Analysis of Pipe-Roof in Tunnel Exiting Portal by the Foundation Elastic Model

The Pasternak double-parameter elastic foundation model of pipe-roof during the construction of tunnel exits is first established. Based on the portal project of Hanjiashan highway tunnel, an inclinometer is adopted to measure the settlement deformation of a pipe-roof and demonstrates the deformation law in tunnel exiting portals. The formulas for calculating the deflection and internal forces are derived to analyze the deformation of the pipe-roof in each excavation stage, and the results are compared with field monitoring data. Finally, the influences of excavation height, excavation footage, and stiffness of the pipe-roof on the support effect are investigated. Analysis indicates that the longitudinal settlement curve shows a groove distribution, which can be divided into five stages: micro, rapid, continuous, resilience, and stable deformation. Moreover, the subsidence rate reaches its maximum at the tunnel face. The influence of tunnel excavation on the deformation mainly has a range of 1.5 times the excavation height. To control the deflection of the pipe-roof, excavation height should be controlled in the range of 3.5 m, excavation footage should be controlled in the range of 1–1.4 m, and the diameter and thickness of the selected pipe-roof should be in the ranges of 89–159 mm and 5–8 mm, respectively.