Abstract: In the tunnel construction process, the displacement monitoring data are usually used as the most important basis for identifying the stability of the surrounding rock, and sometimes even the only one. However, in the tunnel design process, no displacement criterion is established for identifying the stability of the surrounding rock, for the large differences among surrounding rocks with different lithology and geological environments. Furthermore, shape, span and burial depth of the tunnel would probably also effect the displacement of the surrounding rock. On the other hand, statistical analysis of the on-site displacement data indicate that under general conditions of surrounding rock, the relative displacement value of the same grade rock has obvious cohesion compared with obvious differences between the relative displacements values of the different grades. With this characteristic, an identifying rule of stability can be setup for all grades of surrounding rocks with statistical analysis of the on-site displacement data; and then the rule can be used for classification of surrounding rocks. With the wide application of mining method based on the main thought of the new Austrian tunneling method(NATM) and controlling displacement of surrounding rocks in the construction of high-speed railway and expressway tunnels; the above identifying rule of stability and classification method of surrounding rocks will more in line with current technological level of tunnel construction.

Key words: tunnel, displacement monitoring, stability of surrounding rock, statistical analysis, identifying rule