Geotechnical Engineering

DHARM

IDENTIFICATION AND CLASSIFICATION OF SOILS 109

S 1 and S 2 are respectively in the zone of CI and CH (inorganic clays of medium and high
plasticity).
Example 4.2: A soil sample has a liquid limit of 20% and plastic limit of 12%. The following
data are also available from sieve analysis:

Sieve size % passing

2.032 mm 100

0.422 mm 85

0.075 mm 38

Classify the soil approximately according to Unified Classification or IS Classifiction.

(S.V.U.—Four year B. Tech.—June, 1982)

Since more than 50% of the material is larger than 75-μ size, the soil is a coarse-grained

one.

100% material passes 2.032 mm sieve; the material,passing 0.075 mm sieve is also in-

cluded in this. Since this latter fraction any way passes this sieve, a 100% of coarse fraction

also passes this sieve.

Since more than 50% of coarse fraction is passing this sieve, it is classified as a sand.

(This will be the same as the per cent passing 4.75 mm sieve).

Since more than 12% of the material passes the 75-μ sieve, it must be SM or SC.

Now it can be seen that the plasticity index, Ip, is (20 – 12) = 8, which is greater than 7.

Also, if the values of wL and IP are plotted on the plasticity chart, the point falls above A-

line.

Hence the soil is to be classified as SC, as per IS classification.

Even according to Unified Classification System, this will be classified as SC, which

may be checked easily.

Summary of Main Points

1. Certain generalised procedures have been evolved for identification of soils in the laboratory
   and in the field, and for classification of soils. The need for classification arises from the fact that
   natural soil deposits vary widely in their properties and engineering behaviour.


2. The requirements or desirable features of an engineerging soil classification system are so ambi-
   tious that it is almost impossible to evolve an ideal system satisfying all of these.


3. Preliminary classification procedures include descriptive and geological classifications, and also
   classification by structure.


4. Textural classifications are used as part of the more systematic and exhaustive systems such as
   the Unified Soil Classification.


5. The Indian Standard Soil Classification bears many similarities to the Unified Soil Classifica-
   tion although there are a few points of difference, especially with regard to the classification of
   fine-grained soils.


6. Grain-size is the primary criterion for the classification of coarse-grained soils, while plasticity
   characteristics, incorporated in the plasticity chart, are the primary criterion for the classifica-
   tion of fine-grained soils.