Analysis of Soil Displacement Field Caused by Small‐Radius S‐Shaped Tunnel

Small‐radius curved tunnels are increasingly adopted in urban areas to navigate around existing infrastructure. However, their excavation induces more complex and asymmetric soil displacements compared to straight tunnels, especially in S‐shaped alignments where two consecutive reverse curves interact. This study uses 3D numerical modelling (Plaxis 3D) to compare soil settlement patterns from straight, single‐curve and S‐shaped tunnels in soft clay. Results show that the S‐shaped tunnel causes the greatest settlement and the broadest influence zone, with a clear coupling effect between two consecutive reverse‐curvature curves leading to a wavy settlement profile. The settlement field is notably asymmetric, influenced by the intersection angle between the tunnel axis and adjacent infrastructures. The risk assessments of nearby infrastructures are highly dependent on their spatial relationship with the S‐shaped tunnel. These findings highlight the heightened geotechnical risks associated with S‐shaped curved tunnels and underscore the need for tailored design and monitoring strategies in urban tunnelling projects.