Abstract: Herein, a case study is conducted on the Jintang subsea shield tunnel project of the NingboZhoushan railway to investigate the effect of docking distance on the instability mechanism and pattern of the excavation face for a largediameter subsea shield. A finite element model of shield tunnel docking considering seepage is developed. Then, the effect of the docking distance (〖WT5BX〗L〖WT〗〖WT5《TNR》〗) on the instability mechanism of the excavation face and ultimate support pressure is examined. A theoretical model for determining the ultimate support pressure for shield closedistance docking is established using the limit equilibrium method and considering seepage. As the docking distance increases, the shape variation of the excavation face and variation of the ultimate support pressure can be divided into three stages. (1) The rapid growth stage (0<〖WT5BX〗L/D≤0.4)(D〖WT〗〖WT5《TNR》〗 represents the tunnel diameter) : the difference between the condition of this stage and a single tunnel working condition is the most obvious, and the instability zone comprises wedge bodies and silos. At this stage, the docking tunnel causes maximum inhibition, resulting in significant changes in the waterhead field near the excavation face, with the seepage force playing a dominant role. (2) The slow growth stage (0.4<〖WT5BX〗L/D〖WT〗〖WT5《TNR》〗≤0.7): the instability zone comprises logarithmic spirochetes and silos. At this stage, the reduction in the inhibition caused by the docking tunnel leads to a decrease in the influence of the ultimate support pressure on the docking distance and the growth of the ultimate support pressure occurs at a low rate. (3) The gentle growth stage (0.7<〖WT5BX〗L/D〖WT〗〖WT5《TNR》〗≤1.5): the instability zone comprises logarithmic spirochetes and chamfers. At this stage, the inhibition caused by the docking tunnel can be ignored and the ultimate support pressure and instabilityzone shape are comparable to those without docking conditions, which can be considered as a single tunnel working condition. The results obtained from the established theoretical model are close to those obtained by numerical simulations, with an error within 10%, thereby confirming model feasibility.

Key words: subsea tunnel, largediameter shield, docking engineering, excavation face instability, theoretical model