# Experimental Study on Settlement Behavior of Ballasted Tracks with Polymer Compound-Coated Gravel
## 1 Introduction
Ballasts with severe levels of pollution are sent to landfills as waste and incur a great deal of processing costs. Water cleaning using water and detergent and chemical processing methods using hydrogen peroxide and acid can be used as polluted ballast processing methods, but they have several problems, such as low processing efficiency, slow processing speed, and the generation of secondary pollutants. In addition, buried pollutants in the form of leachates and landfill gases adversely affect the environment. In order to resolve these problems, [PERSON] et al. [3] performed a study on manufacturing a new style of dry-cleaning device to purify the ballast. In addition, [PERSON] et al. [4] performed experiments to  
Figure 2: Damaged ballast. study the optimal track settlement properties of ballasted track beds, taking maintenance into account. Analysis of the experimental results with the regression equation of [PERSON]'s track settlement model [5] showed that the effect was slight when the pad was made highly elastic, but the effect was significant when the weight of the ties and the ballast thickness were increased. [PERSON] [6] performed a numerical analysis study according to the ballast particle size distribution, and [PERSON] and [PERSON] [7] used the Python-based open-source code gprMax and the RSA algorithm to identify fouling layers in ballasted track bed ground. [PERSON] et al. [8] performed laboratory box experiments and field experiments on ballasted track settlement properties, taking particle size and load conditions into account. [PERSON] and [PERSON] found that damage and deformation increase, and the ballast's stiffness decreases as the particle size angle increases [9]. [PERSON] and [PERSON] found that the settlement of uniform-grade railway ballast varies widely according to the material's particle size and particle size distribution, but they did not reach a definite conclusion [10]. However, [PERSON] et al. reported that specimens with larger particle sizes experience greater plastic deformation during one-dimensional compression tests at the same level of compression and similar densities. As in the case of fill material, this behavior is due to the breaking of contact points between particles under high loads and the movement of particles to more stable locations [11]. [PERSON] [12] performed experiments by removing and re-tamping the ballast after every 100 million loads over the course of 4 million repeated loads in order to examine the effect of particle size distribution properties on deterioration and settlement. The fine material that occurs as the ballast is broken down is generally known to be a major cause of pollution, and tamping (i.e., compaction) was indicated as a major cause of ballast material breakdown. Single-sized ballasts tend to be preferred, but it was found that a ballast with a better distribution shows better behavior than a ballast with a uniform particle size distribution (single-sized ballast), and it shows less permanent strain than a small-sized ballast. [PERSON] performed Los Angeles abrasion (LAA) and micro-deval attrition (MDA) experiments to examine the behavior of a railway ballast during rail car loading and tamping maintenance, and the results showed that ballast damage is related to tamping [13]. [PERSON] et al. performed field experiments that reinforced a recycled ballast with geocomposite, and the results showed that it was possible to reduce vertical and lateral deformation, thereby reducing maintenance costs [14]. Most of the current research on track bed ballast improvement is related to maintenance activities, such as ballast cleaning, as well as research on track bed ballast crushing and wear. Insufficient research has been pursued on reducing wear and friction between ballast pieces while maintaining uniform ballast quality and ballast shape in the face of impacts, wear, and friction in order to resolve the problems caused by railway vehicle loading, ballast flow, and other issues.