Abstract: There are many choices in design scheme of auxiliary tunnels for mountainous expressway extralong tunnels, and it cost time and labor with high technical requirements. Therefore, the overall plan, structural design, and other key technologies of auxiliary tunnels and underground connecting tunnels are studied and summarized based on the design and consulting practice of multiple extralong tunnels completed. Main conclusions are as follows: (1) The study of auxiliary channel schemes should be determined from the perspectives of auxiliary construction and operational ventilation, and comprehensive scheme comparison and selection should be conducted. (2) For tunnels with small traffic volume, the smoke stroke can appropriately exceed 5 000 m under fire conditions, and full jet longitudinal smoke exhaust should be adopted. (3) The proposed "central underground fan room" scheme can reduce the scale of underground tunnel engineering by approximately 45%, effectively reducing construction risks and engineering costs. (4) The use of "complementary ventilation" can effectively reduce the scale of civil engineering and operating costs.

Key words: highway tunnels, auxiliary channel, underground connecting passage, ventilation plan, smoke itinerary