Critical speed of ballasted railway tracks: Influence of ballast and subgrade degradation

•Field investigation was carried out using Multichannel Analysis of Surface Waves (MASW) to capture the dispersion curve of ballasted track substructure.•Analytical model is presented capturing the influence of track substructure condition such as ballast fouling, ballast thickness, and progressive subgrade degradation on the critical speeds.•Progressive subgrade degradation represented using degradation ratio (δ) reduced critical speed of the track and amplified stresses in subgrade.•For mud pumping prone subgrade soils, higher axle loads caused quicker fluidization even at lower train speeds.•Remedial measures such as addition of capping layer proved to increase critical number of loading cycles for lower axle load trains, but not effective for higher axle load trains. Track substructure layers such as ballast and subgrade involve progressive degradation with time in relation to the loading history. Degradation of track components directly influences the dynamic track response including the displacements and stresses. Determining the influence of real-world substructure degradation problems such as particle breakage, ballast fouling and mud pumping on the critical speed of a given track is essential to design ballasted tracks for high-speed passenger and freight trains. In this paper, a coupled rheological-continuum model is developed to predict the track dynamic response for different train speeds and axle loads. In this novel approach, field dispersion curves obtained from MASW (Multichannel Analysis of Surface Waves) testing can be adopted along with the proposed analytical model to predict the critical speed of track as well as the corresponding dynamic stress state in the substructure layers. Analytical modelling shows that problems such as ballast fouling and mud pumping lead to reduction in the computed critical speed, while also amplifying the dynamic stress response at lower train speeds. The influence of substructure degradation is found to be more prominent for heavy-haul (freight) trains imparting higher axle loads, when compared to passenger trains. Practical insights highlighting the influence of track improvements such as placing a thicker ballast layer and the addition of a capping layer on the critical speed are presented in this study with the objective of rejuvenating future track design.