Page 272 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

232 Engineering Rock Mass Classification

FIGURE 18.1 Planar failure.

unfavorable.” For the toppling failure, unfavorable conditions depend upon the sum of
the dips of joints and the slope bj þ bs.

Values of adjustment factors F1, F2, and F3 for different joint orientations are given in
Table 18.1.

F4 pertains to the adjustment for the method of excavation. It includes the natural
slope, or the cut slope excavated by pre-splitting, smooth blasting, normal blasting, poor
blasting, and mechanical excavation (see Table 18.2 for adjustment rating F4 for different
excavation methods).

l Natural slopes are more stable, because of long-time erosion and built-in protection
mechanisms (vegetation, crust desiccation), so F4 ¼ þ15.

TABLE 18.1 Values of Adjustment Factors for Different Joint Orientations

Case of slope Very Favorable Fair Very
failure favorable Unfavorable unfavorable

P |aj À as| >30 30–20 20–10 10–5 <5

T |aj À as À 180|

W |ai À as|

P/W/T F1 0.15 0.40 0.70 0.85 1.00
<20 20–30 30–35 35–45 >45
P |bj|
W |bi|

P/W F2 0.15 0.40 0.70 0.85 1.00
1.0 1.0 1.0 1.0
T F2 1.0 10–0 0 0 À (À10) <À10
>10
P |bj À bs| 110–120 >120 — —
W |bi À bs| <110 À6 À25 À50 À60

T |bj þ bs|

P/W/T F3 0

P, planar failure; T, toppling failure; W, wedge failure; as, slope strike; aj, joint strike; ai, plunge direction of
line of intersection; bs, slope dip; bj, joint dip (see Figure 18.1); bi, plunge of line of intersection.

Source: Romana, 1985.