Civil Engineering Reference
In-Depth Information
For structures, such as foundations, the design is controlled by the serviceability
limit state. The structure must not move too much and, if it has not moved much,
it is unlikely to fall down. In geotechnical engineering there are two quite different
ways to design structures to limit movements. One is the load factor method where
a load factor
L
f
is applied to the bearing capacity
q
c
, as shown in Fig. 18.1. The
other method for design of foundations is the stiffness method in which movements
are calculated from soil stiffness and applied loads. Whatever factors are applied are
there to account for uncertainties. I will deal with each method and the appropriate
parameters and factors when I deal with slopes, foundations and retaining walls in
later chapters.
18.3 Description and classification
As a start you should always carefully describe the soil and classify it, as discussed
in Chapter 5. The most important things to classify are the nature of the soil and
particularly its grading (is it coarse grained or fine grained?) and the state (is it loose or
dense?). You can observe the nature of the soil in disturbed samples but measurements
of state will require good undisturbed samples.
It is important to distinguish between coarse grained and fine grained soils so you
can determine whether analyses will be drained (effective stress analyses) or undrained
(total stress analyses). There are no hard and fast rules; it is a matter of engineering
judgment. You should draw grading curves and look particularly at the grain size
where the curve is at the 35% fraction. For coarse grained soils you should examine
the grains you can see and describe their shape (rounded, angular, elongated, flaky)
and their surface texture (rough or smooth). For fine grained soils you should measure
the Atterberg limits (liquid limit and plastic limit). These all describe the nature of the
grains and lead to estimates of material parameters.
You should try to determine the state of the soil. This is not so easy because state is a
combination of water content and stress and it should be related to the critical state. In
a sample in your hand the total stresses are zero and the effective stresses are governed
by the suctions which the soil can sustain and the effective stresses in the ground near
a loaded structure may be quite different. You certainly should determine the water
content and unit weight and from these calculate the specific volume, as described in
Sec. 5.5. If you have measured the Atterberg limits you should calculate the liquidity
index. If the soil is coarse grained you should determine its maximum and minimum
specific volumes and calculate the relative density.
You should investigate the structure of the soil in undisturbed samples or in the
ground. Look for bedding and for cracks. You should always put small samples into
a glass of water and examine bonding or dispersion, as described in Sec. 9.11.
18.4 Drained or undrained or consolidation: total or
effective stress parameters
For soils it is essential to separate drained analyses which are done using effective
stresses and pore pressures fromundrained analyses which are done using total stresses.
In Chapter 6 I distinguished carefully between loading (or unloading) events which
were drained from those which were undrained. During an event which is drained
