# Experimental Study on Settlement Behavior of Ballasted Tracks with Polymer Compound-Coated Gravel
## 2 Performance Tests on the Coated Ballast
### Ballast Box Test of Coated Ballast  
The laboratory testing method known as ballast box testing was performed in order to understand the properties of the wear and crushing that are caused by reduced ballast resistance due to railway vehicle operation, poor impact absorption, and track irregularities. The ballast box test specifications were adopted from previous studies [15; 16; 17; 18]. The ballast box test was developed at the University of Massachusetts to understand the properties of settlement, wear, and crushing in railway track beds due to the passage of railway vehicles. The ballast was placed in a steel box, and its behavior was examined by repeatedly applying a load to a piece of railway tie, which is a load-bearing plate. Figure 3: Comparison of normal ballast and coated ballast: (**a**) Coated ballast cross-section. (**b**) Comparison of coated ballast and normal ballast before and after surface roughing. The ballast box has a width of 500 mm, depth of 500 mm, and height of 500 mm, and is constructed so that it can assess the behavior of the ballast during wheel loading. Figure 4 shows the ballast box tests for the normal and coated ballast. Figure 5 shows the state of the ballast after the ballast box tests. As shown in Figure 5a, wear and crushing occurred in the normal ballast. Figure 5b,d show that the coated ballast test results confirm that the polymer compound coating layer can mitigate ballast wear and crushing by reducing the impacts and friction between ballast pieces that occur during repeated loading. In addition, in the specimen with mixed normal ballast and coated ballast, only a few marks occurred on the coated ballast, as shown in Figure 5c. The ballast box laboratory testing program was established to analyze changes in the short- and long-term settlement of the ballast layer under repeated loading, enabling the direct assessment of ballast performance and its short- and long-term maintainability. This assessment was accomplished by obtaining the static load-displacement curve (spring stiffness of ballast underneath sleeper) for each ballast sample test case. Figure 6 shows an overview of the laboratory test results. Figure 4: View of tests in each laboratory experiment: (**a**) normal ballast (300 mm), (**b**) coated ballast (250 mm), (**c**) normal ballast (150 mm) and coated ballast (150 mm), (**d**) coated ballast (100 mm). Figure 5: Ballast before and after laboratory experiments: (**a**) normal ballast (300 mm), (**b**) coated ballast (250 mm), (**c**) normal ballast (150 mm) and coated ballast (150 mm), (**d**) coated ballast (100 mm). As shown in Figure 6a, it was found that the spring stiffness of the normal ballast, in which the shape of the load-displacement curve changes during the testing process, was not constant even under the same test conditions, and the spring stiffness of the normal ballast, which corresponds to the slope of the load-displacement curve, was larger than that of the coated ballast. In Figure 6a, the coated ballast was used after the fatigue test of 1 million cycles. On the other hand, in the case of the coated ballast, it was found that the load-displacement curve was not directly affected by the thickness of the coated ballast and the mixing conditions with the normal ballast, and the coated ballast was able to provide a constant spring stiffness. The load-displacement slope when the coated ballast and the normal ballast were mixed and the load-displacement slope of the coated ballast were similar. In addition, the stiffness of the coated ballast track was confirmed to be flexible, and the damping performance was confirmed by the load-displacement diagram. As shown in Figure 6b, the contact surfaces between particles of the normal ballast were increased, and the amount of ballast settlement increased rapidly according to the number of cycles as the particles were compacted. Compaction occurred as the ballast was fractured by the concentration of stress on the particle contact points.