Effect of Ballast Degradation on Track Dynamic Behavior Using Discrete Element Modeling

Ballast degradation refers to the traffic use of in-service track when aggregate particles become smaller in size and fines are generated because of particle breakage and abrasion to cause geometry defects and influence the serviceability of ballasted track affecting train operations. To evaluate effects of ballast degradation on track substructure maintenance needs, single crosstie models were developed to simulate new and degraded ballast behavior under train loading. Particle size distributions and shape properties of the new and degraded ballast materials studied were adopted from previous laboratory tests conducted at the University of Illinois, U.S. Crosstie vibrations, ballast particle contact force distributions and particle rotations, and localized contact force chain networks were compared among different single crosstie models incorporating new and degraded ballast. The following key observations were made based on the simulation results: (1) crosstie on degraded ballast exhibits larger vibrations and more inconsistent vibration patterns; (2) degraded ballast shows a higher number of localized force chains with more particle contacts while the characteristic trend observed for new ballast is the presence of more symmetric force chain networks, which might be attributed to the new ballast layer’s uniform gradation and good interlock; (3) average normal contact forces are distributed more uniformly in new ballast in both horizontal and vertical directions as opposed to these forces concentrated more in vertical direction in degraded ballast; and (4) more ballast particles experience large rotations in degraded ballast and the related impact zone, which encloses all particles with large rotations, is also broader and deeper.