Influence of Over-Undercut on the Stability of New Mountain Tunnels

Using the Chongqing University City double-track tunnel project as a case study, this study comprehensively applied field measurements and numerical simulation methods to analyze the impact of over-undercuts during tunnel construction on tunnel stability using finite element simulation. Using Intelligent scanning equipment for excavation contours, the over-undercut conditions of the surrounding rock after blasting were measured in detail. The finite element software ANSYS was employed to establish a stratum structure model to simulate and analyze the specific effects of over-undercutting on tunnel stability. The study found that the range of measured overcut amounts varies between 0.02 m and 0.74 m, with an average overcut between 0.29 m and 0.41 m. The average errors for the three experimental sections were 9.4%, 8.6%, and 6.5%. The accuracy and validity of the numerical simulation method were verified by comparing the measured data with the numerical simulation results, with the error range meeting the acceptable fitting requirements. The maximum principal stress of the surrounding rock reached 1.29 MPa, while the minimum principal stress was − 8.18 MPa. Under different conditions, the maximum principal stress occurs at the over-excavation of the arch shoulder, whereas the minimum principal stress is concentrated at the sidewalls. Over-undercutting causes significant stress concentration. The maximum principal stress on the lining ranged from 0.95 MPa to 1.08 MPa, while the minimum principal stress ranged from − 1.90 MPa to − 3.85 MPa. Over-undercutting increases the pressure on the surrounding rock, thereby transferring stress to the support structure. Over-excavation of the arch shoulder and sidewalls causes stress concentration, leading to uneven deformation and development of cracks in the initial support structure, which may result in cracking and deformation. These findings not only provide new theoretical insights into controlling the surrounding rock stability in tunnel engineering, but also offer technical support for the development of underground spaces with significant academic and engineering application value.