Dynamic properties of flooded railway ballast

Railway ballast is granular media derived from crushed rock-based local materials from various sources. Despite the diversity of sources, railway ballast is the main component in ballasted railway track systems. It is installed under the railway sleeper to correctly align the track geometry; absorb dynamic wheel/rail interaction forces, preventing the underlying railway track subgrade from excessive stresses; enable the interlocking of skeleton track onto the ground; and provide lateral track stability. Generally, the multi-body simulation (MBS) of train track interaction idealises ballast as a spring-dashpot system. The dynamic modelling 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 whether the ballast may experience 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 model of rail ballast submerged under the flood condition. By using the impact-excitation responses, best-fitting method is used for modal parameter identification of flooded ballast in a frequency range of 0-500 Hz. This study is the world first to identify dynamic parameters of flooded ballast as the modal mass, dynamic stiffness and dynamic damping coefficient, all of which are necessary for dynamic coupling vehicle-track modelling in an extreme event.