Abstract: To investigate the characteristics and influencing factors of the large deformation in the tuff stratum of the long and highaltitude tunnels located in the QinghaiTibet Plateau, the field investigation, monitoring data analysis, and numerical simulation are adopted to analyze the characteristics of stress distribution, deformation, and failure of large deformation section and discuss the influencing factors for large deformation in highaltitude area. The research and analytical results show the following: (1) The horizontal tectonic stress field is the main stress field in the large deformation section of Ailashan tunnel, and its magnitude is 2.12~2.8 times that of gravity stress field. (2) The deformation failure characteristics of the primary support are mainly manifested as cracking, falling blocks, steel frame distortion, and boundary intrusion. (3) There are 55 cracks with different lengths, widths, depths, and directions on the left and right sidewalls of the secondary lining. (4) The inverted arch is dominated by the central uplift, with a maximum uplift of 382 mm, while the uplift is uneven and the bending failure occurs. (5) The settlement of the crown is 268.80 mm, the maximum horizontal convergence is 924.9 mm, and the maximum daily deformation rate is 110.27 mm. The large deformation is characterized by large amount, fast rate, long duration, and concentrated location. (6) The initial high ground stress field and the lithologic conditions of soft tuff are the main influencing factors for the large deformation of the Ailashan highway tunnel, and the construction method of the original twobench method, the supporting time, and the insufficient stiffness of the supporting structure are the important influencing factors for the large deformation.

Key words: highway tunnel, highaltitude tunnel, tuff stratum, large deformation, failure characteristics