Page 219 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 13 Strength Enhancement of Rock Mass in Tunnels 181

FIGURE 13.5 Plot between qtj and g Á Q0.31 (g in g/cc or T/m3).

Às3 ¼ qtj ð13:22Þ

The tensile strength across discontinuous joints is not zero as generally assumed; it is

found to be significant in hard rocks.
The tensile stress in the tunnel roof of span B will be of the order of gB in the vertical

direction. Equating this with qtj, the span of self-supporting tunnels obtained from
Eq. (13.21) would be 2.9 Q0.31 m. Barton, Lien, and Lunde (1974) found the self-supporting
span to be 2 Q0.4 m. This comparison is very encouraging. Thus, it is understood that the

wedge analysis considering qtj and in situ stress along the tunnel axis may result in a more
accurate value of the self-supporting tunnel span. Equation (13.21) may also be used in

distinct element software.

DYNAMIC STRENGTH OF ROCK MASS

It appears logical to assume that dynamic strain at failure should be of the same order

as the static strain at failure for a given confining stress. Dynamic strain at failure should

be proportional to modulus of elasticity of rock mass (Ee) and static strain at failure
should be proportional to Ed. Therefore, the following hypothesis for dynamic strength
enhancement is proposed.

qcmdyn=qcmass ¼ ðEe=EdÞ0:7 ð13:23Þ

where qcmdyn ¼ dynamic strength of rock mass.
In seismic analysis of concrete dams, dynamic strength enhancement may be quite

high, particularly for a weathered rock mass, because the instantaneous modulus of elas-

ticity (Ee from Eq. 8.19) will be much higher than the long-term modulus of deformation
Ed (Eq. 8.18). Extensive research is urgently needed to obtain more realistic correlations
for dynamic strength enhancement.

RESIDUAL STRENGTH PARAMETERS

Mohr-Coulomb’s theory will be applicable to residual failure as a rock mass would
be reduced to non-dilatant soil-like condition. The mobilized residual cohesion (cr)
is approximately equal to 0.1 MPa and is not negligible unless tunnel closure is more
than 5.5% of its diameter. The mobilized residual angle of internal friction (fr) is approx-
imately 10 degrees less than the peak angle of internal friction (fp), but more than 14
degrees. Rock mechanics helps to judge the support system (Singh & Goel, 2002).