Abstract: As the turning radius of a tunnel decreases, the overcut volume increases during shield tunneling. To reduce ground disturbance in shield tunneling with ultra-small turning radii and explore the technical feasibility of tunneling without over-cutters, a overcut-free tunneling method based on the optimization of shield geometric parameters is proposed. This method requires no over-cutters installed on the cutterhead, and utilizes the diameter difference between the cutterhead and the shield body to create the space required for curve turning. Compared with the conventional overcut mode, it can reduce the overcut on the inner curve side. First, a geometric constraint model for shield tunneling in ultra-small radius curves is established, and a quantitative method for determining the front shield length under overcut-free conditions is derived. Second, the design of key shield parameters is proposed, and subsequently, a tunneling control strategy adapted to the optimized shield is formulated. Finally, a case study is conducted on a tunnel with a turning radius of 110 m, verifying the feasibility of the proposed technical scheme. The results show that: (1) For shield construction in ultra-small radius curves, the overcut-free construction mode can be realized by establishing the geometric constraint model and fully utilizing the diameter difference between the cutterhead and the shield body. (2) Active hinge that allows a large hinge angle is preferred. Setting the hinge joint at the middle part of the shield can guarantee the turning capacity of the tail shield. (3) After the shield enters the curved section, the hinged posture shall be kept stable, and fine attitude adjustments are mainly completed by thrust jacks to reduce control variables. The settlement trend of soil at the tunnel springline changes continuously with shield advancement.

Key words: ultra-small radius tunnel, curved shield tunnel, ground disturbance, geometric constraint model, active hinge