Face stability in tunnels excavated by the drill and blast method

We study the face stability of rock tunnels excavated by the drill and blast method, using an advanced failure mechanism in the framework of limit analysis. First, we study the influence of the geometry of the section and of the excavation process on the stability of the face, comparing a horseshoe section carried out in several stages with its equivalent circular section. Second, we investigate the influence on the collapse pressure of the damage induced in the rock mass during blasting, characterized by the Blast Damage Factor D. Results show that, as expected, multi-stage excavation sequences increase the stability of the face and that decreasing the height of the top-heading reduces the collapse pressure. Moreover, there is evident influence of the disturbance factor "D" on the collapse pressure, so that poor quality blasting can facilitate the collapse of the face.