Track stabilisation with geosynthetics and geodrains, and performance verification through field monitoring and numerical modelling

摘要

All over the world, ballasted railway tracks form one of the major transportation networks designed to provide heavy haul freight and passenger traffic. However, large cyclic loading from heavy axle trains operating at high speeds often causes excessive deformation and degradation of ballast, as well as unacceptable differential settlement of compressible foundation and, or pumping of the soft subgrade soils. The problem becomes more severe under high impact loads due to rail or wheel imperfections, causing accelerated ballast breakage. A proper understanding of load transfer mechanisms and their effects on track deformations are essential prerequisites for minimising maintenance costs. The field trial at Bulli demonstrated that for trains with wheel flats, extremely high stresses were transmitted to the ballast layer. Installing resilient mats such as rubber pads (shock mats) in rail tracks can attenuate impact forces and consequently mitigate particle degradation. In view of this, a series of laboratory tests were carried out using a unique large-scale drop-weight (impact) rig to evaluate the role of shock mats. The field trial also showed that the moderately-graded recycled ballast, when used with a geocomposite layer, could perform well in comparison with traditionally uniform fresh ballast. Both Class A predictions and field measurements at Sandgate proved that relatively short vertical drains would be sufficient to dissipate cyclically induced pore pressures, curtail the lateral movements, and increase the shear strength and bearing capacity of the subgrade. In summary, this invited Special Paper describes in detail the large-scale laboratory tests imperative for material characterisation, fullscale instrumented field trials for performance verification, elasto-plastic finite element analyses for predicting the behaviour of tracks stabilised using shock mats, and geosynthetic products including grids and prefabricated drains.