Damage caused by mechanized tunnel boring in high-stress hard rock

Abstract

Mechanized excavations cause minimal disturbance in the rock mass around the tunnel; however, in hard rock under high stress, changes in the stress level could cause damage and cracking to the rock mass around the tunnel. This is known as spalling and could cause problems when using a tunnel boring machine (TBM), especially of the gripper type. A gripper type TBM must be jacked into the tunnel wall in order to advance; thus, spalling will make a TBM unstable and prevent it from advancing. Additionally, the permeability of rock mass, segment structural design, and specifications of instrumentation around the tunnel will change. This study examined damage in mechanized excavations of different diameters under different stress condition. Prediction of the thickness of the damage zone around an excavation boundary in hard rock was done according to the damage initiation and spalling limit method (DISL). Rock spalling in deep and shallow tunnels under active tectonics (high horizontal-to-vertical stress ratio, K) was studied using 3D numerical modeling. The results indicated that the thickness of the excavation damage zone directly corresponded to an increase in K and the tunnel diameter. Also, the thickness of the highly damage zone directly corresponded to an increase of tunnel depth and the tunnel diameter. The thickness of the highly damaged zone increased for an overburden of over 1000 m at diameters of 3 to 11 m, which would increase the difficulty of operation of a TBM, especially of the gripper type.