Abstract: When the tunnel axis parallels to the trend of steep rock formation, the instability risk of the surrounding rock caused by excavation increases with the increasing excavation span and the decreasing tunnel clear spacing. Accordingly, the reasonable determination of clear spacing for largespan tunnel in steep rock formation is of great significance. A case study is conducted on twintube fourtrack mined Tushan station tunnel of Chongqing rail transit ring line. Then tunnel axis is parallel to trend of steep rock formation, and the two superlarge small clearspacing tunnels are constructed by doubleside drift method. The geometric theory formula that includes a rock fracture angle of θ and the calculation method of minimum tunnel spacing are proposed using the methods of geogeometric analysis and engineering analogies. Then, the reasonable clear spacing of superlarge crosssection tunnels under asymmetrical geological pressure is determined by analyzing the numerical simulation and the monitoring and feedback data of the construction test section. The results show that the clear spacing calculated by setting inside tunnel outside the sliding surface of lateral tunnel is safe and feasible when the tunnel axis is parallel to the trend of steep rock formation.

Key words: large-span mined tunnel, small clear-spacing, reasonable clear spacing, steep dip rocks, surrounding rock stability analysis