Abstract: Due to the complex urban environment, the construction risk of underground excavation is very great, and the transformation from underground excavation to open excavation often takes place.In order to ensure the safety of the tunnel section during the process of converting from underground excavation to open excavation, three schemes were studied for the case where the pipe shed had already been constructed, namely, demolishing the pipe shed before sloping excavation during open excavation construction, demolishing the pipe shed after constructing the retaining pile before excavation, and demolishing the pipe shed after constructing the retaining pile and simultaneously implementing excavation.The results show that the open excavation construction results in a 40% increase in the stress on the pipe shed, and the force of the pipe shed in the first hole is unfavourable to the force of the pipe shed in the second hole.The axial force of the pipe shed in the first tunnel increases by a maximum of 50% during.After the completion of the retaining pile, the bending moment change of the simultaneous demolition and excavation of the pipe shed is about 10 times larger than that of the pipe shed during excavation.The application of retaining pile in foundation pit can reduce the elastic deformation caused by the demolition of pipe shed, and the shape of its deflection curve is changed by the demolition of pipe shed.The maximum deformation before and after the removal of the pipe shed decreased from 0.7 cm to 0.1 cm.It is recommended to set the retaining pile of foundation pit and to remove the pipe shed before excavation after the transformation from underground excavation to open excavation.The research results can provide theoretical basis and construction experience for similar projects.

Key words: tunnel engineering; urban tunnel; pipe shed demolition; pipe shed internal force; pipe shed deformation

摘要： 由于城市环境复杂，暗挖隧道施工风险极大，暗挖转明挖时有发生。为了保障暗挖转明挖施工过程中隧道段安全，通过数值分析，研究了对管棚已施作的情况下，明挖施工时先拆除管棚后放坡开挖、施工围护桩后先拆除管棚后开挖、施工围护桩后拆除管棚和开挖同时实施3种方案。结果表明，明挖施工导致管棚受力增加40%，先行洞的管棚受力不利于后行洞的管棚受力，先行洞的管棚轴力最大增加了50%；在围护桩完成后管棚拆除和开挖同时施工中的管棚弯矩变化比先拆管棚后开挖施工中的管棚弯矩变化大约10倍；基坑围护桩的施作能够减小管棚拆除产生的弹性变形，管棚拆除改变了其位移曲线形状，管棚拆除前后的最大变形从0.7 cm减小到0.1 cm；隧道暗挖管棚段转明挖后建议设置基坑围护桩和先拆管棚后开挖施工的方案。研究结果可为类似项目提供理论依据和施工经验。

关键词: 隧道工程；城市隧道；管棚拆除；管棚内力；管棚变形