Page 214 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

176 Engineering Rock Mass Classification

Singh originally proposed Eq. (13.9) in a lecture at the Workshop of Norwegian
Method of Tunneling in New Delhi, India, in 1993 and reported it later after confirmation
(Singh et al., 1997). Since the criterion for squeezing is found to be surprisingly indepen-
dent of UCS (qc < 50 MPa), in their opinion no correction for UCS (qc) is needed for
weak rocks.

Many investigators agreed with Eq. (13.9) (Grimstad & Bhasin, 1996; Barla, 1995;
Barton, 1995; Choubey, 1998; Aydan, Dalgic, & Kawamoto, 2000; and others). It may be
argued that qcmass should be the same for given RQD, Jn, Jr, and Ja values irrespective of
overburden depth and water pressure in joints. High overburden and water pressure can
cause long-term damage to the rock mass due to induced fractures, opening of fractures,
softening, seepage erosion, and so forth. Hence, Eq. (13.9) is justified logically if Q is
obtained soon after excavation in the nearly dry, weak rock masses.

Eleven cases of tunnels in the squeezing ground have also been analyzed by Singh
and Goel (2002). In poor rocks, the peak angle of internal friction (fp) is back analyzed
and related as follows:

tanfp ¼ Jr þ 0:1 1:5 ðfor Jw ¼ 1Þ ð13:17Þ
Ja

The addition of 0.1 accounts for interlocking of rock blocks. It may be visualized that

interlocking occurs more often in jointed rock mass than in soils due to low void ratio.
Further, Kumar (2000) showed theoretically that the angle of internal friction of
laminated rock mass is slightly higher than the sliding angle of friction of its joints.

Failure of Inhomogeneous Geological Materials

With inhomogeneous geological material, the process of failure is initiated by its weakest
link (zone of loose soil and weak rock, crack, bedding plane, soft seam, etc.). Thus, nat-
ural failure surfaces are generally three-dimensional (perhaps four-dimensional), which
start from this weakest link and propagate toward a free surface (or face of excavation).
As such the intermediate principal stress (s2) plays an important role and governs the
failure and the constitutive relations of the naturally inhomogeneous geological materials
(both in rock masses and soils) in the field. Since micro-inhomogeneity is unknown,
assumption of homogeneity is popular among engineers. Therefore, intuition states that
the effective confining stress is about [(s2 þ s3)/2] in naturally inhomogeneous soils as
well as fault-gouges.

Failure in an inhomogeneous geological material is progressive, whereas a homoge-
neous rock fails suddenly. Hence, the advantage of inhomogeneous materials offered by
nature is that they give advance warning of the failure process by starting slowly from the
weakest zone.

Failure of Laminated Rock Mass

Laminated rock mass is generally found in the roof of underground coal mines and in the
bottom of opencast coal mines. The thin rock layers may buckle under high horizontal in
situ stresses first and then rupture progressively by violent brittle failure. Therefore, the
assumption of shear failure along joints is not valid. As such, the proposed hypothesis of
effective confining stress [(s2 þ s3)/2] may not be applicable in the unreinforced and
laminated rock masses. The suggested hypothesis appears applicable for the rock masses
with three or more joint sets.