Estimation of the dynamic stiffness of railway ballast over a wide frequency range using the discrete element method

Abstract

Ballast is an integral part of railways, serving as a load transfer medium between the rails and the subgrade. In analytical modeling of track dynamics, ballast is idealized as a spring with constant stiffness at all excitation frequencies, which is a reasonable assumption for studying track dynamics at low frequencies up to 100 Hz. For modeling railway noise, the frequency dependency of ballast stiffness and damping at high frequencies is of utmost importance. Since ballast is a granular material whose properties depend on the packing, density and size distribution of the particles, the discrete element method is a promising tool for determining the frequency-dependent ballast stiffness. In the current work, a novel approach is presented to determine the frequency dependency of dynamic stiffness of the ballast used by the Swiss Federal Railways, using Discrete Element Method (DEM). First, a characterization of the Hertzian contact parameters required for modeling the ballast is performed. The effectiveness of the discrete element method for modeling the dynamic behavior of the ballast is evaluated by comparing the results obtained from our calculations with experimental results obtained under two different boundary conditions. The evaluated dynamic stiffness varies significantly with the frequency of excitation, especially above 100 Hz. The validated DEM model is then used to determine the effects of preloads on the dynamic stiffness.