Civil Engineering Reference
In-Depth Information
1.5 Basic characteristics of soils
At first sight soils appear to behave rather strangely. For example, you can pour dry
sand like water and you can pour saturated sand under water in the same way, yet you
can make sandcastles from damp sand that will support loads. Clays can be squeezed
and moulded like plasticine and appear to behave very differently from sands, but very
old slopes in clay have angles comparable to those in sands.
The essential features of soil behaviour which we will examine later in this topic are
as follows:
1. External loads and water pressures interact with each other to produce a stress
that is effective in controlling soil behaviour.
2. Soil is compressible; volume changes occur as the grains rearrange themselves and
the void space changes.
3. Soil shearing is basically frictional so that strength increases with normal stress,
and with depth in the ground. We will find that soil stiffness also increases with
normal stress and depth.
4. Combining these basic features of soil behaviour leads to the observation that soil
strength and stiffness decrease with increasing water pressure and with increasing
water content.
5.
Soil compression and distortion are generally not fully recoverable on unload-
ing, so soil is essentially inelastic. This is a consequence of the mechanism
of compression by rearrangement of the grains; they do not 'un-rearrange' on
unloading.
We will see later that there is no real distinction between sands and clays and that
the apparent differences arise from the influence of pore pressures and seepage of water
in the void spaces between the grains.
1.6 Basic forms of geotechnical structure
The four basic types of geotechnical structure are illustrated in Fig. 1.5; most other cases
are variations or combinations of these. Foundations (Fig. 1.5(a)) transmit loads to the
ground and the basic criterion for design is that the settlements should be relatively
small. The variables of a foundation are the load
V
, the size of the base
B
and the
depth
D
. Foundations may support loads that are relatively small, such as car wheels,
or relatively large, such as a power station. Slopes (Fig. 1.5(b)) may be formed naturally
by erosion or built by excavation or filling. The basic variables are the slope angle
i
and the depth
H
, and the design requirement is that the slope should not fail by
landsliding.
Slopes that are too deep and too steep to stand unsupported can be supported by a
retaining wall (Fig. 1.5(c)). The basic variables are the height of the wall
H
and its depth
of burial
D
, together with the strength and stiffness of the wall and the forces in any
anchors or props. The basic requirements for design are complex and involve overall
stability, restriction of ground movements and the bending and shearing resistance of
the wall. In any structure where there are different levels of water, such as in a dam
(Fig. 1.5(d)) or around a pumped well, there will be seepage of water. The seepage
