilized ballast (USB), and by 38.8% and 26% compared with those of geogrid-stabilized ballast (GSB). Furthermore, the composite stabilization helped to improve the resilient modulus from 216.75 to 280.83 MPa, the damping ratio from 0.230 to 0.340, and the track stiffness from 41.33 to $68.13\mathrm{MN}/\mathrm{m}$ compared with the values for USB. The breakage of ballast also was reduced significantly owing to ballast–polyurethane bonds that remained intact even after 250,000 load cycles. It was found that the geogrid placement location and its corresponding influence zone play a significant role in the overall performance of CSB. The test results further indicated that the geogrid when placed 76 mm from the ballast–subballast interface and a polyurethane treatment depth of 152 mm measured from the sleeper soffit exhibited significantly improved performance under cyclic loading.