Deformation control of a high-fill railway subgrade undercrossed by a three-span frame bridge

The construction of a box-frame culvert under an existing operational railway embankment may trigger deformation of the existing railway and compromise track smoothness, potentially resulting in operational disruptions in severe cases. This study takes the traffic diversion road beneath the Pingnan Railway at Wuhe Station in Shenzhen as the engineering background. A numerical model of a frame bridge underpassing an existing railway line was established. Based on field monitoring data, the disturbance-induced deformation of the overlying existing railway during cast-in-situ frame bridge construction was analyzed under different pile-cutting sequences. Key control techniques in cast-in-situ box-frame culvert underpass construction were then proposed. The calculation results show that, during the cast-in-situ underpass construction of the frame bridge, the track settlement curves under various construction conditions exhibit a generally symmetrical distribution. The minimum track settlement occurs at the supporting pile locations, whereas the maximum settlement appears at the mid-span of the D-beam. An increase in the span of the frame bridge leads to more pronounced horizontal displacement of the railway track within the bridge span. After optimization of the pile-cutting sequence, the track settlement within the area of the SGC10 supporting pile is significantly smaller than that obtained using the original scheme, with the settlement reduced by 49%. Consequently, both the settlement and horizontal displacement of the existing railway track within the entire underpass area are reduced. By cutting the existing supporting piles in stages, the railway track load can be progressively transferred to different frame bridges, enabling multiple stages of load redistribution. This staged load-transfer process can significantly reduce track settlement and improve the smoothness of the existing railway. The calculated deformation patterns of the railway track under different construction conditions are consistent with the field monitoring results, indicating that the proposed modeling approach can be used to predict track deformation during railway underpass construction at grade-separated crossings and provide practical construction experience for the safe implementation of similar railway underpass projects.