Page 216 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

178 Engineering Rock Mass Classification

CRITERION FOR SQUEEZING GROUND CONDITION

Equation (13.14) suggests the following criterion for squeezing/rock burst (s1 ¼ sy, s3 ¼ 0,
and s2 ¼ Po along tunnel axis in Figure 13.1d):

sy > qc mass þ A Á Po ¼ qc0 mass ð13:19Þ
2

Palmstrom (1995) observed that sy/qcmass or sy/RMi may be much higher than 1, that is,
1.5 to 3 for squeezing. Thus, his experience confirmed the proposed criterion (13.19),
which shows that squeezing may occur when the constant A is small (<1.5). There is

a need for in situ triaxial test data for further proof.

Eleven tunnels in the Himalayas showed that squeezing ground conditions are
generally encountered where the peak angle of internal friction (fp) is less than 30,
Jr/Ja is less than 0.5, and overburden is higher than 350 Q1/3 m in which Q is Barton’s
(disturbed) rock mass quality with SRF ¼ 2.5. The predicted support pressures using

Eq. (13.14) agree better with observed support pressure in the roof and walls than those

by Mohr-Coulomb’s theory (Chaturvedi, 1998).

ROCK BURST IN BRITTLE ROCKS

Kumar (2002) observed the behavior of the 27 km long NJPC tunnel and found that the
mild rock burst occurred where A is more than 2.0 and Jr/Ja > 0.5. In 15 sections with
rock cover (H) of more than 1000 m, his studies validated Eq. (13.19) for approximately
predicting mild rock burst/slabbing conditions and estimating rock mass strength qcmass.
If sy/q0cmass > 0.6, then spalling was observed in the blocky rock mass. He also inferred
from 50 tunnel sections that the ratio between tangential stress and mobilized biaxial
strength (sy/q0cmass) is a better criterion for predicting the degree of squeezing condition
than Mohr-Coulomb’s theory (sy/qcmass). Figure 7.3 also showed that the rock burst may
occur where the normalized rock cover HB0.1 > 1000 m, N > 1.5, and Jr/Ja > 0.5.

For safe tunneling, understanding the “post-peak” behavior of a rock mass is often
critical (Figure 3.2; the section Homogeneity and Inhomogeneity in Chapter 3). Unfor-
tunately, costly mistakes are often made because of the lack of understanding of the
actual complex and brittle behavior under high in situ stress, overreliance on analyses,
or lack of experience in low stress conditions. In rock burst conditions, it is necessary to
adopt a robust engineering approach that focuses on a flexible construction process and
ensures that all construction machines work well. Additional uncertainties can be man-
aged by adopting an observational design-as-you-go approach.

Failure of underground openings in hard and brittle rocks is a function of the in situ
stress magnitudes and the characteristics of the rock mass; that is, the intact rock strength
and the fracture network (Figure 13.3). At low in situ stress magnitudes, the failure process
is controlled by the continuity and distribution of the natural fractures in the rock mass.
However, as in situ stress magnitudes increase, the failure process is dominated by new
stress-induced fractures growing parallel to the excavation boundary. This fracturing is
generally referred to as “brittle failure.” Initially, at intermediate depths, these failure re-
gions are localized near the tunnel perimeter, but at great depth the fracturing envelopes the
whole boundary of the excavation (Figure 13.3). Unlike ductile materials in which shear
slip surfaces can form while continuity of material is maintained, brittle failure deals with
materials for which continuity must first be disrupted before kinematically feasible failure
mechanisms can form (Martin, Kaiser, & McCreath, 1999).