## 3 Experiment Analysis of Transition Zones
### Measurement Results  
The moisture condition of three bridge transition zones on a Finnish railway line was measured using GPR. To study the effect of the moisture condition, the longitudinal level (rail height) measured by track inspection coaches in a similar time period using a 5 m-chord method was also collected. Measurement results of an open track section are shown in Figure 6 as a reference case. A photo of the track is shown in Figure 6a, the moisture profile in Figure 6b and longitudinal level (rail height) in Figure 6c. The vertical axis of the moisture profile (Figure 6b) indicates the depth under rails in meters. Although the subballast layer ideally ends 1.2 m, it often settles deeper, especially in transition zones. As a result, the measurement results are collected down to 1.8 m. The ballast, subballast, and subgrade are dry, as indicated by the red colour of the moisture profile. The irregularity of the longitudinal track level is less than 2 mm, which is relatively small. Measurement results from Transition Zone A are shown in Figure 7. This transition zone is composed of embankment and a 41 m long concrete bridge. The photo of the bridge taken by the on-board video camera (Figure 3) during the GPR measurements is shown in Figure 7a. The ballast is laid on the bridge, the depth of which is around 0.6 m beneath rails. GPR data from the concrete bridge is out of the focus of this study and it therefore blocked out. Approach slabs are employed at both ends of the bridge. The slabs are made of concrete and laid diagonally from the ballast layer to the subballast layer. Moisture susceptibility increases before and after the bridge, as can be seen from Figure 7 (see the blue zones indicated by No. 1 and No. 4). The blue zones of No. 2 and No. 5 are caused by approach slabs, since the concrete has a similar expression as highly susceptible moisture areas after post-processing. Likewise, the blue zone from No. 3 is caused by a section of rail, as shown in Figure 8.  
Figure 6: Measurement results of open track: (**a**) photograph of the track; (**b**) the moisture profile; and, (**c**) the longitudinal track level. Figure 7c shows that the irregularity of the track increases significantly before and after the bridge (indicated by No. 6 and No. 7), which reached 3.0 mm and 2.9 mm, respectively. These two irregularities indicate the appearance of differential track settlement, which agrees with the dips found in other transition zones, such as in [4; 6; 7]. Transition Zone A has both increased moisture susceptibility and track irregularity based on the moisture profile and the longitudinal track level data on both ends of the bridge. Measurement results of two more transition zones (named as Transition Zone B and Transition Zone C) are shown in Figures 9 and 10. Their structures are similar to that of Transition Zone A.  
Figure 8: Section of rail in Transition Zone A.  
Figure 7: Measurement results of Transition Zone A: (**a**) photograph of the track; (**b**) the moisture profile; and, (**c**) the longitudinal track level. Moisture susceptibility increases before the bridge in Transition Zone B. The blue area No. 8 (Figure 9) appears at the interface between the ballast layer and the subballast layer, which implies that the water is held at the bottom of the ballast and the drainage system is in poor condition. This is most likely one of the reasons for the track irregularity in the corresponding location (No. 9). However, no significant increase of the moisture can be found above the approaching slab, yet a large dip appears in the corresponding location. This irregularity could be caused by other reasons, such as a rail defect. In the bridge-embankment side of Transition Zone B, the moisture in the ballast and the subballast layer is relatively low. As expected, the track geometry at the corresponding location is much smoother, as shown in Figure 9c. Transition Zone C is in worse condition than Transition Zones A and B.