3D EFFECTS ON EARTH PRESSURE AND DISPLACEMENTS DURING TUNNEL EXCAVATIONS

摘要

Three-dimensional model tests of tunnel excavation and the corresponding numerical analyses were carried out to investigate the influence of tunnel excavation on surface settlement and earth pressure surrounding a tunnel. Numerical analyses were performed with the finite element method using elastoplastic subloading t_(ij) model. Two types of apparatuses were used in the model tests, namely-trap door apparatus and pulling out tunnel apparatus. The minimum excavation length along the excavation direction is 8 cm in the trap door apparatus. However, the process of real tunnel excavation is more continuous. For simulating real tunnel excavation in 3D, a new apparatus was developed, which is called as the pulling out tunnel apparatus. This apparatus can simulate tunnel excavation in sequential way. Both experiments and analyses were conducted with various ground depths for simulating the influence of soil cover on tunnel excavations. Surface settlements are measured at the transverse cross-section of the ground. Earth pressures at the top of the tunnel in the trap door apparatus are measured. However, in the tests performed with the pulling out tunnel apparatus, earth pressures are measured adjacent to the tunnel cross section. In this paper, the effects of 3D excavation on surface settlements and earth pressures are discussed. It is revealed that arching is formed in both transverse and longitudinal directions of tunnel excavation. Numerical results show very good agreement with the results of the model tests. In the three-dimensional analyses performed in a sequential way, earth pressure is almost zero at the excavation front, irrespective of the soil cover. In 2D analyses and 3D analyses using the trap door apparatus, the earth pressure at the excavation front does not vanish, as observed in 3D sequential analyses.