The accurate prediction and evaluation of stress and displacement fields of surrounding rock is the fundamental premise for the deformation control of soft rock tunnels under high geo-stress condition. However, due to the complicated mechanical characteristics of soft rock with confining pressure influence, the current numerical simulation method usually regards the mechanical parameters of surrounding rock as constant and ignores the variation of these parameters in the simulation process, which leads to results that cannot accurately reflect the mechanical behavior of surrounding rock. Therefore, this paper firstly investigates the effect of confining pressure on deformation and strength parameters for soft rock and proposes corresponding variable models for mechanical parameters with the confining pressure influence. Secondly, a transversal loop discriminant update procedure is proposed and introduced into the iteration calculation process of FLAC^3D, thus forming an improved numerical simulation method. This improved method can integrally consider the mechanical parameter variation of surrounding rock with variable confining pressure and realize the automatic update for such a parameter with its variable stress state. Finally, as an application example, an improved expression of longitudinal deformation profile (LDP) for tunnels considering the confining pressure influence is proposed based on numerous simulation results for a soft rock tunnel obtained by this proposed method.

The accurate understanding of the influence of confining pressure on the mechanical characteristics of soft rock and how to comprehensively consider this influence in the elastoplastic analysis of tunnels are the fundamental premises for the effective evaluation of the deformation control and stability of soft rock tunnels. Therefore, this paper firstly investigates the effect of confining pressure on the deformation and strength characteristics of phyllite and slate, using triaxial experiment results and proposed variable models for the mechanical parameters (E, v, c, φ) of soft rock with confining pressure variation. Secondly, according to the second stress state around tunnels and these variable models for the mechanical parameters of soft rock, a new elastoplastic solution for tunnels is devised, which simultaneously considers the effect of confining pressure on the deformation and strength characteristics of the surrounding rock. Finally, with the proposed elastoplastic solution, the effect of multiple factors (initial pressure, supporting force, and tunnel radius) on the stress and displacement of tunnel surrounding rock is analyzed.