Abstract:

 The double sidedrift method used in superlarge crosssection loess tunnels is slow in tunneling advance and costly. To address these challenges, a case study is conducted on the expansion and reconstruction project of the Fenghuangshan tunnel from Qingshuiyi to Zhonghe section of the G30 LianyungangKhorgos expressway, and a fivestep single sidedrift method is proposed. The surrounding rock′s deformation and streel arch′s stress when employing the double sidedrift, fivestep single sidedrift, conventional single sidedrift, and bench methods are comparatively analyzed. Finally, the feasibility of the fivestep single sidedrift method for constructing the Fenghuangshan tunnel is validated through field monitoring data. The results reveal the following: (1) The crown settlement and horizontal convergence of the fivestep single sidedrift method are 3.59 cm and 1.31 cm lower than those of the conventional single sidedrift method and 1.4 cm and 0.59 cm higher than those of the double sidedrift method. The stress of the steel arch is less than that of the double sidedrift method except for the arch shoulder, with a maximum reduction of 70 MPa. The ratio of the maximum stress of the steel arch to the steel yield strength is 0.04 higher than that of the double sidedrift method and 0.08 lower than that of the single sidedrift method. (2) The traditional single sidedrift method is less safe than the fivestep single sidedrift method. The improved fivestep single sidedrift heading method, compared with the double sidedrift method, can accelerate construction progress and reduce construction costs while controlling surrounding rock deformation. (3) The optimal position of the partition wall in the fivestep single sidedrift method is 0.45 times the hole width in the Fenghuangshan tunnel and should be adjusted based on actual conditions for further application.

Key words: superlarge crosssection loess tunnel, fivestep single sidedrift method, numerical simulation, field monitoring