Fracture distribution and deformation mechanism of surrounding rock in two-way extremely unequal pressure soft rock roadway

The failure of butterfly plastic zone in roadway surrounding rock is an important reason for the large deformation of roadway surrounding rock. In recent years, it has attracted the attention of roadway support workers. The understanding on the deformation mechanism of surrounding rock in the butterfly plastic zone is the basis for realizing the control of surrounding rock in soft rock roadway. However, there are limited studies on the deformation mechanism of surrounding rock in the butterfly failure theory. In view of the above problems, the large deformation characteristics of the south transport roadway in a mine in the southern China are taken as the research object. Based on the analysis of the stress environment and geological conditions of the roadway, the distribution of surrounding rock cracks, the deformation mechanism and control methods of the roadway are systematically studied. The results show that the south transport roadway is a typical extremely unequal pressure soft rock roadway. After the excavation of the roadway, the surrounding rock cracks are distributed in a ‘butterfly’ shape. The ‘butterfly fracture zone’ is concentrated in the ‘butterfly plastic zone’. The fractures of the surrounding rock at the butterfly leaf are mainly shear fractures, and the shear fractures at the butterfly leaf are mostly distributed around the roadway in a ‘ring’ distribution, while the tensile fractures are concentrated near the free surface of the surrounding rock of the roadway. There are two main aspects of the stress characteristics of the surrounding rock in the butterfly plastic zone. One is the deflection of the principal stress direction of the surrounding rock, which is manifested in the annular distribution of the maximum principal stress around the roadway. The direction of the minimum principal stress in the upper part of the roadway points to the center of the roadway, and the direction of the minimum principal stress in the lower part of the roadway deviates from the center of the roadway. The second is that the maximum / minimum principal stress ratio of the surrounding rock is large, and the principal stress ratio contour is ‘butterfly’ distributed. According to the distribution characteristics and stress characteristics of surrounding rock cracks in plastic zone, the mechanical dilatancy model of surrounding rock with weak surface is established. It is considered that the dilatancy effect of surrounding rock under this stress characteristic is strong, and the dilatancy effect makes the surrounding rock squeeze into the roadway space, which makes the roadway produce large deformation. The irregular distribution of surrounding rock fissures and the unreasonable original support scheme are the main reasons for the large deformation of the roadway. The study mentioned above is used to suggest a differentiated support system, the heart of which is ‘full-section prestressed short anchor cable + key part long anchor cable strengthening support + grouting’. Field observation demonstrates that the revised support plan can successfully manage the surrounding rock’s deformation and guarantee the stability of the roadway while it is in operation. © 2024 China Coal Society. All rights reserved.