Page 210 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

172 Engineering Rock Mass Classification

UNIAXIAL COMPRESSIVE STRENGTH OF ROCK MASS

Equation 13.3 defines that uniaxial compressive strength of a rock mass is given by

qcmass ¼ qc sn ð13:6Þ

Equation (13.6) underestimates mobilized rock mass strength in tunnels. To use

Eq. (13.3) in tunnels, a value of constant s must first be obtained from Eqs. (13.6)

and (13.9) as follows:

s ¼ h g Q1=3Þ=qci1=n ð13:7Þ
ð7

Ramamurthy (1993) and co-workers (Roy, 1993; Singh & Rao, 2005) conducted
extensive triaxial tests on dry models of jointed rock mass using plaster of Paris
(qc ¼ 9.46 MPa). They varied joint frequency, inclination of joints, thickness of
joint fillings, and so forth and simulated a wide variety of rock mass conditions.
Their extensive test data suggest the following approximate correlation for all rock
masses:

qcmass=qc ¼ ½Emass=ErŠ0:7 ð13:8Þ

where, qcmass ¼ UCS of model of jointed rock mass in s1 direction; qc ¼ UCS of model
material (plaster of Paris) and UCS of in situ block of rock material after size correction;
Emass ¼ average modulus of deformation of jointed rock mass model (s3 ¼ 0) in s1
direction; and Er ¼ average modulus of deformation of model material in the laboratory
(s3 ¼ 0).

The power in Eq. (13.8) varies from 0.5 to 1.0. Griffith’s theory of failure suggests

that the power is 0.5, whereas Sakurai (1994) felt the power in Eq. (13.8) is about 1.0 for

jointed rock masses. Further research at the Indian Institute of Technology (IIT) in Delhi

suggests that power in Eq. (13.8) is in the range of 0.56 and 0.72 (Singh & Rao, 2005).

It appears that the power of 0.7 in Eq. (13.8) is realistic. Equation (13.8) can be used

reliably to estimate UCS of a rock mass (qcmass) from the values of Emass or Ed obtained
from uniaxial jacking tests within openings and slopes.

Considerable strength enhancement of the rock mass in tunnels has been observed

by Singh et al. (1997). Based on the analysis of data collected from 60 tunnels, they

recommended that the mobilized rock mass strength of the actual or disturbed rock

mass is

qcmass ¼ 7 g Q1=3, MPa ðfor Q < 10, 100 > qc > 2MPa, ð13:9Þ
SRF ¼ 2:5, Jw ¼ 1Þ ð13:10Þ

hi
qcmass ¼ ð5:5 g N1=3Þ=B0:1 , MPa ðas per Eq: 7:5Þ

where g ¼ unit weight of rock mass (gm/cc); N ¼ actual rock mass number, that is, stress-
free Barton’s Q soon after the underground excavation; Q ¼ actual (disturbed) rock mass
quality soon after the underground excavation and corrected for SRF ¼ 2.5; B ¼ tunnel
span or diameter in meters and SRF ¼ 2.5 at the time of peak failure of in situ rock mass.
See the section Correlation by Singh et al. (1992); in Chapter 8.

Equation (13.8) also shows that there is significantly high enhancement in the

strength of rock mass. Kalamaras and Bieniawski (1995) suggested the following

relationship between qcmass and RMR: