Research on the Effects of Two-Sided Ground Surcharge/Unloading on Existing Shield Tunnels

For the effect of two-sided sudden surcharge/unloading on existing shield tunnels, an indoor dimensional reduction model test was conducted at a scale of 1:15.5, measuring the ground settlement, lateral convergence of the shield tunnel, and additional surrounding pressure after changing the location and soil quality of surcharge/unloading, and an analysis was carried out. The results showed that the location of the maximum transverse convergence value of the existing shield tunnel is related to the position of the surcharge; when surcharging on both sides of the tunnel, as the position of the left surcharge’s offset increases from 0.2 m to 0.6 m, the transverse convergence value of the tunnel first increases and then becomes smaller; the ground settlement value of each measurement point in the unloading process also increases and then becomes smaller. The settlement value of each measurement point first increases and then decreases, and the alleviation effect of unloading on ground settlement is not obvious; after surcharge on one side of the tunnel, if the other side continues to surcharge, it will lead to a continued increase in the surrounding pressure. Unloading can effectively reduce the additional surrounding pressure of the tunnel, but the additional surrounding pressure cannot be immediately and completely eliminated; compared with dry sand, the maximum value of lateral convergence of the tunnel in wet sand is significantly decreased, and the peak value of ground settlement on both sides is not significant; when both sides are surcharged to 172 kg, the additional soil pressure of the tunnel’s subsoil shows three peaks, one of which occurs at measurement point 4 at the bottom of the tunnel, which may be due to the occurrence of stress concentration at the location.