Engineering Rock Mechanics

320 Design of surface excavations

N

= dip angle

of face

Note that, for each of the instability modes, the lower limit for slope

angle is given by the friction angle, which in this case is 30". The upper

limit for slope angle will be selected during the analysis in order to avoid

instability.

The toppling instability overlay is shown below, with the friction angle

forming one of the limits to the overlay.

To identify the maximum permissible slope angle to avoid instability

at each azimuth, we use each of the overlays in turn (see ERM 1).

For each of the azimuth values under consideration, we identify the

maximum slope angle that will avoid instability for each of the three