Abstract: Engineering disasters caused by harmful gases in tunnels across China are frequent, primarily involving poisoning and asphyxiation, combustion, and chemical corrosion. While a relatively comprehensive survey and evaluation system has been established presently for coal seam gas, it is not well suited for noncoal strata. This inadequacy substantially hinders the high-quality development of noncoal gas tunnel construction. To address this issue, field detection, in-situ testing, theoretical analysis, and onsite measurement and calculation methods are employed to develop a gas risk level evaluation system for harmful gases in noncoal tunnels during survey and construction phases. The main findings are as follows: (1) During the survey phase, the conventional method of midway detection + final hole detection + hole sealing detection can be used to determine the gas grade in general noncoal gas sections through boreholes. In high gas concentration zones, gas accumulation and escape testing enable the determination of gas grade and type based on the gas accumulation rate (ν_g) and the escape curve. (2) During the construction phase, the absolute gas emission (Q_absolute) from the excavation face can be accurately measured using recorded air duct pressure ventilation data and an optimized layout of gas detection points. (3) The Q_absolute of the gas pocket in front of the excavation face can be evaluated using an innovative method of tunnel seismic prediction + advance drilling + detection test, integrated with forward geological prediction work. (4) This gas grade evaluation method for noncoal stratum tunnels has been successfully applied in projects such as the Chengdu-Chongqing high-speed railway and the Chengdu-Meishan urban (suburban) railway line.

Key words: noncoal stratum tunnel, gas grade evaluation, gas accumulation testing, gas escape testing, absolute gas emission, advance geological prediction