Civil Engineering Reference
In-Depth Information
of the soils (see Chapter 5) should be used rather than the geological classifications,
which are based on age (see Chapter 4).
There is no simple answer to the problem of how many holes should be drilled and
to what depths and how many tests should be carried out. Most of the standards and
codes of practice make various recommendations, but really you should do enough
investigation to satisfy everybody that safe and economical works can be designed and
constructed. There will inevitably be uncertainties and these will require conservatism
in design which will lead to additional costs of construction. There is a balance to be
struck between costs of more investigations and savings in construction: it is a matter
of apportioning risk.
Figure 17.1 illustrates a very simplified section along the centre-line of a road. (Notice
that the horizontal and vertical scales are not the same.) The ground conditions revealed
by drilling and other methods have been greatly idealized so that a number of charac-
teristic layers have been identified and the boundaries between them drawn as smooth
lines. The actual soils in the ground within any one layer are likely to be variable,
horizontally and vertically, and their boundaries irregular. Something like Fig. 17.1 is
about the best you can do with a reasonable investigation. Notice that Fig. 17.1 is a
section along the centre-line of the road and to complete the investigation you should
be able to draw cross-sections and sections on either side of the road.
The section shown in Fig. 17.1 is similar to that shown in Fig. 4.4(c) and I have
already discussed the sequence of geological events and processes that formed this
sequence of deposits. Certain features of the nature and state of the various layers can
be estimated from consideration of their depositional environment and subsequent
geological history. The grading and mineralogy of the soft clay and the stiff clay are
the same (so they have the same nature), but their water contents are different (so they
have different states); the soft clay is normally consolidated or lightly overconsolidated
while the stiff clay is heavily overconsolidated.
For each of the principal strata in Fig. 17.1 you will need to determine representative
parameters for strength, stiffness and water seepage flow (i.e. permeability). These will
Figure 17.1
A Simple geotechnical cross-section.
