Impact Analysis for Safety Prevention and Control of Special-Shaped Shield Construction Closely Crossing Multiple Operational Metro Tunnels in Shallow Overburden

With the improvement of the urban rail transit network, crossing construction has become an inevitable situation in the construction of subway tunnels. The special-shaped shield has great advantages in space utilization and the ability to cross narrow roads. However, due to its section characteristics, the special-shaped shield under construction often has a greater impact on the existing operating subway. Based on the background of quasi-rectangular shield tunneling through multiple existing tunnels, this paper proposes a two-stage safety pre-control (prevention before construction and control during construction) treatment method for large cross-section special-shaped shield construction. With three protection or auxiliary measures, the pre-reinforcement and process treatment of the crossing construction section are carried out. The effectiveness of these three measures is evaluated by establishing a 3D finite element model. Moreover, the reasonable range of construction parameters of the quasi-rectangular shield is optimized by numerical calculation. This study exposes the strata displacement law and tunnel deformation characteristics under corresponding protection or auxiliary measures and optimized construction parameters. The analysis proves that the proposed two-stage method can effectively reduce the impact of shield tunneling on the surrounding environment and existing tunnels. After applying the corresponding safety precautions, the surface uplift and tunnel uplift are reduced by about 30% on average, and the surface settlement is reduced by about 80%. Parameter optimization analysis shows that the thrust (face) pressure and grouting volume (filling rate) are more sensitive to environmental impact. It is recommended to select 100–160 kPa thrust pressure, 200% grouting filling rate, moderate grouting pressure, and slow propulsion speed in construction.