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130 Engineering Rock Mass Classification

The support pressure obtained using rock mass number, tunnel depth, and tunnel size in
squeezing ground condition is almost two times that obtained from Barton’s approach.

The difference may occur because of (1) incorrect estimation of the SRF rating, and
(2) the effect of tunnel size in squeezing conditions.

In a mild squeezing condition (Table 9.3 and 1–2% normalized tunnel deformation),
SRF should be 5.0 (Table 8.6). With this SRF, Q ¼ 3.0/5 ¼ 0.6. Accordingly, the vertical
support pressure using Eq. (8.2) is 0.16 MPa, which is almost equal to the support pres-
sure previously obtained from Eq. (9.10). Accordingly, the supports are designed using
Figure 8.5 for Q ¼ 0.6 and equivalent dimension ¼ 9/1 ¼ 9. The supports, thus obtained,
are 10 cm thick SFRS with 3.5 m long rock bolts at 1.6 m center to center (support
category 6).

The above example highlights that the rock mass number (N) approach is found to be
complimentary to the Q-system.

REFERENCES

Abad, J., Caleda, B., Chacon, E., Gutierrez, V., & Hidalgo, E. (1984). Application of geomechanical
classification to predict the convergence of coal mine galleries and to design their supports. In 5th
International Congress on Rock Mechanics (pp. 15–19). Melbourne, (E), Australia.

Barton, N., Lien, R., & Lunde, J. (1974). Engineering classification of rock masses for the designs of tun-
nel supports. In Rock mechanics (Vol. 6, pp. 189–236). New York: Springer-Verlag.

Bieniawski, Z. T. (1976). Rock mass classifications in rock engineering. In Proceedings of the Symposium
on Exploration for Rock Engineering (pp. 97–106 in Bieniawski, 1984). Rotterdam: A.A. Balkema.

Bieniawski, Z. T. (1984). Rock mechanics design in mining and tunneling (p. 272). Rotterdam:
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Cameron-Clarke, I. S., & Budavari, S. (1981). Correlation of rock mass classification parameters obtained
from borecore and in situ observations. Engineering Geology, 17, 19–53.

Daemen, J. J. K. (1975). Tunnel support loading caused by rock failure. Ph. D. Thesis. Minneapolis, MN:
University of Minnesota.

Deere, D. U., Peck, R. B., Monsees, J. E., & Schmidt, B. (1969). Design of tunnel liners and support
system (Final Report, University of Illinois, Urbana, for Office of High Speed Transportation,
Contract No. 3-0152, p. 404). Washington, D.C.: U.S. Department of Transportation.

Goel, R. K. (1994). Correlations for predicting support pressures and closures in tunnels (p. 308).
Ph.D. Thesis. Maharashtra, India: Nagpur University.

Goel, R. K., Jethwa, J. L., & Dhar, B. B. (1996). Effect of tunnel size on support pressure. Technical Note.
International Journal of Rock Mechanics and Mining Sciences—Geomechanics Abstracts, 33(7),
749–755.

Goel, R. K., Jethwa, J. L., & Paithankar, A. G. (1995a). Indian experiences with Q and RMR systems.
Tunnelling and Underground Space Technology, 10(1), 97–109.

Goel, R. K., Jethwa, J. L., & Paithankar, A. G. (1995b). Correlation between Barton’s Q and Bieniawski’s
RMR—A new approach. Technical Note. International Journal of Rock Mechanics and Mining
Sciences—Geomechanics Abstracts, 33(2), 179–181.

Hoek, E., & Brown, E. T. (1980). Underground excavations in rocks (p. 527). Institution of Mining and
Metallurgy. London: Maney Publishing.

Jethwa, J. L. (1981). Evaluation of rock pressure under squeezing rock conditions for tunnels in
Himalayas (p. 272). Ph.D. Thesis. Uttarakhand, India: IIT Roorkee.