Study on CO concentration distribution and ventilation scheme optimization in ultra-long drift-type high-altitude tunnel construction

In high-altitude regions, low atmospheric pressure and air density markedly reduce ventilation efficiency during tunnel construction, causing CO accumulation and severe safety risks. Using the Gongga Tunnel, a national key project, as a case study, this study combines numerical simulation and field experiments to clarify the drift-type ventilation flow and CO migration in the main-pilot tunnel system under high-altitude conditions. The effects of ventilation parameters on system performance were analyzed, the optimal ventilation scheme was proposed, and its effectiveness was validated on site. The results indicate that vortices formed near the working face due to airflow conflict lead to CO concentration peaks 50-125 m from the face. CO migrates toward the tunnel portal as gas masses, expanding in volume and decreasing in peak concentration. Due to cross-sectional shape differences, diffusion in the main-pilot tunnels differs, but both show central CO peaks and lower edge concentrations, matching field tests. Comparative analysis shows that parameter influence on ventilation efficiency follows: duct-working face distance > duct outlet velocity > duct layout position. Optimal performance occurs when duct height is 7.1 m, outlet distance 50 m, velocity 20 m/s, reducing average CO peak concentration by about 22% after 360 s of ventilation. • The internal airflow field shows a "low-center, high-sidewall" velocity profile within 100 m from the working face; beyond that, it stabilizes with nearly uniform cross-sectional wind-speed distribution. • By adjusting three main ventilation parameters, the vortex effect near the working face is attenuated, effectively reducing the accumulation of CO. • Due to cross-sectional shape differences, diffusion in the main-pilot tunnels differs, but both show central CO peaks and lower edge concentrations. • Optimal performance occurs when duct height is 7.1 m, outlet distance 50 m, velocity 20 m/s, reducing average CO peak concentration by about 20% after 360 s of ventilation.