| Summary: | This paper investigates the impact of surcharge/unloading at both sides of the ground surface on an existing underground shield tunnel at varying burial depths. A large-scale indoor model experiment was conducted to study the vertical convergence deformation, tunnel settlement, and additional surrounding pressure of the tunnel at different burial depths when surcharge/unloading occurred at both sides of the ground surface. The effect of tunnel burial depth, surcharge mass, and uneven surcharge at both sides of the ground surface on the additional surrounding pressure of the existing underground shield tunnel was further investigated by theoretical calculation. The results indicate that under the same conditions, greater tunnel burial depth leads to reduced vertical convergence deformation, tunnel settlement, and additional surrounding pressure. Unloading can partially reduce the vertical convergence deformation and tunnel settlement, but it does not completely counteract the effect of the preceding surcharge. In cases of a symmetrical surcharge at both sides of the ground surface above the tunnel, the additional surrounding pressure of the tunnel is symmetrically distributed, with higher pressures at the arch top and arch bottom of the tunnel and lower pressures at the arch waist. This suggests that the arch top and arch bottom of the tunnel are vulnerable points under surcharge and require special attention. The surcharge above the tunnel has the most pronounced effect on the tunnel, followed by the surcharge at both sides of the ground surface, and, finally, the smallest effect from the bias load.
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