# Behavior Characteristics of a Booted Sleeper Track System According to Substructure Deformation
## 4 Conclusions  
In this study, the effect of substructure deformation on the behavior of a sleeper floating track system installed in a subway box tunnel structure was analyzed using a 3D FE model. The mechanism of the sleeper floating track system-substructure interaction due to substructure deformation was investigated, and the possibility of concrete bed cracking was explored. The main findings of this study can be summarized as follows. 1. An analysis of the interaction mechanism between the sleeper floating track system and its substructure under uplift indicated that the resilience pad between the RC sleeper and concrete bed was compressed, potentially resulting in a decrease in the elastic displacement of the track under load. This would lead to an increase in the track support stiffness, thereby increasing the track bearing capacity. In contrast, under substructure subsidence, the occurrence of the \"floating sleeper\" phenomenon could affect the rail fastener bearing capacity. Therefore, to counter the effects of the floating sleeper phenomenon, the spacing between sleepers should be increased in order to increase the flexural stress in the rail and thus relax the stress in the rail fasteners. 2. An analysis of the relative displacement and contact conditions between the rail and the concrete bed according to substructure deformation indicated that the rail and concrete bed deformations were similar under substructure uplift, with the rail deformation being only slightly smaller owing to the compression of the resilience pad  
Figure 13: FE analysis results (shear stress, xz direction) according to magnitude of (**a**) uplift or (**b**) subsidence. at the point of maximum displacement, which caused minor instances of the floating rail phenomenon elsewhere. On the other hand, under substructure subsidence, the rail deformation was noticeably smaller than that of the concrete bed, and the floating sleeper phenomenon occurred extensively. 3. An examination of the tensile stress in the concrete bed due to substructure deformation determined that concrete bed tension cracking may occur at points of maximum uplift and subsidence. Under substructure uplift, it was found that tension cracking could occur within approximately 5 m each side of the point of maximum uplift, on the inner and outer faces of the RC blocks; under substructure subsidence, it was determined that tension cracking could occur within approximately 2.5 m on each side of the point of maximum subsidence, along the rail between the RC sleepers. Thus, substructure uplift was determined to be more likely to cause tension cracking than substructure subsidence. However, because the tensile stresses were observed to be similar at the point of maximum subsidence and at the end of the concrete bed, further analysis of the tension cracking at the end locations remains necessary. 4. An examination of the possibility that RC sleeper rotation due to substructure deformation could cause the shear failure of the concrete bed revealed that the concrete sleepers rotated in opposite directions relative to the point of maximum uplift or subsidence. Concrete shear cracks were likely to occur within 5 m either side of the points of maximum uplift and subsidence. Under either substructure uplift or subsidence, the RC sleepers at the end of the concrete bed were observed to rotate, potentially inducing shear failure owing to contact. Therefore, it is necessary to perform further investigation of shear cracking at the ends of the concrete bed in addition to the points of maximum uplift or subsidence. In future studies, it will also be necessary to analyze the effects of substructure deformation on the dynamic behavior characteristics of floating track systems, such as the natural track frequency, track impact factor, rail fastener bearing capacity, and train stability during travel. Conceptualization, J.-Y.C. and S.-H.K. ; methodology, J.-Y.C. ; formal analysis, D.-H.A. and S.-H.K. ; investigation, D.-H.A. and J.-Y.C. ; writing--original draft preparation, J.-Y.C.