Abstract: To address the issues of excessively high wind safety factors and significant deviations from actual conditions in current gas classification standards for traffic tunnels, the authors propose an optimization method that replaces the conventional fixed safety factor value with the ratio of the maximum to the average wind speed in the tunnel cross-section. Furthermore, numerical simulation techniques are employed to systematically analyze the ventilation flow fields of six traffic tunnels with different cross-sectional areas under four steady return-air wind speed scenarios, focusing on the spatial distribution characteristics of the wind speed safety factor and its relationship with cross-sectional area. The main findings are as follows. (1) The flow field near the tunnel face exhibits pronounced non-uniformity and can be divided into three distinct zones: the vortex zone, the transition zone, and the stable zone. The wind speed safety factor shows considerable fluctuations near the vortex zone but gradually stabilizes with increased distance from the face. (2) Based on the distribution characteristics of the safety factor within the stable zone, a new classification framework is established, categorizing traffic tunnels into small (40－80 m²), medium (80－110 m²), and large (≥110 m²) cross-sectional types, with recommended safety factors of 1.24, 1.20, and 1.16, respectively. (3) By refining the safety factor values, the threshold absolute gas emission rates for micro-gas, low-gas, and high-gas zones in the three tunnel categories are redefined, thereby establishing an optimized gas classification standard for traffic tunnels. These results offer important technical guidance for the design and construction of gas-prone traffic tunnels.

Key words: gas tunnel, gas classification standards, tunnel ventilation, wind speed heterogeneity, numerical simulation, safety factor