Three-dimensional finite element modelling of excavation-induced tunnel wall movement and damage: a case study

A three-dimensional elastoplastic finite element modelling is conducted for the back analysis of a tunnel collapse accident that happened at the Hobart Myer construction site, including the deformation and failure processes of an existing tunnel in close proximity to a basement excavation as well as further excavation-induced tunnel wall damage and collapse. The full three-dimensional tunnelling process and the basement excavation process are modelled using step-by-step approaches through the successive removal and installation of the solid and structural elements, respectively. Furthermore, following the field observations, the storm-induced water pressure in the existing tunnel is taken into consideration and three further small-scale pit excavations without support installation in the sidewall of the basement towards the tunnel are also performed in the numerical modelling process. The obtained results are discussed in comparison to the observed failures at the construction site, to highlight the deformation and failure mechanisms induced by the excavations and stormwater and to examine the trend of the damage and collapse of the tunnel wall and its support system. It is concluded that although the collapse of the tunnel wall is comprehensively the result of nearby basement excavation before and after the heavy rainfall, the storm-induced water pressure is the main factor resulting in the final collapse of the tunnel wall. Further excavations of three small pits only have relatively slight effects on the instability of the tunnel. The well-developed finite element method provides a valuable tool for the study of the stability of shallow tunnels in the presence of nearby excavations under various conditions.