# Experimental Study on Settlement Behavior of Ballasted Tracks with Polymer Compound-Coated Gravel
## 4 Conclusions  
In this study, we experimentally confirmed that the effects of long-term settlement behavior and static spring stiffness performance can be improved by comparatively studying coated and normal ballasts. It is difficult to compare the behavior of ballasts with complex shapes through experimental and numerical analyses. Therefore, for comparison with the experimental analysis, a numerical analysis was performed to confirm whether the variations in the static spring stiffness of the ballasted track, to which the coating material was applied, can be accurately represented. Ballast box tests were performed to examine the performance of the normal ballast and the proposed polymer-coated ballast. The laboratory  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & **Specific Gravity** & **Absorption Rate (\%)** & **Unit Volume (\%)** & **Ratio of Volume (\%)** & **Elastic Modulus (GPa)** & **Tensile Strength** & **Poisson’s Ratio** \\ \hline Ballast & 2.5 & 0.7 & 14.3 & 55.4 & - & - & - \\ Polymer & - & - & - & - & 4.30 & 59.81 & 0.35 \\ Compound & \multicolumn{1}{c}{} & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Material and mechanical properties.  
Figure 8: Results of numerical analysis of the ballast under each load condition.  
Figure 7: Static numerical analysis examples (250 mm): (**a**) ballast modeling, (**b**) load conditions and boundary conditions, (**c**) normal ballast (displacement), (**d**) coated ballast (displacement).  
test results indicated that the shape of the load-displacement curve of the normal ballast under static load changed compared to the coated ballast, and it was confirmed that its material spring stiffness was not constant. In the laboratory tests, rapid settlement occurred under cyclic loading as the ballast particles were rearranged. This indicates that lateral deformation and settlement occurred owing to the wear at the contact points between ballast particles, as well as breakage, particle rotation, slippage, and rearrangement as repeated train loads were applied under low-constraint conditions. This settlement phenomenon is affected by various factors, such as the ballast material properties, ballast layer thickness, and train loads; however, improving the ballast, which is one of the factors that affects settlement, and using a coated ballast that minimizes wear and breakage in the ballast layer can effectively reduce ballasted track maintenance and repairs.  
In addition, the behaviors of the normal and coated ballast samples were examined by numerical analysis simulations that were similar to the ballast box tests. The results confirmed that the coated ballast provided a constant stiffness that was 8.8% lower than that of the normal ballast. It was confirmed through experiments and numerical analysis that the coated ballast had a rigidity of constant slope. The rigidity of the coated ballast was constant even when the load was applied. The durability of the ballast samples was examined by applying cyclic loading, and the results confirmed that the coated ballast exhibited superior durability compared to the normal ballast. In the future, we plan to model the actual shape of the ballast using a 3D scanner and numerically analyze the ballast according to particle size. Moreover, studies will be conducted using the coated and normal ballasts in the field to examine the field applicability of the coated ballast.  
This research received no external funding.  
Not applicable.  
This study was supported by a grant from Dong Yang University in 2019.  
The author declares no conflict of interest.