Experimental study on influences of surface surcharge on existing shield tunnels buried in soft and hard soils

To explore the influences of surface surcharge on the existing shield tunnels buried in soft and hard soil strata, the scaled model tests on the interaction between shield tunnels and strata are carried out, and the tunnel deformation, earth pressure and soil settlement under surface surcharge are measured. Analysis of model test results shows that under the same surface surcharge, larger deformation is induced for the tunnel buried in soft soils than in hard soils. As the compression modulus of the soil strata is small, the overlaying soils over the tunnel exhibit passive earth arch pressure under surface surcharge. Under the large compression modulus of the soil strata, the overlaying soils over the tunnel show active earth arch pressure. The relation between the vertical convergence deformation of the tunnel and the vertical compression of the soil strata indicates that both the stiffness of cross section of shield tunnel and the compression modulus of soil strata determine the settlement state of the overlaying soils over the tunnel, and thus decide the influences of surface surcharge on the existing shield tunnels. The research results qualitatively reveal the reason of the mechanism why the oversized deformation is easily induced for the existing shield tunnels under surface surcharge.