Civil Engineering Reference
In-Depth Information
Stability of slopes
21.1 Introduction
The surface of the earth is very rarely flat and so there are slopes nearly everywhere.
Even relatively flat ground often has rivers and drainage channels with side slopes.
Slopes may be natural, due to erosion by rivers or the sea, or man-made by excavation
or fill. Man-made slopes for roads and dams are permanent, but temporary slopes are
required during construction of foundations and underground structures.
The geometry of a slope may be characterized by its angle
i
and height
H
, as shown
in Fig. 21.1. The loads on the slope are due to the self-weight of the soil and to
external loads, whichmay come from foundations at the top or water in the excavation.
A special case of a slope is a vertical cut, such as the sides of a trench, where
i
90
◦
.
In the soil behind any slope there will be shear stresses and these are required to
maintain the slope. Materials that cannot sustain shear stresses cannot have slopes, so
water in a glass has a level surface.
During excavation of a slope the mean normal total stresses will be decreased due to
removal of soil from the excavation, while during construction of an embankment the
mean normal total stresses will increase as more fill is placed. In both cases, however,
the shear stresses increase as the height and/or slope angle increase. I will call any kind
of slope construction
loading
because the shear stresses increase irrespective of what
happens to the mean normal total stress.
If a slope is too steep or too high it will fail and there will be a slip or landslide, as
illustrated in Fig. 21.2. The landslide will stop when the height and angle are critical
(
H
c
and
i
c
) and the slope has a factor of safety of unity. Rock slopes can be very steep,
but soil slopes are much more modest, with angles from 10
◦
to 30
◦
and, for steeper
angles, heights up to 20 m. The best laboratory to study slope stability is at the seaside
where you should dig a hole in the beach and construct a sandcastle.
=
21.2 Types of instability
Slope instabilities involve large ground movements and usually require a mechanism
of slip surfaces. Mechanisms can have a number of different configurations and some
typical ones are illustrated in Fig. 21.3. In Fig. 21.3(a) and (b) the soil is homogeneous
and the position of the slip surface (deep or shallow) is governed largely by the pore
pressures. In Fig. 21.3(c) the geometry of the slip surface is controlled by a weak layer.
Figure 21.3(d) illustrates a mud flow where there are very large homogeneous strains.
