Classification and Physical Properties of Soils - Elements of Soil Mechanics - page 21

Soil A : From the grading curve it is seen that this soil consists of 57% gravel and 43%
sand and is therefore predominantly gravel. The curve has a horizontal portion indicating
that the soil has only a small percentage of soil particles within this range. It is therefore
gap graded.
The soil is a gap graded sandy GRAVEL. Group symbol GP g S.
Soil B : From the grading curve it is immediately seen that this soil is a sand with most
of its particles about the same size.
The soil is a uniformly graded SAND. Group symbol SP u .
Soil C : It is interesting to note that, as the whole of soil C passed the 425 μ m sieve,
there would be no need to remove any of the soil before subjecting it to the consistency
limit tests. From the grading curve, by considering particle sizes only, the soil is a mixture
of 10% sand, 50% silt and 40% clay. The soil is undoubtedly fine and the group symbol
could be F, although, as the silt particles are more dominant than the clay, it could be
given the symbol MC. The liquid limit of the soil is 48% which, according to BS 5930,
indicates an intermediate plasticity. The group symbol of the soil could therefore be
either FI or MCI.
However, for mixtures of fine soils BS 5930 suggests that classification is best carried
out by the use of the plasticity chart shown in Fig. 1.8. The liquid limit of the soil = 48%
and the plasticity index, (w L − w P ) = 27%. Using Fig. 1.8 it is seen that the British system
classifies the soil as an inorganic clay with the group symbol CI.
1.7.2 Description of soils
Classifying and describing a soil are two operations which are not necessarily the same. An operator who
has not even visited the site from which a soil came can classify the soil from the information obtained
from grading and plasticity tests carried out on disturbed samples. Such tests are necessary if the soil is
being considered as a possible construction material and the information obtained from them must be
included in any description of the soil.
Further information regarding the colour of a soil, the texture of its particles, etc., can be obtained in
the laboratory from disturbed soil samples but a full description of a soil must include its in situ , as well
as its laboratory characteristics. Some of this latter information can be found in the laboratory from undis-
turbed samples of the soil collected for other purposes, such as strength or permeability tests, but usually
not until after the tests have taken place and the samples can then be split open for proper examination.
Other relevant information, such as bedding, geological details, etc., obtained from borehole data and
site observations should also be included in the soil's description.
Further information is available in BS 5930 Code of Practice for Site Investigations , and Clayton et al .
( 1995) .
1.8 Soil properties
From the foregoing it is seen that soil consists of a mass of solid particles separated by spaces or voids.
A cross-section through a granular soil may have an appearance similar to that shown in Fig. 1.10a .
In order to study the properties of such a soil mass it is advantageous to adopt an idealised form of
the diagram as shown in Fig. 1.10b . The soil mass has a total volume V and a volume of solid particles
that summates to V s . The volume of the voids, V v , is obviously equal to V - V s .