Stability classification and construction method analysis of subway stations in upper-soft and lower-hard strata

•Statistical analysis of the parameters of soft and hard strata in China, as well as subway cross-sections and construction methods.•Stability grading criteria for subway stations in soft and hard strata.•Propose instability prevention measures for each stability grade of subway stations in soft and hard strata. In congested urban underground space, a subway station may have to be constructed within an unfavorable complex geological formation comprising upper-soft and lower-hard strata, at a higher risk of structure damage and ground surface subsidence during the construction process. When the subway station is constructed using the undercutting method in a complex urban environment, there is currently lack of theory or guidance on the selection of appropriate construction method for subway stations in different upper-soft and lower-hard strata conditions. This paper conducted a series of numerical simulation, model test, and field test to specifically investigate the stability classification of large-span underground subway stations in upper-soft and lower-hard strata. Results indicate the following: (1) The thickness of the overlying soft rock serves as an indicator for the stability classification of large-span subway stations in upper-soft and lower-hard strata. (2) The stability of large-span subway stations in upper-soft and lower-hard strata can be generally divided into five levels (W-I, W-II, W-III, W-IV, and W-V) from very stable to unstable. (3) For each classification, appropriate construction method was proposed and validated through typical engineering case studies. The research conclusions of this paper can provide effective reference for the selection of construction safety and stability methods for subway station excavation.