Surface Settlement of Small Clear Distance Tunnel in Soft Soil Layer during Shield Propulsion

Shield tunneling has become a widely adopted method for urban underground transport construction. However, research on the impact of small-clearance shield tunnel construction in urban soft soil layers is still in its early stages. Understanding the effects of such construction on surface settlement and summarizing temporal settlement patterns is crucial for reducing construction risks and minimizing disturbances to the surrounding areas. This paper investigates the impact of two-lane small-clearance tunneling on surface settlement through finite element modeling and analysis, focusing on the Nanjing Heyan Road Crossing as a case study. The study also evaluates the feasibility of different construction distances in successive two-lane shield tunneling. Numerical simulations and on-site measurements were conducted to explore the surface settlement behavior under varying construction distances. The research examines the vertical and lateral settlement characteristics in the backfilled area, located 32 m from the tunnel initiation well. The findings reveal the settlement behavior throughout the shield tunneling process under different construction distances. The results indicate that the vertical settlement profiles of shield tunnel construction in soft soil layers conform to the Mindlin settlement curve, while the lateral settlement profiles follow the double-funnel Peck curves. Additionally, the closer the construction distance between the two tunnels, the greater the disturbance to the ground surface, whereas increasing the construction distance results in higher peak surface settlement. Soil settlement around the tunnel is significantly affected by construction disturbances, structural stiffness, lateral unloading bias, the extrusion effect of tube sheet weight, and shield machine movement. The use of synchronous grouting effectively mitigates excessive surface settlement. This study provides valuable insights and serves as a reference for similar shield tunnel construction projects in urban soft soil layers.OPEN ACCESS Received: 18/03/2025 Accepted: 16/05/2025 Published: 22/09/2025