Abstract: In order to quantitatively evaluate the stability of tunnel surrounding rock and guide the support design, the concept of critical stable section of tunnel and the stability analysis method of tunnel surrounding rock based on critical stable section are proposed. The main contents are as follows: 1) The critical stable section of a tunnel is the largest section with the same central buried depth and similar geometry as the designed excavation section, and the surrounding rock can be self stable without support. 2) When the design excavation section is smaller than the critical excavation section, the surrounding rock between the critical stability section and the design excavation section can be used as a support structure; and when the safety factor meets the design requirements, it is considered that the surrounding rock can be self-stable for a long time, except for local protection, no system support is required, otherwise supplementary engineering support measures are required. 3) When the design excavation section is larger than the critical stable section, the "artificial" support is required. 4) A method for calculating the design support force is proposed, in which, the surrounding rock in the damage area is considered to be loose, and the design support force should be able to maintain the stability of the loose body with a certain safety factor. This method is employed to analyze two typical sections of the critical stable sections of railway tunnels, and to calculate the self-bearing safety factor of surrounding rock and the required ‘artificial’ support force under the conditions of different surrounding rock grades and different burial depths.

Key words: critical stable section, stability of surrounding rock, safety factor, supporting force, instability criterion, quantitative design