Page 157 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 9 Rock Mass Number 125

TABLE 9.4 Important Empirical Approaches and Their Recommendations

Approach Results based on Recommendations
Terzaghi (1946) Support pressure increases with
a. Experiments in sands the opening size
Deere et al. (1969) b. Rectangular openings
Support pressure increases
Wickham et al. with flat roof with the opening size
(1972) c. Qualitative approach
RSR system Support pressure increases
Barton et al. (1974) a. Based on Terzaghi’s with the opening size
Q-system theory and classification
on the basis of RQD Support pressure is independent
Unal (1983) using of the opening size
RMR of Bieniawski a. Arched roof
(1976) b. Hard rocks Support pressure increases
c. Quantitative approach with the opening size
Singh et al. (1992)
a. Hard rocks Support pressure is observed
b. Arched roof to be independent of the opening
c. Quantitative approach size (2–22 m)

a. Coal mines
b. Rectangular openings

with flat roof
c. Quantitative approach

a. Arched roof (tunnel/cavern)
b. Both hard and weak rocks
c. Quantitative approach

Source: Goel et al., 1996.

Influence of Rock Mass Type

Support pressure is directly proportional to the size of the tunnel opening with weak or
poor rock masses, whereas in good rock masses the situation is reversed (Table 9.4).
Hence, it can be inferred that the applicability of an approach developed for weak or poor
rock masses has a doubtful application in good rock masses.

Influence of In Situ Stresses

Rock mass number (N) does not consider in situ stresses, which govern the squeezing or
rock burst conditions; instead, the height of overburden is accounted for in Eqs. (9.9) and
(9.10) for estimation of support pressures. Thus, in situ stresses are indirectly considered.

Goel et al. (1995a) evaluated the approaches of Barton et al. (1974) and Singh et al.
(1992) using the measured tunnel support pressures from 25 tunnel sections. They found
that the approach of Barton et al. (1974) is unsafe in squeezing ground conditions
and the reliability of the approaches of Singh et al. (1992) and Barton et al. (1974) depend
upon the rating of Barton’s SRF. Also found is that the approach of Singh et al. (1992) is
unsafe for larger tunnels (B > 9 m) in squeezing ground conditions (see the section
Correlation by Singh et al. (1992) in Chapter 8). Kumar (2002) evaluated many classifica-
tion systems and found rock mass number to be the best from the case history of the NJPC
tunnel in India.