Page 376 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 26 Geological Strength Index 331

FIGURE 26.4 Tunneling problems associated with different levels of strain. (From Hoek, 2001; Singh
and Goel, 2006)

Example 26.1

In a major hydroelectric project in dry quartzitic phyllite, the rock mass quality (Q) is in
the range of 6 to 10. The joint roughness number Jr is 1.5 and joint alteration number Ja is
1.0 for critically oriented joints in the underground machine hall. The unit weight of
phyllite rock is 2.78 gm/cc. The upper bound strength envelope between s1 and s3 from
triaxial tests gave UCS (qc) ¼ 80 MPa, fp ¼ 32, mr ¼ 5.3, and Er ¼ 11.6 GPa when the
plane of schistocity is horizontal. The average UCS for various angles of schistocity is
40 MPa. The GSI is estimated to be about 55 as rock mass is micro-folded and joints
are very rough and unweathered. With these values, it is required to consider the engi-
neering parameters of the undisturbed (D ¼ 0) rock mass for the machine hall cavity
(width 24 m and height 47 m).

The average rock mass quality is √(6 Â 10) ¼ 8 (approximately). Other calculations
are presented in Table 26.6 for the undisturbed rock mass. The peak angle of internal
friction works out to be 27 from Figure 26.3 and 32 from triaxial tests and 56 from
the Jr/Ja value. Thus, a value of fp ¼ 32 appears to be realistic. A blast damaged zone
of about 2 m depth may be assumed in the computer modeling all around the cavity with
half the values of cp, qcmass, Ed, and G.

It may be emphasized that Table 26.6 suggests parameters for the first iteration only
in the computer modeling. The more realistic model and parameters may be back cal-
culated from the observed displacements of the cavity during upper half excavation.