Analysis on mechanical model and impact factors of unloading failure of rock surrounding in soft rock roadway

In order to study the mechanical response and large deformation instability mechanism of deep soft rock roadway under excavation unloading effect， the excavation and unloading mechanical model of surrounding rock of roadway is established based on the unified strength criterion and rock three line segments mechanics model.According to the distribution characteristics of secondary tangential stress and equivalent shear stress of surrounding rock after roadway excavation， the mechanical structure of surrounding rock is divided into shallow bearing circle， deep bearing circle， stable-self bearing circle， main bearing circle and key bearing circle.Through theoretical analysis， the analytical solutions of secondary stress field， displacement field and the scope of softened zone and broken zone are obtained after the soft rock roadway excavation.Combined with the mechanical parameters related to a soft rock roadway， the influencing factors of surrounding rock yield stress， surrounding rock fracture and softened zones and surrounding rock bearing structure under the roadway excavation unloading are analyzed.The results show that the yield stress of surrounding rock decreases with the decrease of initial cohesion and internal friction angle and the increase of excavation speed，and the surrounding rock is more likely to yield and damage.The scope of softening zone of surrounding rock is related to the initial mechanical parameters of surrounding rock， and the scope of softening zone of surrounding rock decreases with the increase of initial cohesion.The scope of fractured zone decreases with the softening modulus of cohesion，shear expansion angle and excavation speed.The range of the main bearing circle and the key bearing circle， the peak value of the radial stress and the axial stress increase with the increase of the softening degree of surrounding rock， and transfer to the roadway radial depth.