关键词: 围岩松动圈; 初期支护; 应力释放; 拉应变;

Abstract: After tunnel excavation, the surrounding rock stress will be released gradually, accompanied with increase of surrounding rock deformation and decrease of pressure.At this time, if the optimal timing of the primary support of the tunnel can be effectively determined, the self-supporting capacity of the surrounding rock can be put into full play to reduce requirement on strength of supporting structure and to control the rock deformation, ensuring the construction safety.Taking the criterion that the tensile strain of surrounding rock reaches the ultimate tensile strain and the rock mass begins to generate loose circle as the optimal timing of the primary support, we established a theory of determining the timing of the primary support based on the loose circle theory.Meanwhile, the change rules of stress, strain, displacement and loose circle of the surrounding rock in process of stress release were analyzed by using Midas GTS finite element software.Based on the loose circle theory, it is determined that the optimal timing of primary support is stress release reaching 60%,40% and 20% for the surrounding rock of grade Ⅲ,Ⅳ and Ⅴ respectively.

Key words: loose circle of rock mass; primary support; stress release; tensile strain;