Evaluation of the Additional Stress on Adjacent Tunnel Shafts Induced by Foundation Pit Excavation

The prevalence of T-shaped structures comprising interconnected tunnels and shafts is steadily increasing. Through a two-stage analysis approach, the influences of unloading and barrier action of the supporting system under the bedding and the sidewall of the foundation pit were analyzed, and a mechanical model considering the additional stress produced by the tunnel shaft under the action of the foundation pit excavation was developed. Utilizing the Mindlin solution, this study computed the additional stress on the adjacent tunnel shaft caused by foundation pit excavation, and the optimal equation for calculating the additional stress was identified using MIDAS GTS-NX software. Through single-factor variable analysis, the influencing factors, such as excavation length, width, excavation depth, minimum distance between the shaft and foundation pit, and stress loss rate, were varied, and the influence of unloading at the bottom and sidewall of the foundation pit on the additional stress of the shaft was calculated and examined. The following conclusions can be obtained: (1) when bedding is excavated on the side of the tunnel shaft, the influence of unloading on the bottom of the foundation pit is greater than that on the sidewall; (2) when the vertical shaft is subjected to a horizontal stress load on the side of the foundation pit along the depth direction, the joint of the tunnel and vertical shaft is subjected to the maximum stress; (3) when the turning point of vertical stress changes (from upward to downward) occurs below the surface, this indicates that the assembled pipe joint structure of the shaft experiences the risk of tension disconnection under the action of foundation pit unloading; and (4) sensitivity analysis of the horizontal and vertical additional stress factors reveals that the sensitivity of the distance between the shaft and the pit is the highest, followed by the excavation depth.