Page 217 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides
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Chapter 13 Strength Enhancement of Rock Mass in Tunnels 179
FIGURE 13.3 Examples of tunnel instability and brittle failure (highlighted gray squares) as a function
of rock mass rating (RMR) and the ratio of the maximum far-field stress (s1) to the UCS (qc). (Modified
from Hoek et al., 1995, and Martin et al., 1999)
In brittle failing rock mass where stress-induced failure leads to the creation of a
zone of fractured rock or cavity near excavation (Figure 13.3), tunneling basically
involves three aspects: (1) retention of broken rock near excavation, (2) control of
deformations due to bulking of fractured rocks, and (3) dissipation of strain energy
if failure occurs violently. The fracturing may degrade rock mass quality drastically,
but the process of fracturing or spalling is fortunately mostly self-stabilizing similar
to squeezing grounds. The depth (df) of local spalling is correlated with maximum
tangential stress (sy) as follows (Kaiser, 2006):
