Objective Currently researches on the relationship between rise-span ratio and failure approach index remain relatively limited. To determine the optimal rise-span ratio for large-span mined station tunnels under sandy mudstone conditions, it is necessary to conduct corresponding studies.

Method Using Kuangjiatang Station on Chongqing Rail Transit Line 24 as research object, a 3D numerical model is established to simulate and calculate displacement and deformation of large-span structures, failure approach index, and the proportion of surrounding rock at different failure approach levels under various rise-span ratios. The influence of the rise-span ratio on surrounding rock damage in the station is then analyzed.

Result & Conclusion  The vertical displacement caused by vault settlement gradually increases as the rise-span ratio decreases, while the horizontal displacement of tunnel sidewalls gradually decreases with a reduction in the rise-span ratio. When the rise-span ratio decreases from 1:2.0 to 1:3.0, the vertical displacement increases from -0.97 cm to 1.2 cm, and the horizontal displacement decreases from 0.75 cm to 0.64 cm. The failure approach index varies significantly around the station, ranging approximately from 0.106 to 0.783, with its maximum located in the surrounding rock near the sidewalls. When the rise-span ratio is between 1.0:2.0 and 1.0:2.4, the maximum failure approach index decreases rapidly; when the ratio is between 1.0:2.4 and 1.0:3.0, the decrease of above index becomes slower, and the proportion of surrounding rock with a failure approach index ≥0.7 is also relatively small. The maximum horizontal depth of this surrounding rock exhibits a monotonic decrease followed by a sudden increase as the rise-span ratio decreases. Analysis of parameters such as displacement and failure approach index indicates that the optimal rise-span ratio for the large-span structure in the study area is 1.0:2.8, at which the stress condition of the surrounding rock structure is most favorable.