Dynamic behaviour of railway ballast exposed to flooding conditions

Railway ballast is one of the main components in ballasted railway track systems. It is installed under the railway sleeper to absorb dynamic wheel/rail interaction forces, preventing the underlying railway track subgrade from excessive stresses, enabling the interlocking of skeleton track onto the ground and providing lateral track stability. Generally, the dynamic modeling of ballast gravels relies on the available data, which are mostly focused on the condition at a dry condition. Recent findings show that railway track could significantly experience extreme climate such as long-term flooding. This phenomenon gives rise to a concern that the ballast may experience a higher level of moisture content than anticipated in the past. On this ground, a test rig for estimating the dynamic properties of rail ballast has been devised at the University of Birmingham. A non-destructive methodology for evaluating and monitoring the dynamic properties of the rail ballast has been developed based on an instrumented hammer impact technique and an equivalent single-degree-of-freedom system approximation. This investigation focuses on the dynamic single-degree-of-freedom (SDOF) model of rail ballast submerged under the flood where the dependent effects of frequency can be distinguished. Based on the impact-excitation responses, the analytical state-dependent model was applied to best fit the experimental modal measurements that were performed in a frequency range of 0-500 Hz. The curve fitting gives such dynamic parameters as the modal mass, dynamic stiffness and dynamic damping constant, all of which are required for modern numerical modeling of a railway track.