Influenced zones for deformation of existing tunnels adjacent to excavations

The unloading effect of excavations can cause deformation of the adjacent existing tunnels, which may seriously influence the operation and safety of the tunnels and hence should be strictly controlled. Based on the field measurements of a project in Tianjin and a large number of case histories, the finite element method considering the small strain of soil is adopted to conduct parametric studies on the relationship between the deformation of the existing tunnels and the excavation construction. According to various deformation controlling criteria for the existing tunnels, the retaining side of excavation is zoned with different deflection modes and maximum horizontal displacements of retaining structures. It is shown that the influenced zone can be simply characterized by a right trapezoid, which can be used to predict the deformation of the existing tunnels approximately according to the relative locations of the tunnels, deflection modes and maximum horizontal displacements of retaining structures. For a given deflection mode of the retaining structures and a deformation controlling criterion, the influenced zone expands as the maximum horizontal displacement of the retaining structure increases. Under the same criteria and maximum horizontal displacement of the retaining structures, the range of the influenced zone increases with the variation of deflection modes in the order of cantilever type, convex type, composite type and kick-in type.