Design and
78 analysis I of
surtace
excavations
In the last chapter, some of the idealized models were presented that have
been developed to provide a basic grasp of the instability mechanisms
associated with slopes and foundations in rock. Although the models are
highly simplified, they do assist understanding and indicate the important
parameters, together with their sensitivity. When faced with the design and
analysis of an actual surface excavation, one has to go beyond these
primary models and match the model with the site reality and the nature
of the rock.
Thus we should ask what is the exact design objective, what mechanisms
are likely to be operating, which data are required as a consequence, and
does the model incorporate the discontinuous, inhomogeneous, anisotrop-
ic and non-elastic behaviour of the ground, together with factors such as
the effects of blasting, rainfall, seismic risk and so on? The approach should
therefore be to examine the potential for instability mechanisms and to
gradually refine the design and analysis, from an initial skeletal approach
through to a comprehensive finely-tuned design. In this book we are
discussing principles and hence the techniques which may be applied
during the initial approach to all projects. Any further development should
be tailored to the rock, site and project circumstances-in the knowledge
that the initial analyses have been performed thoroughly.
18.1 Kinematic analysis of slope instability
mechanismsOne of the best examples of an initial approach is establishing the pos-
sibility of instability by the method of kinematic analysis of slopes. 'Kine-
matics' refers to the study of movement, without reference to the forces that
produce it. For some geometries of slope and discontinuities, movement
is possible (i.e. the system is kinematically feasible). For other geometries,
movement is not possible (i.e. the system is kinematically infeasible).
A method based on checking the kinematic feasibility of a rock slope-dis-
continuity system will provide a 'first pass' analysis, although kinematic
feasibility checks are but the first in a long line of design and analysis tools.