Page 53 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

36 Engineering Rock Mass Classification

For obtaining the vertical support pressure from the rock load factor (Hp) Terzaghi
suggested the following equation:

pv ¼ g : Hp ð5:1Þ

where pv is the support pressure, g is the unit weight of the rock mass, and Hp is the height
of loose overburden above the tunnel roof (Figure 5.1). Terzaghi’s theory is limited be-
cause it may not be applicable for tunnels wider than 6 m.

The roof of a tunnel is assumed to be located below the water table. If it is located
permanently above the water table, the values given for Classes IV to VI in Table 5.2 can
be reduced by 50% (Rose, 1982).

If the joints in a blocky and seamy rock do not contain clay, the pressure of the rock on
the tunnel support may be as high as one-half of the pressure exerted by the same rock
on the same tunnel at a considerable depth below the water table. On the other hand, if

the joints are partially or entirely filled with clay, a nominal support may be sufficient to
hold up the roof during the dry season; in a dried-out state the clay acts as a cementing
material. However, during long wet spells the clay ceases to act as an effective binder and

the pressure on the tunnel support becomes as heavy as if the joints were lubricated
(Proctor & White, 1946).

Because of this, several large tunnels, which were mined and supported during the dry
season, caved in soon after the rains. If it is uncertain whether or not the rock located
above the tunnel will remain dry throughout the year, it is advisable to design the tunnel
supports on the basis of the values obtained by the equations given in Table 5.2 regardless
of the appearance of the rock during mining operations.

Deere et al. (1970) modified Terzaghi’s classification system by introducing the rock

quality designation (RQD) as the lone measure of rock quality (Table 5.3). They have
distinguished between blasted and machine excavated tunnels and proposed guidelines
for selection of steel set, rock bolts, and shotcrete supports for 6- to 12-m diameter
tunnels in rock. These guidelines are listed in Table 5.4.

Deere et al. (1970) also considered the rock mass as an integral part of the support
system; Table 5.4 is only applicable if the rock mass is not allowed to loosen and disin-
tegrate extensively. They assumed that machine excavation reduced rock loads by
approximately 20 to 25%.

Limitations

Terzaghi’s approach was successfully used when conventional drill and blast methods
of excavation and steel-arch supports were employed in tunnels of comparable size.
This practice lowered the strength of the rock mass and permitted significant roof
convergence that mobilized a zone of loosened rock mass above the tunnel roof.
The height of this loosened rock mass, called “coffin cover,” acted as dead load on
the supports. Cecil (1970) concluded that Terzaghi’s classification provided no quan-
titative information regarding the rock mass properties. Despite these limitations, the
immense practical value of Terzaghi’s approach cannot be denied, and his method is
still applied under conditions similar to those for which it was developed.

With the advent of the New Austrian Tunnelling Method (NATM) and Norwegian
Method of Tunnelling (NMT), increasing use is made of controlled blasting and machine
excavation techniques and support systems employing steel fiber reinforced shotcrete
(SFRS) and rock bolts. Even in steel-arch supported tunnels, wooden struts have been