Study on Techniques for Crossing Under Main Lines of Long-Span and Flat Tunnels by the Superly Long Pipe-Shed Method

The stability of underground structures and the deformation control of surrounding environments are key and difficult subjects in shallow tunnels crossing below important infrastructures,and the pipe-shed method is an important technical measure, which can improve the stability for the environment of rock and soil. Combined with an engineering practice of a long-span, flat and superly shallow tunnel in Fujian Province, which was used the superly long pipe-shed method to cross below main lines, the working principles of the pipe-shed system were analyzed, and the construction methods and crucially technical points of superly long pipe-sheds in ultra shallow tunnels were summarized and put forward. Based on the Winkler elastic foundation beam theory,considering the construction process of three-step tunnelling to reserve core soil,the laws for subsidence of the earth’s surface and the internal force distribution characteristics of the tunnel support structure were analyzed to guide the setting of support parameters and support measures on site. Based on the measured results, the rationality of the calculation results and support measures were vindicated. Combined with the construction process and environmental characteristics, the principle of the subsidence was discussed in detail. Finally,based on the measured subsidence,the subsidence characteristics and influencing factors caused by superly long pipe-shed construction were analyzed systematically. According to the actual engineering characteristics,the control measures of rail deformation were proposed,and the subsidence control methods were proposed from key points of construction techniques.