# Idealisations of Dynamic Modelling for Railway Ballast in Flood Conditions
## 5 Conclusions  
Modern railway tracks have widely adopted railway ballast, which is granular media from crushed rocks, as one of their critical components over centuries. The railway ballast is generally installed under the railway sleeper to align track geometry; absorb dynamic wheel-rail interaction forces; prevent the underlying railway track subgrade from excessive stresses; and enable the interlocking of skeleton track onto the ground providing lateral track stability. Current practices in dynamic coupling vehicle-track simulations idealise track components into MDOF systems. However, to date, only dynamic parameters of dry ballast condition have been investigated. Recent findings show that railway track could significantly experience extreme climate such as long-term flooding. Therefore, it is necessary to identify modal parameters of railway ballast exposed to flooding conditions. This study is the first to highlight such critical conditions. Analytical and experimental studies have been conducted to address such the pressing issue. The modal experimental studies reveal an unprecedented insight into the dynamic properties of the flooded ballast. The flood condition can reduce the stiffness of the track system, whilst also increasing the damping or energy dissipation of the track. It is important to note that this study considered a flash flood case only. In reality, the flood condition can also reduce the load carrying capacity and stiffness of the subgrade layer. In addition, the experiments in this study reveal that a frequency-based condition monitoring technique might have certain limitation in practice. Future work will highlight the development of new SDOF model that is more realistic and more capable of defining state-dependent characteristics of the railway tracks submerged under flood conditions. The influence of impulse energy as well as the track mass will also be investigated in the near future. In summary:  
* This research explored the dynamic properties of the railway ballast under various conditions, and identified that their dynamic properties can change significantly under flood conditions compared with a dry state. * The dynamic modal parameters of the identical ballast material in the dry state are consistently the same regardless of the scale of track masses. Under flood conditions, the resonance frequency shifts are relatively small, the damping increases with the increase of the flooding level, and the stiffness exhibits a decreasing trend. This phenomenon is especially obvious when the conventional concrete sleeper is used for testing the natural ballast. * The changes in water temperature can slightly affect the test results, high temperature can decrease the dynamic stiffness, and low temperature can increase the dynamic stiffness, then the resonance frequency will change as well. * The correlation between different masses has a proportional relationship to their dynamic stiffness divided by the square of the natural frequency. * The small mass resonance frequency occurs in the high-frequency band, and the larger resonance occurs in the low-frequency band. The experimental results show that the low-frequency band data is more accurate. * The increase in the flooding level will change natural frequency and decrease the FRF. The system stiffness will decrease with the decrease of the natural frequency. * The properties of the material have different effects on the dynamic characteristics of the study. Meanwhile, the buoyancy will also have an interference effect on the system, especially where the test is conducted by using a lighter track mass. * In order to obtain data for the experiment, a total of six dynamic models were established for this study, and the first three were discussed in detail, among which two reasonable models (Models 1 and 3) were found. The mean values of the experimental data curve fittings in Model-1 and Model-3 were 97.45% and 96.95%, respectively. The state-dependent model (Model 3) is found to be reasonable but needs further optimisation.