Abstract: To ensure the safe and efficient operation of the Longmenshan tunnel with a length of approximately 24 km and a buried depth of nearly 2 500 m, two cross-sectional layout schemes, two drill-and-blast main tunnels + middle TBM parallel guide tunnel, and drill-and-blast main tunnel + TBM main tunnel are proposed considering the requirements of tunnel operation safety and tunnel construction capabilities. Subsequently, theoretical calculations and analysis are conducted on the operating speed limit, design speed, building clearance, and inner contour dimensions in ultra-long tunnels. The following conclusions are drawn. (1) Based on the difference in brightness inside and outside the tunnel and driving behavior characteristics, the tunnel is divided into sections with different speed limits. The section from tunnel entrance to the first parking zone has a speed limit of 80 km/h, and the tunnel section—100 km/h. With a limited cost increase, the entire tunnel can be operated at a speed limit of 100 km/h. (2) The tunnel body section adopts a building limit widened from 75 to 150 cm on the left side for the passage of firefighting motorcycles, whereas the entrance section adopts a building limit without widening, with a designed speed of 100 km/h. Considering the need to increase the inner contour of the entrance section by ±4% of the maximum height and seismic fortification level 4, one inner contour is adopted for the entire tunnel, which can be horizontally offset by only 27 cm from the first parking strip at the entrance. (3) The TBM middle parallel guide tunnel is equipped with a dedicated smoke exhaust duct at the top, and its middle and lower parts are used as evacuation passages for personnel and vehicles, with a shielded TBM diameter of 9.2 m. The TBM main tunnel is equipped with a dedicated smoke exhaust duct at the top and a dedicated evacuation passage for personnel and small vehicles at the bottom, with a shielded TBM diameter of 13.3 m. The middle parallel guide scheme of TBM tunnels is recommended for better evacuation and rescue, as well as easier handling of the release of rockburst-induced TBM jamming.

Key words: Longmenshan ultra-long highway tunnel, cross-section layout, TBM, construction clearance, lateral widening