Abstract: Rational geometric parameters of spiral highway tunnels are essential for traveling safety of vehicles. Herein, data research and theoretical analysis are employed to establish the relationship between tunnel alignment and accident rates. Then, factors affecting tunnel slope, length, and curvature radius are identified, and the threshold values for slope and slope length under different influencing factors are analyzed. The results show the following: (1) Under the same longitudinal slope, downhill sections exhibit a higher accident rate than uphill sections. Under the same slope, trucks are more affected by slope length, and the accident rate increases sharply when the tunnel′s curvature radius is less than 600 m. (2) The factors affecting tunnel slope primarily include locomotive climbing performance, climbing balance speed, allowable climbing speed, and operational ventilation. The factors affecting slope length is brake temperature, while the factors influencing curvature radius primarily include vehicle headlight scattering angle, sight distance, cross slope, driving safety, and operational ventilation. (3) Based on factors affecting tunnel slope, the threshold for the longitudinal slope of spiral tunnels with large longitudinal slope and small-radius is determined. Using a brake hub temperature of 200 ℃ as a control benchmark, and in combination with safety evaluation criteria, tunnel slope length thresholds for different slopes and design speeds are provided. Additionally, based on factors influencing curvature radius, the curvature radius threshold for spiral tunnels with large longitudinal slope and small-radius is established. (4) Based on the threshold values for slope, slope length, and curvature radius, the maximum allowable longitudinal slope, design speeds for different slopes, and geometric design recommendations for spiral tunnels with large longitudinal slope and small-radius are provided.

Key words: spiral highway tunnel, linear design, slope, slope length, curvature radius, geometric parameter design