Railway Subgrade Deformation Control of Shield Tunnel Underpassing with Pipe Roof Support Method

[Objective] Under the disturbance of adjacent construction, the shield tunneling underpassing construction will have a certain impact on the subgrade settlement. The control effect of the pipe roof on the subgrade settlement varies under different circumferential layout scopes and spacing conditions. Therefore, it is necessary to study the control effect of advanced pipe roof support on the settlement above the railway subgrade. [Method] In the case study of Chengdu Metro Line 5 underpassing the railway subgrade, a double-layer pipe roof support scheme is proposed, and the FLAC 3D software is used for numerical simulation. According to the subgrade settlement standards, based on the three indexes of the volume of the tunnel surrounding rock plastic zone, the subgrade settlement and the segment displacement , and combined with the on-site monitoring data, the support effect of the pipe roof under different circumferential layout scopes and spacing is studied. [Result & Conclusion] Compared with the circumferential spacing of the pipe roof, a reasonable arrangement of the circumferential scope can better exert the support effect of the pipe roof. For the general strata, the control requirements of the segment displacement can be met when the pipe roof layout is in a form of 150°arch part circumferential scope and the circumferential spacing is 40 cm. When the circumferential spacing is 30 cm, as the circumferential scope of the pipe roof increases from the arch angle of 90°to 180°, the maximum value of the subgrade settlement decreases from 50.00 mm to 2.50 mm, less than the subgrade settlement warning value of 3.75 mm. At this time, the volume of the plastic zone of the surrounding rock is close to 0 with a smaller segment displacement, and the monitoring result of the subgrade settlement meets the requirements of the settlement warning value. The determined optimal pipe roof layout scheme is that the circumferential spacing is 30 cm, and the arch angle is 180°.