## 5 Discussion  
A higher moisture level in the ballast and subballast layers of the measured transition zones was detected with field measurements. According to the experience of others, e.g., [3], wet ballast has often been observed in transition zones. High moisture in the ballast indicates that the drainage system is not fully functional. The intent of a drainage system in railways is to intercept subsurface water entering the area of the track substructure, to intercept surface water approaching the track structure from the sides, and to remove water draining out of the ballast and subballast [36]. When the drainage system does not work properly, it may bring problems that fall [36] into two categories. 1. Changes in physical properties of tracks, such as: pore pressure increases under cyclic load causing increase in plastic strain accumulation, decrease in stiffness. 2. Damage to track components, such as: subgrade attrition and slurry formation from ballast action; ballast degradation from slurry abrasion, chemical action, and freezing of water; sleeper attrition from slurry abrasion. The first category problems can be observed as the degradation of track alignment, which is shown by the measured longitudinal track level in Section 3 and explained in Section 4. Moreover, track irregularities have also been reported in transition zones, e.g., [1; 3; 4; 6; 7]. An example of a \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Peak** & **Wheel 1** & **Wheel 2** & **Wheel 3** & **Wheel 4** \\ \hline \(-7\) & 4.7 & 2.9 & 4.4 & 1.1 \\ \(-6\) & 3.2 & 1.9 & \(-0.3\) & \(-0.7\) \\ \(-5\) & 2.5 & 3.5 & 1.6 & \(-0.1\) \\ \(-4\) & 3.2 & 2.4 & 1.7 & \(-3.3\) \\ \(-3\) & 3.4 & 2.9 & 4.5 & 4.5 \\ \(-2\) & 1.6 & \(-8.5\) & 3.0 & \(-5.4\) \\ \(-1\) & 3.5 & 1.0 & 1.4 & \(-1.0\) \\ \(1\) & \(-2.6\) & 3.2 & \(-4.8\) & 2.2 \\ \(2\) & 3.7 & \(-2.5\) & 5.0 & 1.2 \\ \(3\) & \(-1.0\) & 1.9 & \(-2.7\) & 2.4 \\ \(4\) & 5.1 & 0.8 & 4.0 & 3.4 \\ \(5\) & \(-1.4\) & \(-1.0\) & 0.0 & \(-2.9\) \\ \(6\) & 10.0 & 0.8 & 5.9 & \(-1.0\) \\ \(7\) & 3.1 & 3.5 & 3.6 & 2.1 \\ Average & 2.6 & 0.8 & 1.7 & 0.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Increase percentages of the dynamic wheel loads in the high moisture case comparing to the reference case. transition zone with large track irregularities is shown in Figure 16. The second category problems result in maintenance, such as ballast cleaning (undercutting) by ballast cleaners [36; 56], or ballast adding by track stoneblower [56; 62]. When the problems are not solved timely, frost heave and mud pumping problems may occur. All of the problems cause a significant increase in track maintenance costs and reduction in passenger comfort. A possible solution is to improve the drainage system in transition zones at the design stage, since currently most codes regard the drainage system in transition zones the same as that in free track, e.g., in [63]. Numerical simulations have demonstrated how increased moisture (represented as reduced ballast stiffness) causes dynamic wheel loads to have a greater impact on transition zone structures (ballast/subballast) during loading (train traffic). This may result in a variation of ballast distribution and consequently accelerate ballast settlement. Since most numerical studies of transition zones only consider the difference of the stiffness and settlement in transition zones, e.g., [6; 8; 11; 64], it is suggested to also consider the effect of ballast moisture in future studies. In addition to the reduction in stiffness, it is also possible to model the ballast with high moisture as a higher growth rate of settlement. In [29], static one-dimensional tests of ballast were conducted to study the effect of the addition of water on ballast deformation. Specimens with added water had a 40% increase in settlement when compared to dry samples. Similar findings from laboratory tests can also be found in [30], where the additional settlement caused by water for fresh and recycled ballast is 39% and 42%.