Page 160 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

128 Engineering Rock Mass Classification

GRC Using Eq. (9.10)
In Eq. (9.10), as described earlier, f(N) is the correction factor for tunnel closure. For
different values of permitted normalized tunnel closure (ua/a), different values of f(N) are
proposed in Table 9.3. The first step is to choose any value of tunnel wall displacement
(ua) in column 1 of Table 9.6. Then the correction factor f(N) is found from Table 9.3 as
shown in column 2 of Table 9.6. Finally, Eq. (9.10) yields the support pressure in the roof
(pv) as mentioned in column 3 of Table 9.6. Using Table 9.3 and Eq. (9.10), the support
pressures pv(sq) have been estimated for the assumed boundary conditions and for
various values of ua/a (column 1) as shown in Table 9.6. Subsequently, using the value
of pv (column 3) and ua/a (column 1) from Table 9.6, GRC has been plotted for ua/a up
to 5% (Figure 9.3).

This approach is simple, reliable, and user friendly because the values of the input
parameters can be easily obtained in the field.

TABLE 9.6 Showing Calculations for Constructing GRC Using Eq. (9.10)

Assumed ua/a (%) Correction factor (f) pv(sq) from Eq. (9.10) (MPa)
(1) (2) (3)

0.5 2.7 0.86
1 2.2 0.7
2 1.5 0.475
3 1.2 0.38
4 1.0 0.317
5 0.8 0.25

FIGURE 9.3 GRC obtained from Eq. (9.10).