Evolution of trackbed performance and ballast degradation due to passages of million train wheel axle loads

Abstract

The increases in train speeds and wheel axle loads produce significantly higher dynamic responses and ballast breakage in ballasted tracks, which could cause reduced trackbed performance and the deterioration of the ballast particles under long-term train traffic loads. To evaluate trackbed performance under long-term moving train loads, a full-scale ballasted track experiment with eight sleepers was designed and tested on a validated physical model test platform (ZJU-iHSRT).Sleeper support stiffness is a qualitative index of the ballasted track in the evaluation of trackbed performance, which was studied based on the sleeper displacement measured by a laser displacement sensor during the test.Then, ballast breakage analysis was conducted to quantify the ballast degradation of ballast particles after every 400,000 passes of train wheel axle loads through the sieving test of particle size distribution.Vibration velocity sensors and “SmartRock” wireless sensors were installed at specific locations in the ballast layer to compare the ballast dynamic responses before and after the implementation of long-term train traffic loads. The test results showed that both the static and dynamic sleeper support stiffness increased tremendously with the densification of trackbed caused by long-term train loads; especially, heavy train loads increased the BBI by 60% and doubled the static sleeper support stiffness. The evolution of static sleeper support stiffness and ballast breakage showed a similar trend, which significantly intensified with the increase in train speed, then stabilized with the number of train wheel axle loads, and increased sharply again due to the increase in axle load. Ballast breakage and particle rearrangement made the small particles fill the ballast voids, which enhanced the densification of ballast particles, thereby reducing the trackbed elasticity.