INFLUENCE OF RIGIDITY OF SOIL SURROUNDING SHIELD TUNNEL UPON EQUIVALENT RIGIDITY OF THE TUNNEL IN AXIAL DIRECTION

Abstract

The purpose of this study is to investigate the interaction between ground surrounding a shield-driven tunnel and segments, especially their ring-joint portion. When a seismic force acts on an underground structure such as shield tunnel, the structure is influenced by the deformation of the ground surrounding the structure during the earthquake. In the analysis, the shield-driven tunnel is usually regarded, for simplifying the calculation, as a mechanically continuous tubular structure with uniform stiffness in the axial direction. This uniform stiffness is termed the equivalent rigidity. This paper describes a numerical simulation of the relation between mechanical properties of the ground and equivalent rigidities. From earthquake observations on real shield-driven tunnels, it has been noted that the ring-joints between segments open and shut during earthquakes, reducing the strain of the segments. A severe disturbance of the mechanical condition, that is, slip and crack, is supposed to occur at the ground just near the ring-joints. In the numerical simulation to study the disturbance, the finite element method is adopted, taking into account strain-dependence of rigidity of the soil, initial stress due to the dead load and the prestressing, and non-linearity of the spring of the ring-joint between segments. Furthermore, joint elements are set between segment and soil to represent slipping, and also on the cross section plane at the ring-joint point to represent cracks of the soil. The numerical simulation shows that the equivalent rigidity increases with the stiffness of grout, but it is influenced little by the strain level of the ground.