Cyclic and Postcyclic Triaxial Testing of Ballast and Subballast

Ballast and subballast are two layers of noncohesive, granular materials with different grain sizes filled in the track substructure to bear traffic loadings from train passages. The performance and sustainability of a track substructure depend on both of these two granular layers. This paper presents the experimental results of cyclic triaxial testing and postcyclic triaxial testing on individual ballast and subballast materials. Also, to investigate mechanical behavior at the ballast/subballast interface, cyclic tests were performed on composite specimens consisting of the upper half ballast layer and underlying half subballast layer. These cyclic tests indicated that the existence of the ballast/subballast interface depresses the development of permanent axial or volumetric deformation at the interface zone, and this effect vanishes as the initial compaction and confining pressure increase. Postcyclic failure tests show that the ballast/subballast interface zone's strength is close to that of the ballast material but lower than that of the subballast material.