Influenced zones for deformation of underlying metro tunnels induced by braced deep excavation in soft strata

Based on the statistics of collected case histories, the finite element method considering small strain behaviors of soils is adopted to study the influences of excavation on deformation behaviors of underlying metro tunnels in soft silty clay. Then, through the analysis of deformation isoline of the tunnels, combined with the three-level tunnel deformation control standards of 20, 10 and 5 mm, the influenced zones for deformation of underlying tunnels are divided. According to the features of the influenced zones, by defining their determination parameters, a simplified description of the scope of the influenced zones is realized. The results indicate that the influenced zones of tunnel deformation can be divided into "primary", "secondary", "general" and "weak" influenced zones by adopting the three-level tunnel deformation control standards. The scope of the influenced zones of underlying tunnels can be simplified as a right trapezoid, the isoline of tunnel deformation can be simplified as a straight line, and by defining the two coordinates on the straight line: depth coefficient N₁ and depth coefficient N₂, the scope of the influenced zones can be quickly determined.