Tunnel reinforcement in columnar jointed basalts: The role of rock mass anisotropy

The extent of the loosening zone in tunnels excavated through columnar jointed basalts is studied with the numerical discontinuous deformation analysis (DDA) method. The structure of the rock mass and tunnel geometry are modeled on the basis of a real field case study of deep tunneling performed in such a rock mass in south west China. Slender prismatic keyblocks formed by the intersection of broadly-spaced, gently-dipping breccia layers and closely-spaced, orthogonally oriented, steeply dipping columnar joints result in a highly anisotropic rock mass structure and give rise to sliding and toppling failure modes in the sidewalls and to an excessive height of loosening zone in the roof, the geometry of which is shown to be controlled by the orientation of the steeply inclined columnar joints. Results of displacement monitoring performed during tunnel excavation in a similar rock mass with multiple point borehole extensometers confirm the numerically obtained depth of the loosening zone both in the sidewalls and the roof. We find that the height of the loosening zone in the roof as obtained with DDA is greater than would have been predicted by Terzaghi's empirical rock load classification for "blocky" rock masses, and show that its shape and orientation are controlled by the anisotropy of the rock mass structure. Moreover, we demonstrate that dimensioning rock bolt reinforcement using well-established empirical criteria without consideration of the anisotropic nature of the rock mass may lead to un-conservative design.