Investigation into the influence of excavation of twin-bored tunnels within weak rock masses adjacent to slopes

Abstract

The focus of this investigation is twin tunnelling projects designed and constructed adjacent to slopes. To this end, several state-of-the-art two-dimensional (2-D) and three-dimensional (3-D) numerical models for multiple scenarios were utilized. The novelty of this research (further highlighted by two case studies) explicitly demonstrates that for 2-D and 3-D elastic and plastic analyses for weak rock masses, the interaction between the two tunnel branches for various conditions show how the pillar width minimally influences such interaction past two diameter widths of separation. As well, the 3-D sequential longitudinal tunnel excavation of the twin tunnel does not significantly influence the crown displacements of the two tunnels prior to construction of the mid-span of the twin tunnel. Slope stability analysis was conducted by employing the novel shear strength reduction (SSR) method. These findings confirm that changes in the location of the twin tunnels with respect to the slope and one another in combination with the natural angle of the slope can improve the overall stability without any other special measures taken. As well, by having the twin tunnel pillar width constant, the vertical translation of the tunnels (i.e., increase in the overburden) is more beneficial than that of a horizontal translation, resulting in a higher factor of safety.