Engineering Rock Mechanics

3 1 2 Design and analysis of surface excavations

I Dip direction of slope

Lower angle limit: friction

Upper angle limit: slope

Use with poles

(b) Use with poles

Design slope \-2oo

30" friction and 75" slope dip

Figure 18.2 Construction of instability overlay for plane sliding.

is critical: any major increase or decrease in the dip will significantly alter

the degree of instability, because slope dip angles around this value

coincide with the dip of the majority of discontinuities in set B. Finally, the

orientation of the slope itself is critical: were we to be able to alter the dip

direction of the slope by +30", the potential for plane instability would be

reduced considerably.