Characterisation of rigid polyurethane foam reinforced ballast through cyclic loading box tests

Abstract

Train speeds and heavy haul axle loads are constantly increasing the forces and stresses experienced by track structures. This is especially true for track transitions that generate high dynamic forces on both the track and vehicles as a result of differing track stiffness values on either side of the track transition. Reducing differential settlement between the two track structures at a track transition is one method of improving the life of the track and increasing maintenance intervals. Ballast attrition and breakdown at these track transition zones is also of major concern as ballast fouling can lead to reduced drainage performance of the ballast as well as a potential loss of strength as the ballast becomes increasingly fouled. In this study rigid polyurethane foam was used as a means to reinforce ballast. Various tests were conducted using a dynamic load hydraulic load frame in a large ballast box test at heavy haul axle loads. Unreinforced, reinforced and 50 % reinforced ballast layers of 300 mm depth were tested to approximately 5,000,000 load cycles. The results showed that rigid polyurethane foam reinforced ballast exhibited in the order of 60 % less settlement for a fully reinforced layer and 42 % less settlement for a half reinforced layer. The increase in layer stiffness with increasing load cycles was also observed for the reinforced ballast layers which is contrast with the decrease in layer stiffness for conventional unreinforced ballast. The use of rigid polyurethane foam (RPF) to reinforce ballast has a number of benefits which could result in better track geometry and longer maintenance cycles resulting in lower overall costs.