Page 158 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

126 Engineering Rock Mass Classification

TABLE 9.5 Effect of Tunnel Size on Support Pressure

S. No. Type of rock mass Increase in support pressure
due to increase in tunnel
span or diameter from
3 to 12 m

A. Tunnels with arched roof

1 Non-squeezing ground conditions Up to 20% only

2 Poor rock masses/squeezing ground 20–60%

conditions (N ¼ 0.5 to 10)

3 Soft-plastic clays, running ground, 100–400%

flowing ground, clay-filled moist

fault gouges, slickensided shear

zones (N ¼ 0.1 to 0.5)

B. Tunnels with flat roof (irrespective of ground conditions) 400%

Source: Goel et al., 1996.

New Concept on Effect of Tunnel Size on Support Pressure

Equations (9.9) and (9.10) have been used to study the effect of tunnel size on support
pressure, which is summarized in Table 9.5.

It is cautioned that the support pressure is likely to increase significantly with the
tunnel size for tunnel sections excavated in the following situations:

1. Slickensided zone
2. Thick fault gouge
3. Weak clay and shales
4. Soft plastic clays
5. Crushed brecciated and sheared rock masses
6. Clay-filled joints
7. Extremely delayed support in poor rock masses

Further, both Q and N are not applicable to flowing grounds or piping through seams.
They also do not consider mineralogy (water sensitive minerals, soluble minerals, etc.).

CORRELATIONS FOR ESTIMATING TUNNEL CLOSURE

The behavior of concrete, gravel, and tunnel-muck backfills, commonly used with steel
arch supports, has been studied. Stiffness of these backfills has been estimated using
measured support pressures and tunnel closures. These results have been used to obtain
effective support stiffness from the combined support system of steel ribs and backfill
(Goel, 1994).

Based on measured tunnel closures from 60 tunnel sections, correlations have been
developed for predicting tunnel closures in non-squeezing and squeezing ground condi-
tions (Goel, 1994). These correlations are given in Eqs. (9.11) and (9.12).