Experimental study on the characteristics of rockburst occurring at the working face during tunnel excavation

Rockburst occurs not only at the sidewall but also at the working face during the excavation of a deeply buried tunnel according to engineering practice. In our study, we try to obtain a good understanding of the occurrence process and characteristics of the rockburst occurring at the working face (WF rockburst). An experimental study, tunnel model test on the cubic (150 mm x 150 mm x 150 mm) granite specimen with non-through circle holes, was conducted. The characteristics of WF rockburst were first investigated. Then, the influences of the tunnel axis stress and rock pillar thickness (the distance between the two opposite working faces during tunnel construction using the two-way excavation method) on WF rockburst were studied. The experimental results indicated that WF rockburst can be successfully simulated by conducting the tunnel model test. WF rockburst exhibited differences compared with the rockburst occurring at side wall (SW rockburst). WF rockburst occurred before SW-rockburst. The scale of WF rockburst was larger than that of SW rockburst, but the intensity of WF rockburst was generally less than that of SW rockburst. In addition, the tunnel axis stress controls the occurrence of SW rockburst. A higher tunnel axis stress decreases the difference between the local energy release rate (LERR) at working face and that at sidewall. When LERR at working face increases to that at sidewall under an enough high tunnel axis stress condition, WF rockburst can be triggered. In addition, WF rockburst may still occur when the thickness of the rock pillar decreases to 1 times the diameter of the tunnel even when the tunnel axis stress is low, which may be due to the superposition of the stress concentration at the opposite two working faces. Furthermore, with a decrease in the rock pillar thickness, the intensity of the WF rockburst increases.