We investigate the influence of sequential excavation on stress distribution around tunnels excavated in discontinuous rock masses. To understand the governing physical mechanisms, we begin with an analytical approach, employing a closed-form stress distribution solution alongside simplified finite elements simulations. This allows us to examine stress rearrangement in the tunnel cross-section following excavation, comparing single-sequence versus multiple-sequence drifts. Our findings highlight a significant variation in stress magnitude and distribution across different tunnel opening layouts due to the application of multiple sequential excavations.
To validate our predictions for a discontinuous rock mass, we utilize the numerical Discontinuous Deformation Analysis (DDA) method, incorporating real field data from a well-documented tunnel collapse case history in Shibli, Iran. Further, to assessing the effect of discontinuous rock mass conditions on deformation during sequential excavation, we apply rock mass quality scaling based on the Geological Strength Index (GSI), running DDA simulations for GSI levels ranging from 50 to 80. We evaluate three excavation scenarios for each GSI level: full-face, top-heading and bench, and multiple drifts. In each case, we analyze stress distribution and magnitude while modifying the original DDA code to meet our research objectives.
Our results show that the excavation pattern directly affects stress redistribution around the tunnel. At lower GSI levels, transitioning from single to multiple drifts increases the spatial extent of the loosening zone by up to 20%, while arching stresses increase by as much as 50%. This indicates that in weak rock masses, the excavation sequence plays a critical role in controlling stress distribution and stability. In contrast, at higher GSI levels, the excavation pattern primarily increases the magnitude of arching stresses.