# Interlocking Settlement Induced by Widening Subgrade of Railway Line
## 1 Introduction  
With increasingly rapid economic development in many countries, including Korea, the demand for rail transportation, such as freight and passenger railway transport, has also increased over the past decades. As the demand for additional railway transportation increases, along with the construction of new lines, it is necessary to increase its capacity by widening existing subgrades to add tracks [1]. A generally used method for adding railway tracks to existing embankment subgrades is to construct new embankments on existing subgrade slopes. When widening embankments, differential settlements and cracks between existing and new embankments may occur, due to the difference in soil properties and compressibility [2, 3]. Field investigations [4], small centrifuge tests [5], and numerical analyses [6, 7] have been conducted to analyze the deformation characteristics and mechanisms of widened embankments. As a result, georelite reinforcements [8, 9], geosynthetic clay liners [10], foundation columns [11], lightweight stacking materials [12, 13], and combinations of these methods [14, 15] have been proposed to solve problems that may occur in widened embankments.  
Widened embankments not only cause additional stresses and deformations in the extension but also affect existing embankments [16]. Additional stress occurs at the bottom of the existing subgrade owing to the load of the extension. Accordingly, an additional settlement of the existing subgrade (interlocking settlement) occurs, as shown in Figure 1. Excessive interlocking settlement reduces the safety of existing tracks, causing safety problems, such as train derailment, possibly requiring excessive repair and maintenance costs. In addition, economic problems, such as the need for additional sites [17, 18], civil complaints due to land occupation, and supply and demand of a large number of soil materials, may occur depending on the conditions of the construction site.  
Reinforced subgrade for railways (RSR) is an efficient method to widen the embankment subgrade without additional sites. As shown in Figure 2, RSR is a construction method in which a backfill reinforced with nets of welded rebars and geogrids is reconstructed, with the rigid facing wall post-constructed after the deformation of the backfill converges during the stabilization period. Integrating the rigid facing wall and geogrid secures the stability of the roadbed, with an excellent deformation control performance when using short geogrids with a length of 35% of the height [19; 20; 21; 22; 23]. In addition, it has excellent applicability in small spaces since this method uses a small foundation and short geogrids. The embankment subgrade can be widened without additional sites, significantly reducing the amount of soil material used, as shown in Figure 3[24].  
Figure 1: Concept drawing of interlocking settlements.  
Figure 3: _Cont._  
Figure 2: Concept drawing of RSR.  
Although the RSR method has several advantages, studies on the effect on existing lines, such as interlocking settlement due to the widened subgrade, are few. In particular, it is difficult to obtain measurement data on the long-term deformation of existing operating lines considering safety, cost, and time. In this study, the embankment subgrade of an existing operating line was extended without an additional site by installing an RSR on the slope. The long-term settlements of the RSR and existing lines were evaluated by measuring the settlement during and after the construction of the RSR. The long-term settlements of the RSR and existing embankment during and after construction and the interlocking settlement occurring during the widening of the embankment were evaluated and analyzed.