Page 374 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 26 Geological Strength Index 329

ROCK PARAMETERS FOR INTACT SCHISTOSE

In argillaceous or anisotropic rocks (shales, phyllites, schists, gneisses, etc.), the UCS of
rock material qc depends upon the orientation of the plane of weakness. Both GSI and
RMR take into account the orientation of joints. To avoid double-accounting for joint
orientation in both UCS and GSI, it is a common engineering practice to use the upper
bound value of qc and corresponding mr for rock cores with nearly horizontal planes of
weakness for estimating mb, s, and Ed for jointed rock masses.

Cohesion along joints is needed for wedge analysis or computer modeling. Cohesion
along bedding planes or planar continuous joints (longer than 10 m) may be negligible.
However, cohesion along discontinuous joints (assumed continuous in the wedge anal-
ysis) may be the same as cohesion (c) of the rock mass. The cohesion of the rock mass is
due to the cohesion of the discontinuous joints. The ratio of c and cohesion of rock ma-
terial (Figure 26.2) may be of the same order as the area of intact rock bridges per unit
area of discontinuous joints.

ESTIMATION OF RESIDUAL STRENGTH OF ROCK MASSES

To extend the GSI system for estimation of rock mass residual strength, Cai et al. (2007)
proposed an adjustment of the original GSI value based on the two major controlling fac-
tors in the GSI system, block volume (Vb) and joint condition factor (JC), to reach the
residual values.

The difference between the peak and residual strength of a rock mass with non-
persistent joints is larger than that of a rock mass with persistent joints. The implication
is that a drop of GSI from peak to residual values is larger for rock masses with non-
persistent joints. Besides rock bridges, rock asperity interlocking also contributes to
the difference between peak and residual strengths.

Residual Block Volume

If a rock experiences post-peak deformation, the rock in the broken zone is fractured
and consequently turned into a poor and eventually “very poor” rock (Figure 7.2).
The properties of a rock mass after extensive straining should be derived from the rock
class of “very poor rock mass” in the RMR system (Chapter 6) or “disintegrated” in the
GSI system.

For the residual block volume, it is observed that the post-peak block volume is small
because the rock mass has experienced tensile and shear fracturing. After the peak load,
the rock mass becomes less interlocked and is heavily broken with a mixture of angular
and partly rounded rock pieces.

Detailed examination on the rock mass damage state (before and after the in situ
block shear tests at some underground cavern sites in Japan) revealed that in areas
not covered by concrete, the failed rock mass blocks are 1–5 cm in size. The rock mass
is disintegrated along a shear zone in these tests. As such, Cai et al. (2007) suggested the
following residual block volume Vbr :

l If Vb > 10 cm3, Vbr (in disintegrated category) ¼ 10 cm3
l If Vb < 10 cm3, Vbr ¼ Vb