Foundation and
77 slope instability
mechanisms
17.1 Near-surface instability
Rock foundations can be on a small scale, such as that of the concrete
pier illustrated in the Frontispiece, or on a large scale, such as those of
the majestic Hoover Dam, illustrated in Fig. 17.1, which is 221 m high
and located on the Colorado River in the USA.
Rock slopes can be created for a variety of purposes. In Fig. 17.2a,
we illustrate one region of a large open-pit copper mine. The slope
geometry comprises a stepped series of benches, formed as the rock
is excavated during the mining process. The height of each bench is
governed by blasting and mining machinery parameters. In Fig. 17.2b,
there is a highway cutting with rock slopes on either side, and a
terminal slope where the road enters a tunnel. The methods of studying
slope instability in these two cases may well be the same, but the
consequences of slope failure in the two cases will be different: a single
block rolling onto the road may well cause public alarm in the civil
engineering case, but have little consequence in the mining engineering
case.
Also, the methods of analysing slope instability will be a function of
the rock type. For the granite mass illustrated in Fig. 17.3a, failure will be
caused by the presence of the fractures: rock blocks will slide and topple,
but a new failure plane is most unlikely to be generated through the
strong intact granite. For the chalk illustrated in Fig. 17.3b, failure may
similarly be caused by the blocky nature of the chalk, but it may also
occur by a new failure plane being generated through the chalk mass
because the chalk is relatively weak.
Foundations and slopes near the ground surface are often character-
ized by complex geological conditions, because of complex fracturing, a
long history of groundwater movement, and other types of weathering.
Moreover, compared to an underground excavation in a rock mass, the
rock mass near to the surface is loaded by a 'soft system', often the dead
weight of the rock mass itself, and there is greater kinematic potential for
movement than there is around an underground excavation. All these