Spatiotemporal Stability Responses of Tunnel Excavation Under Cyclical Footage Impact: A FLAC3D-Based Numerical Study

Cyclical footage, a fundamental parameter in tunnel construction, has received limited attention despite its critical role in ensuring excavation stability and safety. To investigate the underlying mechanisms by which different cyclical footage values influence the stability of deeply buried tunnels, the Fast Lagrangian Analysis of Continua in Three Dimensions (FLAC3D) was employed to simulate the spatiotemporal evolution of tunnel responses during excavation. The simulation results reveal that each cyclical footage scenario produces a “segmental decline” pattern in tunnel displacement. Unlike shallow tunnels, where peak displacement typically occurs near the tunnel face, in deep tunnels the maximum Y-direction displacement appears at the center of the excavation face. Furthermore, as cyclical footage increases, the displacement extent first expands and then contracts. The stress release process is found to be minimally affected by cyclical footage, with vertical strain in the tunnel roof nearly fully released within 1 m ahead of the excavation face. Therefore, deploying tunnel support structures beyond this 1 m zone may enhance their effectiveness. This study provides a theoretical reference for optimizing excavation parameters in deep tunnels and contributes to reducing the likelihood of support failure during underground construction.