Construction optimization of the arch cover method for super-large-section subway stations in shallow-buried strata

Hongyancun Station on Chongqing Rail Transit Line 9 has a maximum burial depth of 106.37 m. It is a two-story underground mined station, with the concourse level on the first underground floor and the platform level on the second underground floor. The station has a total length of 262.3 m and a total width of 21.8 m. Adopting a single-arch double-layer structure, the tunnel features composite lining, with a net excavation width of 24.34 m, an excavation height of 21.23 m, and a cross-sectional area of 436 m². The tunnel crown consists of strongly weathered silty mudstone classified as Grade Ⅳ surrounding rock, with rock mass integrity coefficients ranging from 0.70 to 0.78 and a saturated compressive strength of 10.5 MPa that exhibits obvious strength variability. The tunnel body is composed of moderately weathered silty mudstone classified as basic Grade Ⅲ surrounding rock, with rock mass integrity coefficients ranging between 0.76 and 0.81 and a saturated compressive strength of 31.5 MPa. The arch cover method was adopted to overcome construction difficulties and ensure safety for the shallow-buried, super-large-section metro station in upper-soft and lower-hard rock strata.

Field monitoring was carried out by installing measuring points on the ground surface, tunnel vault, and both side walls to measure ground subsidence and tunnel deformation. The stress and deformation characteristics of the optimized arch cover construction method were then analyzed.

The research results show that the arch cover construction method was optimized. Following the principle of “upper part prior to lower part, side parts prior to central part”, the super-large-section excavation was divided into 11 parts. Large arch feet were utilized to bear upper loads and form an arch cover stress system, thereby eliminating stress concentration that occurred before the primary support of conventional large-section stations reached the base. The optimized sequential excavation and timely closure into a ring structure considerably reduced the temporary support workload, created space for mechanized operations during middle and lower bench construction, and enabled safe and rapid construction of super-large-section tunnels. This construction method fully mobilized the self-bearing capacity of the surrounding rock and lowered stress on the primary support. A single-pass excavation technology for the left and right pilot tunnels at the tunnel arch was also proposed.

Monitoring data indicate that during bench base excavation, the excavation sequence of central grooving and side wall surrounding rock exerts minor influences on ground subsidence, surrounding rock deformation, rock stress, and primary support safety. The innovative construction method has potential for wider application to similar engineering projects.