Abstract: In the process of tunnel construction under high stress, the problem of large deformation of weak surrounding rocks is severe. Although large deformation can be controlled and improved by support to a certain extent, the selection of the support type in actual engineering is difficult. Accordingly, the support types commonly used in the current soft rock tunnels with large deformation are summarized, namely, strong, prestressed, and yielding supports. According to the various bearing characteristics of a support system, these support types are classified into two: active and passive supports. Then, based on the deformation law of the surrounding rocks after tunnel excavation under various support strengths, the applicability of the passive support mode is explored. A selection method of active and passive support modes with the extrusion factor 〖WT５《TNR#I》〗N〖WT５《TNR》〗c as the control index is proposed. When 〖WT５《TNR#I》〗N〖WT５《TNR》〗c≥0.2, the current strong support mode can effectively support the surrounding rocks, and when 〖WT５《TNR#I》〗N〖WT５《TNR》〗c<0.2, the active support can be considered according to the deformation of the surrounding rocks on the site. Additionally, based on the material characteristics of the anchorage system, further analysis of the applicability range of active support shows that a prestressed anchor bolt system can be used when the applied prestress does not exceed 200 kN, whereas a prestressed anchor cable system is recommended when the applied prestress exceeds 200 kN. Based on the prestressed anchor cable system, a selection method of active and activeyielding support types with the cable elongation 〖WT５《TNR#I》〗ε〖WT５《TNR》〗a as the control index is proposed. That is, the active support can be used when 〖WT５《TNR#I》〗ε〖WT５《TNR》〗a in a working state does not exceed 2.3%, and when it exceeds 2.3%, it is suggested to adopt the activeyielding support with greater deformation capacity. Besides, relying on the typical large deformation section of the Muzhailing highway tunnel, the support types were selected based on the aforementioned basis and corresponding field tests were performed. The results somewhat verify the feasibility of the proposed selection basis.

Key words: highway tunnel, soft rock large deformation, support type, anchorage system type, squeezing factor, elongation