Page 63 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

42 Engineering Rock Mass Classification

conditions, but the tunnel size could have considerable influence on the support
pressure in squeezing ground conditions. For more in-depth coverage on this subject,
see Chapter 9.

The estimated support pressures from Table 5.2 have been compared with the
measured values with the following conclusions:

1. Terzaghi’s method provides reasonable support pressure for small tunnels (B < 6 m).
2. It provides over-safe estimates for large tunnels and caverns (diameter 6–14 m).
3. There is a very large range of estimated support pressure values for squeezing and

swelling ground conditions.

MODIFIED TERZAGHI’S THEORY FOR TUNNELS AND CAVERNS

Singh, Jethwa, and Dube (1995) compared support pressure measured from tunnels
and caverns with estimates from Terzaghi’s rock load theory and found that the support
pressure in rock tunnels and caverns does not increase directly with excavation size as
assumed by Terzaghi (1946) and others. This is due mainly to the dilatant behavior of
rock masses, joint roughness, and prevention of rock mass loosening by improved tunnel-
ing technology. They have subsequently recommended ranges of support pressures as
listed in Table 5.5 for both tunnels and caverns for those who still want to use Terzaghi’s
rock load approach. They observed that the support pressures are nearly independent of
size of opening.

It is interesting to note that the recommended roof support pressures turn out to be
the same as those obtained from Terzaghi’s rock load factors when B and Ht are
substituted by a tunnel width of 5.5 m. The estimated roof support pressures from
Table 5.5 were found to be comparable with the measured values irrespective of
the opening size and the rock conditions (Singh et al., 1995). These authors have
further cautioned that the support pressure is likely to increase directly with the ex-
cavation width for tunnel sections through slickensided shear zones, thick clay-filled
fault gouges, weak clay shales, and running or flowing ground conditions where inter-
locking blocks are likely to be missing or where joint strength is lost and rock wedges
are allowed to fall due to excessive roof convergence because of delayed supports
beyond stand-up time. It should be noted that wider tunnels require reduced spacing
of bolts or steel arches and thicker linings since rock loads increase directly
with the excavation width, even if the support pressure does not increase with the
tunnel size.

REFERENCES

Barton, N., Lien, R., & Lunde, J. (1974). Engineering classification of rock masses for the design of tunnel
support (NGI Publication No. 106, p. 48). Oslo: Norwegian Geotechnical Institute.

Brekke, T. L. (1968). Blocky and seamy rock in tunnelling. Bulletin of the Association of Engineering.
Geologists, 5(1), 1–12.

Cecil, O. S. (1970). Correlation of rock bolt—Shotcrete support and rock quality parameters in
Scandinavian tunnels (p. 414). Ph.D. Thesis. Urbana: University of Illinois.

Deere, D. U., Peck, R. B., Parker, H., Monsees, J. E., & Schmidt, B. (1970). Design of tunnel support
systems. Highway Research Record, No. 339, 26–33.