## 1 Introduction  
In transition zones, which are the weakest zones in the railway roadbed, track irregularities frequently occur because of differential settlements, which are induced by the difference in the vertical stiffness of the concrete abutment and the earth transition zone [1]. To mitigate differential settlement, several design codes suggest various measures, such as the implementation of approach blocks, approach slabs, and reinforced materials nd structures.  
The approach block is used to mitigate differential settlement; material with medium stiffness is placed between the abutment concrete structure and the subgrade on the back of the abutment, thereby smoothing the change in stiffness of the subgrade. Studies related to the approach block were conducted which focused on several materials [2; 3; 4] and ground-reinforcement strategies [5; 6]. Cement-treated gravel is a material typically used for the construction of approach blocks [7; 8; 9]. When graded gravel is mixed with a certain amount of cement (3-5%) and compacted, it exhibits a stiffness greater than that of graded gravel (without cement) [10; 11]; thus, the difference in stiffness between backfill and concrete structures can be mitigated.  
The approach slab is a concrete structure in the shape of a slab and is supported at one end by the bridge abutment and at the other end by the embankment soil [12]. By connecting the abutment and embankment with a slab, this structure is used to reduce the difference in stiffness of the subgrade. A nonlinear finite element analysis [13], a field survey [14], and dynamic analysis using a three-dimensional finite element model [15] have been performed to investigate the approach slab. These studies reported on the effect of reducing differential settlement in transition zones.  
In another approach to improve the performance of the transition zone, geosynthetics-based reinforcement methods have been explored. [PERSON] et al. performed shaking table tests of an abutment reinforced with a geogrid and discovered that the structural stability may be secured by utilizing the tension of the geogrid [16]. Moreover, [PERSON] et al. investigated the performance of a geogrid-reinforced and pile-supported bridge approach via field monitoring [17]. In addition, the performance and stability of geosynthetic-reinforced soil bridges have been evaluated via numerical analyses [18] and their life cycle assessment [19]. However, only a few studies have focused on the installation of geosynthetic-reinforced retaining walls at the transition zone of the railway abutment.  
The above studies have something in common that they make the change of stiffness smooth in the transition zone. Because the transition zone composed of soil material has a lower stiffness than a bridge with concrete, structural and material methods have been employed to increase the stiffness of the transition zone.  
In this study, the low compressibility and applicability of the geosynthetic-reinforced retaining wall using cement-treated gravel as a backfill material in the transition zone was evaluated. For this study, a reinforced subgrade for railway (RSR) was constructed in an abutment transition zone at the Osong railway test line in Korea, and long-term measurement was performed during and after the construction.