FIELD IDENTIFICATION &
CLASSIFICATION OF SOILS
Suryakanta | March 28, 2014 | Geotechnical, Soil, Soil Investigation | No Comments
The principle terms used by civil engineers to describe soils are
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Gravel (particle size larger than 4.75 mm)
Sand (particle size within 4.75 mm to 0.075mm)
Silt & Clay (particle size less than 0.075mm)
Most natural soils consist of a mixture of two or more of these constituents and may contain an
admixture of organic material in a partly or fully decomposed state. The mixture is given the name of
the constituent that appears to have the most influence on its behavior, and the other constituent are
indicated by adjectives.
For example, silty clay has predominantly the properties of clay but consists a significant amount of
silt.
Field
Identification & Classification of Soils
While identifying and classifying the soils in the field, we need to use the following steps in a logical
sequence.
STEP-1 (IDENTIFYING AS COARSE GRAINED SOIL OR
FINE GRAINED SOIL)
In the first step we try to identify the soil as coarse grained soil (CGS) or fine grained soil (FGS).
Coarse grained soils are those soils whose individual particles are visible by the naked eye. Sandy
soil and gravelly soil fall in this group.
Fine grained soils are those soils whose individual particles are not visible by the naked eye. Silty
soil and clayey soil fall in this group.
To know more about coarse grained soil and fine grained soil please read my post what are the
primary differences between coarse grained soil & fine grained soil
The following procedure is adopted in the field to identify the soil as CGS or FGS
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Take a representative sample of soil and spread it on a flat surface or palm of the hand.
Remove all particles whose sizes are larger than 75 mm.
If the soil contains more than 50% of visible particles then the soil is said to be a coarse grained
soil.
If the soil contains less than 50% of visible particles then the soil is said to be a fine grained
soil.
Once the soil is identified as coarse grained soil or fine grained soil, then we further sub divide it on
the basis of percentage of different sizes present in the soil mass. Coarse grained soil can be sub
divided into gravelly soil or sandy soil.
STEP-2 (CLASSIFYING COARSE GRAINED SOIL)
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A coarse grained soil is said to be gravelly soil if the percentage of gravel is greater than sand.
If percentage of fines (i.e. silt & clay) present in a gravelly soil is less than 5%, then it is
identified as clean gravel. Clean gravel can be either classified as well graded gravel
(GW) or poorly graded gravel (GP).
GW– if there is good representation of all particle sizes
GP – if there is an excess or absence of intermediate particle sizes.
If percentage of fines (i.e. silt & clay) present in a gravelly soil is more than 12%, then it is
identified as dirty gravel. Dirty gravel can be either classified as silty gravel (GM) or clayey
gravel (GC).
GM– if the fines have little or no plasticity
GC– if the fines are of low to medium to high plasticity.
Gravels containing 5 to 12% fines are given boundary classification, which is generally done at
laboratory.
A coarse grained soil is said to be sandy soil if the percentage of sand is greater than gravel.
If percentage of fines (i.e. silt & clay) present in a sandy soil is less than 5%, then it is identified
as clean sand. Clean sand can be either classified as well graded sand (SW) or poorly
graded sand (SP).
SW– if there is good representation of all particle sizes
SP – if there is an excess or absence of intermediate particle sizes.
If percentage of fines (i.e. silt & clay) present in a sandy soil is more than 12%, then it is
identified as dirty sand. Dirty sand can be either classified as silty sand (SM) or clayey sand
(SC).
SM– if the fines have little or no plasticity
SC– if the fines are of low to medium to high plasticity.
Sand containing 5 to 12% fines are given boundary classification, which is generally done at
laboratory.
STEP-3 (CLASSIFYING FINE GRAINED SOIL)
Before classifying the fine grained soils we need to carry out some filed tests, such as
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Dilatancy
Dry strength
Toughness
Dispersion
After conducting these tests we can use the table given below to classify the soil as silt or clay. If you
want to know the detail about these filed tests then read my post how to identify silt and clay in the
field.
Typical
Name
Dry
Strength
Dilatancy Reaction
Toughness
of plastic
thread
Time to
settleIn
Dispersion
test
Sandy silt
None to very
low
Rapid
Weak to
friable
30 sec to 60
min
Silt
Very low to
low
Rapid
Weak to
friable
15 to 60 min
Clayey
silt
Low to
medium
Rapid to slow
Medium
15 min to
several hours
Sandy
clay
Low to high
Slow to none
Medium
30 sec to
several hour
Silty clay
Medium to
high
Slow to none
Medium
15 min to
several hour
Clay
High to very
high
None
Tough
Several hour to
days
Organic
silt
Low to
medium
Slow
Weak to
friable
15 min to
several hour
Organic
clay
Medium to
very high
none
Tough
Several hour to
days