With the continuous increase in urban traffic, more and more existing tunnel reconstruction and expansion projects have been put on the agenda. It is of great significance to carry out theoretical research on the reconstruction and expansion of existing tunnels. After simplifying the existing tunnel and the tunnel to be expanded as equivalent circles, the evolution theory of the loose zones of the existing tunnel and the tunnel to be expanded is studied using the Mogi-Coulomb strength criterion. The secondary development of the Mogi-Coulomb criterion is realized based on the finite difference software FLAC3D. After verifying the theoretical calculation results and numerical simulation results through engineering examples, the relevant influencing factors are analyzed, and the following conclusions are drawn: (1) The influence of intermediate principal stress σ₂ must be considered during the reconstruction and expansion of deep-buried tunnels. σ₂ not only affects the development process of surrounding rock displacement u and plastic zone R_Ⅱp/R_Ⅱ and loose zone R′_Ⅱ/R_Ⅱ, but also affects the final results of u, R_Ⅱp/R_Ⅱ and R′_Ⅱ/R_Ⅱ; (2) When the tunnel is buried deeper and the cross-section of the expanded tunnel is larger, the final deformation u of the surrounding rock of the expanded tunnel is greater. When the cross-section of the expanded tunnel is larger, the influence of the tunnel burial depth on u is greater. Only the tunnel burial depth has a greater influence on the range R′_Ⅱ/R_Ⅱ of the loose area.(3) The influence of the size of the existing tunnel section and the strength of the existing supporting structure on the surrounding rock of the expanded tunnel is mainly concentrated on the advance deformation in front of the tunnel face. The smaller the existing tunnel section and the smaller the bearing capacity of the existing supporting structure, the greater the advance deformation. During the construction process, the influence of the advance deformation in front of the tunnel face should be given due attention.

The excavation of the cavern is accompanied by the deterioration of the parameters of the rock mass, and it is of great significance to describe the deterioration of the parameters to guide the engineering practice. In order to describe the deterioration of the deformation parameters, based on the Hoek-Brown criterion, the damage threshold is used to characterize the degree of deterioration of the elastic modulus, and the equilibrium equation with the damage threshold is simplified by the Taylor series to obtain the analytical solution of the radius of the plastic zone and the damage threshold is given. The value method, combined with the finite difference software FLAC3D, verifies the rationality of the formula. Based on the two assumptions of constant elastic strain and variable elastic strain, the plastic zone displacement expression including the damage threshold is obtained by combining the uncorrelated flow criterion. Finally, the variation law of the radius of plastic zone and the displacement of the cave wall under different parameters is analyzed by an example. The results show that the more serious the deterioration is, the easier the cavern is to be unstable. As the damage threshold increases, the displacement difference and plastic zone radius under the two assumptions gradually increase, and the displacement obtained by the constant elastic strain assumption is smaller. In order to avoid instability and damage of the cavern, support should be provided in time.