Displacement Monitoring of Parallel Closely Spaced Highway Shield Tunnels in Marine Clay

The interaction between parallel closely spaced tunnels is inevitable especially when tunneling in soft soil. In this research, a displacement monitoring was performed on two parallel closely spaced highway tunnels in soft marine clay excavated by slurry shields in Shanghai, China. Soil movement between the tunnels and liner displacement of the first constructed tunnel induced by the construction of the second tunnel were analyzed and discussed. It was found that the soil between the tunnels moved in directions both perpendicular and parallel to the tunnels due to the effects of thrust, extrusion, and grouting by the second shield tunneling. The maximum horizontal displacements of the soil between the tunnels occurred at 0.2D (D = outer diameter of the tunnels) above the tunnel springline, which forced the upper semicircle of the existing tunnel liner deformed downward while lower semicircle deformed outward, and the semicircle close to the second tunnel deformed more than the opposite semicircle. The discrepancy between displacements of the existing tunnel liner and the soil in the middle of the tunnels increased with increases of displacements due to increasing stiffness of the liner.