Design Methods - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 342

This is shown by means of the example of a 8 m wide traffic tunnel with an overburden of
80 m located in jointed rock with a structural model that was established for this rock mass
as illustrated in Fig. 12.9 (Sommer 2009). The existing three orthogonal discontinuity
sets according to the Q system are characterized by parameters J n , J r , J a and J w as spec-
ified in Fig. 12.9. Assuming an unconfined compressive strength of the intact rock of
σ cIR = 10 MPa the stress reduction factor is SRF ≈ 5. As demonstrated in Fig. 12.8, for
the same rock mass, RQD values from 19% to 100% can result from core drills with dif-
ferent orientations of the borehole with respect to the discontinuity system. Thus, Som-
mer (2009) evaluated the Q system support recommendation for RQD values of 19%
and 100% yielding Q = 0.2 and 1.1, respectively. Q = 0.2 leads to a Q system support
recommendation consisting of 1 m spaced rock bolts and a 20 - 40 cm thick reinforced
shotcrete membrane. Q = 1.1, however, yields a support consisting of 1 - 1.5 m spaced
rock bolts and a 5 - 10 cm thick reinforced shotcrete membrane (Fig. 12.9).
Figure 12.9 Influence of the RQD on the Q system support recommendations (Sommer 2009)