Classification and Physical Properties of Soils - Elements of Soil Mechanics - page 15

improved systems allowed for the plasticity characteristics of soil and a modified form of the system pro-
posed by Casagrande in 1947 is the basis of the soil classification system used in the UK.

The British Soil Classification System (BSCS)

The British Standard BS 5930 ( 1999) , Code of practice for site investigations , gives a full description of
the BSCS and the reader is advised to obtain sight of a copy.
The system divides soil into two main categories. If at least 35% of a soil can pass through a 63 μ m sieve
then it is a ine soil . Conversely, if the amount of soil that can pass through the 63 μ m sieve is less than
35% then it is a coarse soil . Each category is divided into groups, depending upon the grading of the soil
particles not passing the 63 μ m sieve and upon the plasticity characteristics of the soil particles passing
the 425 μ m sieve.
A summary of the BSCS is shown in Table 1.1 and its associated plasticity chart in Fig. 1.8.
To use the plasticity chart it is necessary to plot a point whose coordinates are the liquid limit and the
plasticity index of the soil to be identified. The soil is classified by observing the position of the point rela-
tive to the sloping straight line drawn across the diagram.
This line, known as the A-line, is an empirical boundary between inorganic clays, whose points lie above
the line, and organic silts and clays whose points lie below. The A-line goes through the base line at I P = 0,
W L = 20% so that its equation is:

I = 0 73 . ( w − 20 %)

P L

The main soil types are designated by capital letters:

G Gravel M Silt, M-soil

S Sand C Clay

F Fine soil, Fines Pt Peat

The classification 'F' is intended for use when there is difficulty in determining whether a soil is a silt or a
clay.
Originally all soils that plotted below the A-line of the plasticity charts were classified as silts. The term
'M-soil' has been introduced to classify soils that plot below the A-line but have particle size distributions
not wholly in the range of silt sizes.
Behind the letter designating the main soil type additional letters are added to further describe the soil
and to denote its grading and plasticity. These letters are:

W Well graded L Low plasticity

(w L < 35%)

P Poorly graded I Intermediate (35 ≤ w L ≤ 70)

P u Uniform H High plasticity (50 ≤ w L ≤ 70)

P g Gap graded V Very high (70 ≤ w L ≤ 90)

O Organic E Extremely high (w L > 90%)

The letter O is applied at the end of the group symbol for a soil, no matter what type, if the soil has a
significant amount of organic matter within it.
Examples of the use of the symbols are set out below.

Soil description Group symbol

Well graded silty SAND SWM

Organic CLAY of high plasticity CHO

Sandy CLAY of intermediate plasticity CIS

Uniform clayey sand SP u F