Page 211 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 13 Strength Enhancement of Rock Mass in Tunnels 173

RMR À 100
qcmass ¼ qc Á exp 24 ð13:11Þ

Barton (2002) modified Eq. (13.9) on the conservative side for calculating QTBM for tun-
nel boring machines (TBMs; according to Eq. 14.2):

scm ¼ 5 g ðQ Á qc=100Þ1=3 MPa ð13:12aÞ

where qc ¼ Is/25 for anisotropic rocks in MPa (schists, slates, etc.) and Is ¼ standard point
load strength index of rock cores (corrected for size effect for NX size cores). Barton

(2005) clarified that Eq. (13.12) should be used only for QTBM (Chapter 14).
The correlations of Barton (2002) for the underground openings are

cp ffi RQD Á 1 Á qc , MPa ðSRF ¼ 2:5Þ ð13:12bÞ
Jn SRF 100

tanfp ffi JaÁ Jw þ 0:1 Eqð13:12cÞ
Jr

∴ cpÁ tanfp ffi 1
k

ffi Qc ¼ Q Á qc
100

where cp ¼ peak cohesion of rock mass in MPa; fp ¼ peak angle of internal friction of
rock mass; k ¼ permeability of rock mass in lugeon (10-7 m/sec); and Qc ¼ normalized
rock mass quality.

The last term in Eq. (13.12c) is added by Choudhary (2007) because fp for the rock
mass is more than fj for its joints due to the interlocking of rock blocks. He analyzed 11
cases of squeezing in tunnels in the Himalayas in India, and found Eqs. (13.12b),
(13.12c), and (13.14) to be realistic (with SRF ¼ 2.5 in the elastic zones).

Based on block shear tests, Singh et al. (1997) proposed the following correlation for

estimating the UCS of the saturated rock mass for use in rock slopes in hilly areas:

qcmass ¼ 0:38 g Á Q1=3, MPa ð13:13Þ

Equation (13.13) suggests that the UCS of rock mass would be low on slopes. This is
probably because joint orientation becomes a very important factor for slopes due to uncon-
strained dilatancy and low intermediate principal stress, unlike tunnels. Further, failure
takes place along joints near slopes. In slopes of deep opencast mines, joints may be tight
and of smaller length. The UCS of such a rock mass may be much higher and may be found

from Hoek’s criterion (Eq. 13.5) for analysis of the deep-seated rotational slides.

Equations (13.8) and (13.9) are intended only for 2D stress analysis of underground

openings. The strength criterion for 3D analysis is presented in the next section.

REASON FOR STRENGTH ENHANCEMENT IN TUNNELS
AND A NEW FAILURE THEORY

Consider a cube of rock mass with two or more joint sets as shown in Figure 13.1.
If high intermediate principal stress is applied on the two opposite faces of the cube, then
the chances of wedge failure are more than the chances of planar failure as found in the