Primary Support Design for Sequentially Excavated Tunnel Junctions in Strain-Softening Hoek–Brown Rock Mass

Modern highway and railway tunnel projects include cross passages at regular intervals along the main tunnel length as escape routes. Such areas have complex geometry and incorporate three-dimensional stress redistributions in the tunnel support system and the rock mass. The sequential procedure of excavation and primary support installation of a tunnel junction area can be approximated only with complex three-dimensional numerical simulations. This study presents a practical methodology for the estimation of the displacements and loads induced to the main tunnel lining, after the construction of the escape tunnel. The methodology is based on the evaluation of three-dimensional numerical simulations results. All simulations incorporate sequential excavation and support cycles in confinement depended strain-softening Hoek–Brown rock mass. The results indicate that the area of the main tunnel lining in proximity to the crown of the escape tunnel is severely unloaded and tensile axial forces are possible to appear, while the main tunnel lining in proximity to the side walls of the escape tunnel is compressively overloaded. Finally, suitable support measures are proposed.