Engineering Rock Mechanics

Kinematic analysis of slope instability mechanisms 3 1 3

N

Figure 18.3 Example assessment for a slope of orientation 295"/75"-plane
instability.

Armed with this information, it is necessary to check that the data
for discontinuity set B have been correctly recorded. Then, can the dip
direction and dip angle of the slope be altered? If so, the potential for
instability can be eliminated by re-orientating the slope. If not, further
analysis has to be conducted to decide on the optimal excavation and
support techniques. In the event of detailed numerical analyses being
required, the variation associated with the orientation of the individual
discontinuities within a set must always be considered. One might be
tempted to try to produce one factor of safety using a single discontinuity
orientation but, as the overlay method clearly demonstrates, the variation
within a set means that such an approach is meaningless without
qualification.
There is also the interaction between the design process and the rock
mass. There are three options:

the design can be altered to account for the rock mass;

a fixed design can be applied to the fixed rock mass and the structure

the rock mass can be altered to account for the design.

The power of the kinematic feasibility overlays lies in an 'immediate
appreciation' of the primary parameters and design possibilities.

engineered accordingly; and

78.7.2 Wedge instubihy

An overlay for wedge instability potential can be constructed by the same
method used for plane instability. Wedge instability can be considered as