Three-dimensional numerical simulation of a tunnel excavated using NATM - Brasilia underground case

During the first half of the 90's a subway tunnel was dug in porous clay using NATM in 'Brasilia, the Brazilian capital city. The main observed features were excessive settlement and an unexpected trend, with displacements increasing from the tunnel crown towards the surface. Many unsuccessful attempts to simulate this behaviour using two-dimensional numerical analyses have been reported. The first two authors, using some numerical artifices to simulate the structural collapse of Brasilia's porous clay, obtained partial success. This paper presents a three-dimensional numerical analysis using the FEM, in which the construction sequence of Brasilia's underground was carefully matched. The soil behaviour was simulated using elastoplastic (Drucker-Prager) and critical state (Cam clay) models. The Drucker-Prager model was unable to reproduce the observed pattern, even when a non-associated flow rule was adopted. Compatible results were obtained with a three-dimensional analysis using the Cam clay model. Two-dimensional analyses, even with the Cam clay model, are unable to reproduce the registered results. These later analyses cannot account for the load transfer along the excavation direction. This arching effect, together with the appropriate constitutive model, seem to be the key to reproduce the observed field behaviour.