Numerical Simulation Analysis of Surrounding Rock Instability Mechanism in Shallow Tunnel Excavation

The buried depth of shallow tunnel is shallow, and it is easy to cause collapse disaster caused by surrounding rock instability during construction. In order to study the instability mechanism of surrounding rock of shallow buried tunnel, this paper establishes a three-dimensional mechanical model of shallow buried tunnel excavation by FLAC3D numerical simulation software, analyzes the evolution law of displacement, stress and plastic zone during shallow buried tunnel excavation, discusses the influencing factors of surrounding rock instability, and reveals the instability mechanism of surrounding rock of shallow buried tunnel. The results show that the vault displacement increases slowly with the tunnel excavation, and increases sharply after the tunnel face passes through the monitoring section, with an increase of 152.55%. The vault stress continues to release, and the maximum principal stress decreases from 350 kPa to 4.46 kPa. At the same time, there is a significant shear stress concentration area, and the maximum shear stress reaches 348.51 kPa. The bearing capacity of surrounding rock is gradually lost, and finally a ring-through plastic zone is formed around the tunnel. With the increase of shear modulus ratio and thickness ratio of hard and soft rock above the tunnel and the decrease of buried depth of the tunnel, the stability of surrounding rock is improved. For shallow buried tunnel construction projects, geological exploration should be conducted in a timely manner to predict the safety of tunnel construction under the geological conditions and make reasonable improvements to the rock mass in advance, in order to enhance the shear modulus and the self stabilization ability of tunnel surrounding rock.