Abstract: To tackle the problems of water inrush, collapse and other disasters in the construction of the unfavorable geological segment of water-rich large-sectional tunnel using traditional construction methods, we propose an advanced semi-closed double pilot step method (ASDP step method for short) based on engineering geological investigation and analysis on water inrush and collapse mechanism. Subsequently, we simulate the construction process of ASDP step method by using finite difference software FLAC^3D. In association with field monitoring data, we found that the excavation of the right and left pilots contributes little to the vertical deformation of the arch crown and the arch bottom. Even if the inverted arch of pilot is excavated in advance in this method, the arch bottom uplift and overall tunnel deformation caused by soil unloading can be well controlled by quickly applying I-beam to form a semi-closed structure. After adopting ASDP step method, we successfully completed the construction of unfavorable geological segment of Guangshan No.1 Tunnel. The ASDP step method is featured with reasonable connection among procedures, high footage efficiency, good drainage effect and low cost, thus can be used as the preferred method for the construction of water-rich large-sectional tunnel in unfavorable geology.

Key words: highway tunnel, advanced semi-closed double pilot step method, water-rich, large-section, water inrush