UNLOADING TEST FOR ROCKBURST MECHANISM IN TUNNEL MODEL

With research on "surrounding rock-roadway" system, the unloading rockburst can be recreated by the operation of horizontal stress unloading. The results indicate that the unloading rockburst has experienced evolution process of from "nonchalant stage to small particles ejection stage to flake off stage and to rockburst".The tunnel model produces a number of the process from rockburst to stress adjust then to stress adjust failure and to another rockburst. At the end it forms "V"shape rockburst hole. In the condition of unloading, it exists some relationship between horizontal stress and fracture model. The unloading operation of tunnel model of distribution stress can be obtained from theoretical derivation. Vertical crack stress in different horizontal stresses(8.9MPa, 13.3MPa, 17.8MPa) is taken by modified Griffith failure criteria. The calculation model of "V"shape is established. At the stress point(13.3MPa) when the horizontal stress is under 13.3MPa, the relationship between horizontal stress and vertical crack stress has a positive correlation. When the horizontal stress is above 13.3MPa, the relationship between the horizontal stress and the vertical crack stress is a negative correlation. When the horizontal stress is at 13.3MPa, the strain energy concentrated area is the largest, and distance between this area and side wall is the minimum.