Applicability of the Initial Support Arch Cover Method in Subway Stations in Upper Soft and Lower Hard Strata

The subway station excavation project faces complex and variable geological conditions, and the choice of excavation method is currently a focus of attention. In conventional excavation methods, the cut-and-cover method occupies a large amount of land and generates noise pollution; the double-side wall pilot tunnel method has excessive strength reserves; the arch cover method has good adaptability to the upper soft and lower hard strata, but there is a problem with water seepage at construction joints. Qingdao City is located in a special geological environment of upper soft and lower hard strata, and most subway stations are built in the granite layer. The above construction methods are not very suitable for Qingdao, which has abundant groundwater resources. In order to study the applicability of the initial support arch cover method in subway stations in such strata, this paper uses midas GTS NX software to model and simulate a station on Qingdao metro line 1, and analyzes the laws of arch crown settlement, tunnel internal net convergence, and ground surface settlement combined with real-time monitoring data analysis. The results show that the initial support arch cover method can effectively ensure construction safety in such upper soft and lower hard strata, with the key construction link being the removal of the initial support and the excavation of the core soil. Timely support is crucial for maintaining the stability of the chamber and preventing ground subsidence.