# Experimental Study on Settlement Behavior of Ballasted Tracks with Polymer Compound-Coated Gravel
## 1 Introduction  
Railway ballasted track is one of the most-often constructed track types in urban railways, general railways, and high-speed railways. The ballast used as track bed material consists of crushed gravel with a size of 4-5 cm, and has been used since the beginning of railway history 200 years ago. Approximately 80% of railway tracks in South Korea are ballasted tracks. Ballasted tracks transmit load from the rails and ties to the subgrade during railway vehicle travel and feature good drainage, as shown in Figure 1.  
Ballast is a core track component that directly affects the overall track support performance and elastic behavior, and it has the advantage of a short construction period and inexpensive construction cost. However, ballasted tracks experience plastic deformation (such as ballast flow), track deterioration, ballast pollution, and reductions in water permeability due to mud-pumping as a result of the repeated load of the railway vehicles. These disadvantages affect the riding safety of the railway vehicles and comfort of the passengers. As such, ballast has the disadvantage of requiring railway operators to periodically measure track deformation and perform repair work if safety thresholds are exceeded. In addition, the status and performance of the ballast create significant differences in the impact and vibration levels that act upon the track during train car travel. If the ballast's performance  
Figure 1: Diagram of ballasted track bed pressure distribution. is reduced, track deterioration is exacerbated, which causes track irregularities (such as alignment, longitudinal level, cross level, and twist), which directly affect the railway vehicle' riding safety and comfort and increase track maintenance costs. Currently, in South Korea and abroad, track beds require continuous maintenance and management, such as adding new ballasts and cleaning, because of changes in the overall functionality of ballasted track beds that are caused by changes in the cross-sections of the ballast due to internal and external impacts, as load is directly transferred from the railway vehicles. Therefore, in order to reinforce the current track bed ballast, railway operators are using methods such as laying ballast mats under ballasted track beds and injecting resin-based ballast stabilizers and non-shrink mortar into the ballast to cause bonding and solidification among ballast pieces. However, methods that inject track bed stabilizers and non-shrink mortar are based on the concept of removing the elastic functions of the ballast and converting it into a concrete track bed. Currently, this may be effective at resolving ballasted track maintenance problems such as mud-pumping and nonuniform settlement, but this approach is unavoidably limited in terms of function and effectiveness, and it fundamentally cannot prevent or reduce ballast wear, deformation, and damage. Furthermore, it still cannot resolve the problem of reduced track performance and degradation caused by a damaged ballast. In tracks where railway vehicles travel, the ballast pieces are deformed by the vibrations caused by the railway vehicle load, and these tracks have the structural property of progressive track irregularity. When the wearing and crushing of the ballast is accelerated and the ballast is damaged by repetitive railway vehicle operation and track repair work, as shown in Figure 2, the track's support capacity is reduced, and track irregularities occur due to fouling. In addition, if the ballast directly below the ties becomes pulverized, it can cause the track bed to solidify by mixing with the soil [1,2]. When rainwater and freeze-thaw cycles act upon the track bed, mud-pumping occurs due to poor drainage, and the track bed becomes polluted. Due to the nonuniform track settlement and track irregularities that occur when the ballast is worn and crushed, continuous maintenance such as tamping, cleaning, and replacement are required.