Page 273 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 18 Slope Mass Rating 233

TABLE 18.2 Values of Adjustment Factor F4
for Method of Excavation

Method of excavation Value of F4
Natural slope þ15
Pre-splitting þ10
Smooth blasting þ8
Normal blasting or mechanical 0
excavation
Poor blasting À8

Source: Romana, 1985.

l Normal blasting applied with sound methods does not change slope stability
conditions, so F4 ¼ 0.

l Deficient blasting or poor blasting damages the slope stability, so F4 ¼ À8.0.
l Mechanical excavation of slopes, usually by ripping, can be done only in soft and/or

very fractured rock and is often combined with some preliminary blasting. The plane
of slope is difficult to finish. The method neither increases nor decreases slope
stability, so F4 ¼ 0.

The minimum and maximum values of SMR from Eq. (18.1) are 0 and 100, respectively.
It is needless to mention that the slope stability problem is not found in areas where the
discontinuities are steeper than the slope; therefore, this condition is not considered in the
empirical approach.

Romana (1985) used planar and toppling failures for his analysis. The wedge failures
have been considered as a special case of plane failures and analyzed in forms of
individual planes, and the minimum value of SMR is taken for assessing the rock slopes.
Dip bi and dip direction ai of the intersection of these planes should be taken as bj and aj,
respectively; that is, bj ¼ bi and aj ¼ ai where wedge failure is likely to occur
(Figure 18.2).

FIGURE 18.2 Wide angle wedge failure.