Objective The strata in Qingdao exhibit distinct upper-soft and lower-hard characteristics. Setting the station arch within the underlying hard rock stratum offers numerous advantages, such as reducing support costs and alleviating construction schedule pressures. Therefore, it is necessary to conduct research on the reasonable rock-span ratio of stations and synchronously revise the deformation control standards.

Method MIDAS finite element software is used to establish a numerical analysis model. The finite element strength reduction method is utilized to determine the reasonable rock-span ratio for the self-stability of the unsupported surrounding rock. The distribution characteristics of the equivalent plastic strain in the surrounding rock and the mechanical behavior of soft-hard interface are studied. The reliability of the numerical analysis results is verified through engineering monitoring data, and corresponding rock deformation control standards for surrounding rock is simultaneously proposed.

Result & Conclusion  The reasonable \omega -\eta distribution curve satisfies a directly proportional linear relationship, and the reasonable rock-span ratio value \omega _\mathrmh under any coverage-span ratio \eta can be obtained using a fitting function. Its lower and upper limits are 0.089 and 0.241, respectively, with the corresponding hard rock thicknesses H_r being 1.83m and 4.95m. There are two potential fracture surface sliding traces in the surrounding rock, and the failure of the inverted V-shaped sliding surface at the arch occurs prior to that of the arc-shaped sliding surface at the side wall. Insufficient hard rock thickness can easily lead to the overlying soft strata collapsing into the interior of the station. The maximum principal stress direction at the soft-hard interface undergoes an abrupt change before and after excavation, gradually deflecting toward the tangential direction of the excavation contour. Compared to a temporary rock-span ratio \omega _\mathrml , the soil-arching effect with stronger bearing capacity will be formed in the surrounding rock under a reasonable rock-span ratio \omega _\mathrmh . The revised control limits for the vault settlement decrease by 30.00% to 53.33% compared to the current normative standards, and its rationality and reliability are successfully verified by engineering monitoring data.