Engineering Rock Mechanics

Questions and answers: design of surface excavations 3 1 9

Set 1 2 3 4

Dip direction/Dip angle 292164 151 137 052176 02011 6

The rock mass can be considered dry,

and the angle of friction for all frac-

Plane tures is 30°. Consider the primary po-

sliding tential modes of instability (plane,

wedge and direct toppling) at 15O in-

tervals of dip direction (i.e. Oo, 15O,

30°, ..., 345O, 360O) and use kin-

ematic feasibility techniques to pre-

pare a table showing the steepest

safe slope and the respective critical

failure mode at each azimuth.

Wedge

sliding

A18.3 The fracture data are first plot-
Toppling ted on the hemispherical projection. We
need to plot both fracture normals (for
the analysis of plane sliding and dir-
ect toppling) and great circles (in order
to locate the intersections, which are re-
quired for both the analysis of wedge sliding and direct toppling). This
plot is shown below.

Three kinematic feasibility overlays are required: one for each of the

instability modes of plane sliding, wedge sliding and direct toppling. For

a given slope dip direction, the plane sliding and wedge sliding overlays

lie on opposite sides of the hemispherical projection, and this means that

they can both be drawn on the same sheet of tracing paper and used

simultaneously. This combined overlay is shown below, see ERM 1.