Civil Engineering Reference
In-Depth Information
1.5.3 Granular soils - particle size distribution
A standardised system helps to eliminate human error in the classification of soils. The usual method is
based on the determination of the particle size distribution by shaking an oven dried sample of the soil
(usually after washing the sample over a 63
μ
m sieve) through a set of sieves and recording the mass
retained on each sieve. The classification system adopted by the British Standards Institution is the Mas-
sachusetts Institute of Technology (MIT) system. The boundaries defined by this system can be seen on
the particle size distribution sheet in Fig.
1.2.
The results of the sieve analysis are plotted with the particle
sizes horizontal and the summation percentages vertical. As soil particles vary in size from molecular to
boulder it is necessary to use a log scale for the horizontal plot so that the full range can be shown on
the one sheet.
The smallest aperture generally used in soils work is that of the 0.063 mm size sieve. Below this size (i.e.
silt sizes) the distribution curve must be obtained by sedimentation (pipette or hydrometer). Unless a
centrifuge is used, it is not possible to determine the range of clay sizes in a soil, and all that can be done
is to obtain the total percentage of clay sizes present. A full description of these tests is given in BS 1377:
Part 2.(BSI,
1990)
.
Examples of particle size distribution (or grading) curves for different soil types are shown in Fig.
1.9.
From these grading curves it is possible to determine for each soil the total percentage of a particular
size and the percentage of particle sizes larger or smaller than any particular particle size.
The effective size of a distribution, D
10
An important particle size within a soil distribution is the effective size which is the largest size of the
smallest 10%. It is given the symbol D
10
. Other particle sizes, such as D
60
and D
85
, are defined in the same
manner.
Grading of a distribution
For a granular soil the shape of its grading curve indicates the distribution of the soil particles within it.
If the shape of the curve is not too steep and is more or less constant over the full range of the soil's
particle sizes then the particle size distribution extends evenly over the range of the particle sizes within
the soil and there is no deficiency or excess of any particular particle size. Such a soil is said to be
well
graded
.
If the soil has any other form of distribution curve then it is said to be
poorly graded
. According to their
distribution curves there are two types of poorly graded soil:
●
if the major part of the curve is steep then the soil has a particle size distribution extending over a
limited range with most particles tending to be about the same size. The soil is said to be
closely graded
or, more commonly,
uniformly graded
;
●
if a soil has large percentages of its bigger and smaller particles and only a small percentage of the
intermediate sizes then its grading curve will exhibit a significantly flat section or plateau. Such a soil is
said to be
gap graded
.
The uniformity coefficient C
u
The grading of a soil is best determined by direct observation of its particle size distribution curve. This
can be difficult for those studying the subject for the first time but some guidance can be obtained by
the use of a grading parameter, known as the uniformity coefficient.
D
D
60
C
u
=
10
