Abstract: The complicated working procedure and frequent stress transformation at the intersection between main and auxiliary tunnels are big challenges in the traditional roof-lifting method. To improve the safety of the roof-lifting construction of the auxiliary tunnel entering the main tunnel in soft stratum and to effectively control the primary supporting deformation at the stress concentration position, a case study is conducted on the construction of Zhaotong tunnel inclined shaft No. 2 entering the main tunnel of Chongqing-Kunming high-speed railway. The roof-lifting technology of the upper connecting hole + portal frame method is studied based on the full analysis of the geological characteristics of expansive soil in the inclined shaft section. An independent upper guide tunnel is set behind the intersection between the auxiliary and main tunnels to excavate the main tunnel crown. Meanwhile, multiple working faces are performed synchronously with the roof of the main hole and the lining of the inclined shaft. Additionally, the application of the close-row gantry roof-lifting technology can effectively fit the excavation contour of the main tunnel, realizing a safe and fast roof-lifting operation. The construction results show that the interaction effect of the auxiliary and main tunnels can be minimized, obtaining a synchronous, safe, and fast construction.

Key words: soft stratum, auxiliary tunnel, tunnel intersection, upper connecting hole, portal guide tunnel, roof lifting