Cost Effective Design Of Rail Structural Fill Zone And Its Application In Southern Sydney Freight Line

Rail track structure is a layered system. Between the rail ballast and the subgrade, a subballast layer (comprising a capping layer and a structural zone fill layer) is often required to reduce the induced stress and accumulated strain at the top of the subgrade. Design of the subballast layer, in particular the structural zone fill layer, has normally been carried out based on empirical rules or authority guidelines which have standard, and often exclusive, specification for the material. For example, Railcorp and ARTC Earthworks Standards specify a structural zone fill CBR value of 8% and provide required structural zone thickness based on different subgrade strength/stiffness. The use of higher quality structural zone fill material with CBR values in the order of 20% to 40% may significantly reduce the rail structural fill zone thickness and hence generate cost savings while not compromising the long term performance of the rail structure. However, no design guidelines are available for design. This paper presents the development of a new design method for design of structural zone thickness considering the use of higher quality structural zone fill material, and the application of this design method in Southern Freight Link (SFL).