ABSTRACT
This chapter presents the description of micromechanical based Discrete Element Modelling (DEM) that has been implemented in computer software PFC2D along with a detailed investigation on the modelling of irregular ballast particles and densification and breakage of ballast during cyclic loading. The calculation cycle in PFC2D is a time-stepping algorithm that consists of the repeated application of the law of motion to each particle, a force-displacement law to each contact, and a constant updating of wall positions. The DEM keeps track of the motion of individual particles and updates any contact with neighbouring particles by using a constitutive contact law. The constitutive model acting at a particular contact consists of three parts: a stiffness model consisting of a linear or a simplified Hertz-Mindlin Law contact model, a slip model, and a bonding model consisting of a contact-bond and/or a parallel-bond model.
