Influence of Surface Loads and Construction Sequence on Ground Response Due to Tunnelling

Abstract

ABSTRACT Two-dimensional model tests of tunnel excavation and the corresponding numerical analyses were carried out. Numerical analyses were performed with the finite element method using the elastoplastic subloading tij model, which can simulate typical soil behavior. Model tests and numerical analyses were performed for three series of tunnel excavations. To investigate the influence of tunnel excavation on surface settlement and earth pressure surrounding a tunnel, both the model tests and numerical simulations were conducted considering the ground as green field. It is found that earth pressure decreases at the place of excavation, while increasing adjacent to the excavation due to an arching effect. The results confirm the validity of the constitutive model including the initial condition of the ground. To investigate the effects of building loads in tunnel excavation, model tests and numerical analyses were carried out as well. In this series, the initial dead load was applied adjacent to the tunnel. It is demonstrated that the building loads control surface settlements such that the maximum surface settlement does not always occur above the centerline of the tunnel, but at the position of the existing building. The deformation mechanism and the earth pressure during the tunnel excavation for the ground disturbed by the building loads are different from those for the greenfield situation. The results of this series validate the model regarding the boundary condition and initial state of the ground. In a third series of tests, two different excavation sequences were considered for a tunnel with a large cross-section. In the first case, the middle section was initially excavated and later two side-sections were excavated. In the other case, two side-sections were excavated before excavation of the middle section. The results of this series show that final surface settlements vary slightly with excavation sequences, whereas final earth pressures differ considerably for different excavation sequences. The results of this series prove that the model can consider construction sequences perfectly in finite element analyses. Numerical simulations agree not only qualitatively but also quantitatively with the model tests results for both deformations and earth pressures.