Field Identification & Classification of Soils

The principle terms used by civil engineers to describe soils are

  • 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
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

  • 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)

  • 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

  • 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

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