Civil Engineering Reference
In-Depth Information
Laboratory testing of soils
7.1 Purposes of laboratory tests
Testing soil samples in the laboratory plays an important role in soil mechanics research
and civil engineering practice. Almost all we know about soil behaviour has been
learned from laboratory tests. Tests may be carried out on small samples of soil to
examine the characteristics of the soil or on models of soil structures to examine how
slopes, walls and foundations deform and collapse. In this chapter we will consider
tests on soil samples. Laboratory tests are carried out for a number of purposes, the
most important being:
1. For description and classification of a particular soil (see Chapter 5).
2. To investigate the basic mechanical behaviour of soils and to develop theories for
soil behaviour (see Chapters 8 to 13).
3. To determine design parameters (i.e. numerical values for strength, stiffness and
permeability) for geotechnical analyses (see Chapters 18 to 25).
Laboratory tests may be carried out on samples that are intact or have been com-
pletely reconstituted. In reconstituted samples the soil has been mixed at a relatively
large water content and then recompressed. In this way any structure developed in
the soil in the ground due to deposition or ageing is removed and the tests measure
the fundamental behaviour and properties of the soil. Intact samples are recovered
from the ground with minimum disturbance (see Chapter 17); they contain the
in situ
structure and retain the properties of the soil in the ground.
Most of the analyses of geotechnical structures described in Chapters 18 to 25 and
used for routine design were developed for simple soils which behave more or less like
the theories described in Chapters 8 to 13. These analyses may not be applicable to soils
whose behaviour differs significantly from these simple theories, in which case special
methods will be required which are outside the scope of this topic. An important and
often neglected purpose of soil testing is to examine soil for unexpected or strange
behaviour. This is best done by comparing the behaviour of intact samples with the
basic theories and with the behaviour of the same soil reconstituted and recompressed
to the same state.
