Design Methods - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 344

Figure 12.11 Classification of a limestone according to the Q system (Barton et al. 1974)
To check this support recommendation and to investigate the influence of discon-
tinuity orientations, two-dimensional FEM stability analyses using the program
system FEST03 (Section 10.7.1) were carried out (Sommer 2009). The overbur-
den of the tunnel was assumed to be 50 m and no groundwater was assumed to
be present in the rock mass. Two cases were investigated. In case 1, horizontal
bedding-parallel discontinuities and vertical joints striking parallel to the tunnel
axis are assumed. In case 2, the bedding-parallel discontinuities are assumed to
dip vertically and to strike parallel to the tunnel axis (Fig. 12.12). A full-face
excavation and a support according to the above mentioned Q system support
recommendations were simulated. The rock mechanical parameters specified in
Fig. 12.12 were derived according to the Q system (Sommer 2009).
In Fig. 12.13, characteristic results of these analyses are represented. In case 2,
the zones in which the shear strength along the bedding-parallel discontinuities is
exceeded are much larger than in case 1 and reach up to the ground surface. As a
consequence, the maximum normal thrusts and displacements at the contour of the
tunnel in case 2 are larger by a factor of more than two compared to case 1.
The design of the shotcrete membrane on the basis of these analyses leads to the result
that for case 1 a minimum reinforcement is sufficient, which is in agreement with the Q
system support recommendations. In case 2, however, a 30 cm thick reinforced shotcrete
membrane is required. Thus, in this case the required support evaluated on the basis of
the Q system is insufficient.