Optimization of Shield Construction Sequence and Excavation Face Longitudinal Spacing in Small-Clearance Three-Lane Tunnels

With the rapid development of urban rail transit, a large number of small-clearance tunnel shield projects have emerged, among which the theory of small-clearance tunnel construction design for three-lane tunnels has not yet been well established. Based on an actual project, this paper conducted an optimization analysis of the construction sequence, designed four excavation conditions, and examined the influence of the longitudinal spacing between tunnel excavation surfaces on surface settlement, segment displacement, and surrounding rock pressure. The results showed that: the sinkholes under the four conditions were all V-shaped, with a width of approximately 55 m (9D); the peak settlement in Case 4, where the middle tunnel was excavated first, was 10.16 mm, smaller than that in Case 2, where the tunnels on both sides were excavated first (11.10 mm); the range of ground surface settlement extended from 2.5D behind the excavation face to 1.5D ahead of the excavation face in the case of singlelane construction; when adjacent lines were constructed simultaneously and the axis spacing was less than 20 m, the working faces of the two tunnels maintained at least 16 m (2.5D) longitudinal spacing. When existing tunnels were present on the other side of the first tunnel, the longitudinal spacing of the excavation faces was kept at least 12 m (2D) to minimize the impact on ground surface settlement, segment displacement, and surrounding rock pressure.