EVOLUTION MECHANISM OF DAMAGE ZONE IN SURROUNDING ROCK MASS DURING EXCAVATION OF DEEP TUNNELS UNDER HIGH GEOSTRESS CONDITION

As the development activities of human underground space goes to deep earth, the high geostress has become one of the typical geological characteristics of underground rock mass engineering and has an important effect on the safety and stability of surrounding rock mass. In this paper, the distribution regularities of geostress field in our country are firstly analyzed. And then, the damage of surrounding rock mass induced by quasi-static unloading and transient unloading of geostress is studied by the methods of theoretical calculation and numerical simulation. At last, according to the testing results of damage in surrounding rock mass in Canada URL underground laboratory and diversion tunnels of Jinping Ⅱ hydropower station, the influence of unloading way of geostress on the damage formation of surrounding rock mass is analyzed. Results show that comparing with quasi-static unloading of geostress, transient unloading of geostress can produce an extra dynamic stress in surrounding rock mass and enlarge the unloading effect in radial and loading effect in tangent, which makes the surrounding rock mass easier to damage. In addition, the increase of lateral pressure coefficient can increase the damage range of surrounding rock mass induced by quasi-static unloading and transient unloading of geostress augments. In the direction of minimum principal stress, the main damage form of surrounding rock mass is shear failure. While in the direction of maximum principal stress, it shows as tension failure.