Objective Since shield tunneling undercrossing in upper-soft and lower-hard strata tends to induce deformation of existing tunnels, it is necessary to conduct in-depth analysis on similar engineering projects.

Method Taking a shield tunneling project undercrossing the existing tunnel in the upper-soft and lower-hard strata of Guangzhou as an example, an analytical solution for existing tunnel deformation induced by shield tunneling based on the energy method is modified through the equivalent layer method theory. Comparative analysis is performed using FLAC 3D numerical simulation software to systematically study the influence of shield undercrossing on existing tunnel deformation in upper-soft and lower-hard strata.

Result & Conclusion  The equivalent layer method theory is applicable to the upper-soft and lower-hard strata condition, and the difference between the modified analytical solution of the variational method of total potential energy and the numerical simulation results is small, validating the rationality of the proposed research method. The maximum settlement value of the existing tunnel in the upper-soft and lower-hard strata is 2.51 mm, which falls between the settlements in homogeneous soft rock strata and homogeneous hard rock strata. The larger the elastic modulus of the lower stratum, the smaller the overall settlement of the existing tunnel, but the non-uniform deformation increases slightly. Within a certain range, increasing the clearance between the new and existing tunnels will expand the stratum disturbance range. Compared with the upper-soft and lower-hard strata conditions, and the gradient change amount of the clearance between the new and existing tunnels, the gradient change amount of the new tunnel’s diameter accounts for the largest proportion of the influence on the existing tunnel settlement. When the new tunnel’s diameter decreases by 1.0 m, the reduction in the maximum settlement of the existing tunnel can account for 56.83%.