## 2 Design and Construction of RSR Abutment and Transition Zone  
By initially constructing the backfill which is reinforced with a geogrid, followed by a rigid facing wall, the resultant RSR can minimize the residual settlement, as illustrated in Figure 1[20; 21]. Through the integration of the geogrid and the rigid facing wall, the bearing capacity of the subgrade can be increased, and horizontal earth pressure can be reduced [22; 23]. Moreover, unlike segmental retaining walls reinforced with geosynthetics, the RSR eliminates any occurrences of local failure; thus, this subgrade is suitable for supporting large and frequent railway loads.  
subsectionDesign of RSR Abutment and Transition Zone  
By issuing a field change request, the conventional reinforced concrete (RC) retaining wall was changed to an RSR. In the previous design, because of the short distance between the retaining wall and the adjacent road, the foundation of the retaining wall intruded onto the adjacent road. In addition, owing to the proximity of the site to a high-speed railroad, there were concerns that large pieces of equipment such as a crane may fall toward the high-speed railroad; thus, the construction was conducted at night during a 3 h period, from 01:00 to 03:00 AM, to obey Korean regulations. Nevertheless, the RSR could be constructed in a narrow working space without the need for any large equipment.  
The site investigation results are listed in Table 1. The landfill and sedimentary layers comprised clayey sand and sandy clay, respectively. Although the ground contains clayey soil, it was in a stiff state with a high \(n\) value (\(n\) = 8-13), but the layer thickness was not thick.  
The RSR was constructed at a height of 2.6-7.4 m and, including abutment transition zones, the total length was 156.0 m in STA.1k662.90-1k820.00 of the Osong railway test line in Korea. In this study, the STA.1k665 cross section in the abutment transition zone was selected for stability analysis, and its behaviors were subsequently evaluated.  
As depicted in Figure 2, the bench cut size for the foundation is reduced, and instead of installing steel piles with a length of 11 m, the ground stability was improved by installing cast-in-place (CIP) concrete piles with lengths of 1.5-2.0 m. The length of the short geogrids is 2.60 m (35% of the wall height), and the length of the long geogrids is selected to satisfy the criteria for safety against circular failure, sliding, and overturning. These geogrids provide a tensile strength of 100 kN/m and a design tensile strength of 50 kN/m. [PERSON] (2017) verified the stability of this RSR section during the dry and wet seasons, as well as during earthquakes using the RSR design program developed by the Korea Railroad Research Institute, as summarized in Table 2[24].  
\begin{table}
\begin{tabular}{c c c c} \hline
**Depth** & **Thickness (m)** & **Layer Descriptions** & **
\begin{tabular}{c} **\(n\) Values 1** (Number \\ **of Blows/cm)** \\ \end{tabular} \\ \hline
0–1.4 & 1.4 & Landfill layer & 13/30 \\  & & Medium stiff clay sand & \\
1.4–3.7 & 2.3 & Sedimentary layer & \\  & & Stiff sandy clay & 8/30–12/30 \\
3.7–12.0 & 8.3 & Weathered soil layer & \\  & & Soft to very stiff silty sand & 16/30–50/9 \\  & & Weathered rock layer & \\
12.0–15.0 & 3 & Very stiff weathered & 50/10–50/7 \\  & & Granite & \\ \hline \end{tabular} \({}^{1}\) The number of hammer blows required to penetrate the sampler by 0.3 m in the standard penetration test.  
\end{table}
Table 1: Site investigation results.  
Figure 1: Construction of reinforced subgrade for railways (RSR) [23].