The influence of damaged zone on stability of concrete lined tunnels excavated in Mohr-Coulomb rock mass

Rock excavation using drill and blast method is commonly used in mining, quarrying and tunneling world-wide. Although, drill and blast method has witnessed significant technological advancements, it has inherent disadvantage of deteriorating surrounding rock mass due to development of a blast induced damage zone with reduced strength and stiffness parameters. Traditional tunnel analysis adopts a single value of blast damage factor D for the Hoek-Brown (HB) criterion for the entire rock mass, leading to the underestimation of tunnel stability. However, the blast damage zone with finite thickness is significant in tunnel design and stability analysis. In this paper, the behavior of tunnels under different damage conditions is examined. In this regard, a fully analytical solution is used. The solution is presented for tunnels excavated in elastic–brittle–plastic rock masses with Mohr-Coulomb failure criterion. The damaged zone is assumed to have cylindrical shape with varied damage Factor D. The results obtained by the proposed solution indicate that, the effects of the alteration of rock mass properties in the damaged zone may be considerable.