Page 372 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 26 Geological Strength Index 327

FIGURE 26.3 Friction angle (f) of rock mass for D ¼ 0 for different GSI and mr values. (From Hoek
and Brown, 1997)

The Hoek et al. (2002) correlations for s are valid for rock slopes and open pit mines, but
not for structurally controlled rock slopes and transported rockfill slopes. For tunnels and
caverns, there is an enormous strength enhancement (Chapter 13).

MODULUS OF DEFORMATION

Hoek and Diederichs (2006) found a useful correlation for modulus of deformation (Ed)

of rock mass based on approximately 496 in situ tests.

1 À D=2
Ed ¼ Er 0:02 þ 1 þ expðð60 þ 15D À GSIÞ=11Þ , GPa ð26:14Þ

where Er ¼ modulus of elasticity of intact rock in GPa.
The elastic modulus (Ee) is obtained from the unloading cycles of the uniaxial jacking

tests. It is correlated for both dry and saturated rock mass as follows (Chapter 8 and

Eq. 8.19):

Ee ¼ 1:5Q0:6 Er0:14, GPa ð26:15Þ

where Q ¼ rock mass quality.

Equation (26.15) is suggested for the dynamic analyses of concrete dams during a

major earthquake and machine (generator) foundations on the rock masses.

The original equation proposed by Hoek and Brown (1997) has been modified by the

inclusion of factor D to allow for the effects of blast damage and stress relaxation. The

strength and deformation parameters estimated from the GSI system are very close to

those obtained from in situ tests (Cai et al., 2004). Back analysis of observed displace-

ments in openings may give more realistic values of the design parameters including the

disturbance factor by trial and error.