Mechanism analysis and type determination of the rockburst of the Gaoloushan tunnel based on a study of rockburst fragments

The Gaoloushan Tunnel is a control project for Longnan city and Jiuzhaigou, Sichuan Province. With the characteristics of "three high and one large," this project is a typical representative of a deeply buried, long highway tunnel under complex geological conditions. In this dissertation, two types of rockburst in the field are taken as the research objects. By impacting a rockburst experimental system and setting different stress paths, an image acquisition system recorded the entire process of impact rockburst and slab buckling rockburst in real time to analyze the characteristics of spalling and ejection. Rockburst due to static loading was performed and compared with rockburst caused by static loading + dynamic loading. Then, the quality, scale distribution, shape, and fractal dimension of slab buckling rockburst test fragments (rockburst fragments 1), impact rockburst test fragments (rockburst fragments 2), and unknown types of rockburst fragments (rockburst fragments 3) collected in the field were compared. On this basis, the image change process of the rockburst test was combined to deepen the understanding of the causes of different types of rockburst fragments and the rockburst mechanism. The results show that (1) slab-buckling rockburst and impact rockburst differ in the damage dominant mechanism, one being tension damage dominant, the other being tension-shear damage dominant. V-shaped and pan-shaped rockburst pit shapes are consistent with the on-site rockburst situation, which proves the rationality of this experiment. Slab-buckling rockburst has attenuation characteristics during the rockburst process, unlike the impact rockburst. From an analysis of debris ejection velocity and rockburst pit shape, impact rockburst is more severe than slab buckling rockburst. (2) Type 1 rockburst fragments are dominated by medium-grained slate fragments, which are more easily broken in the length direction and have a much larger mass than rockburst fragments 2, which is closely related to the incubation mechanism of slab buckling rockburst with a slabbing failure structure formed by vertical stress concentration. (3) Because of the intervention of dynamic load, type 2 rockburst fragments are obviously sheared, so they are easier and more broken in the thickness direction, forming fragments dominated by coarse-grained flake fragments, which may be more harmful because of their large mass, large volume, and long ejection distance in on-site rockburst. (4) The above comparison and analysis show that type 3 rockburst fragments probably correspond to impact rockburst, and the flaky and "V"-shaped characteristics are a unique fragment type of this rockburst.