On-Site Identification & Classification of Soil

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.

While identifying and classifying the soils in the field, we need to use the following steps in a logical sequence.

1. Identifying as Coarse Grained Soil or Fine Grained Soil
2. Classifying Coarse Grained Soil
3. Classifying Fine Grained Soil

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]

Fine grained soil is primarily of two types i.e. silt & clay. The distinction between silt & clay cannot be based on particle size because the significant physical properties of the two materials are related only indirectly to the size of particles. Furthermore, since both are microscopic, physical properties other than particle size must be used as criteria for field identification. There are 4 nos of field test that we can easily conduct on the field to describe or classify silt & clay. These are

• Dilatancy
• Dry strength
• Toughness
• Dispersion

Let us discuss each one of them one by one.

1. Dry Strength Test

The dry strength provides one basis for distinction. A small briquette of the soil is molded and allowed to dry in the air. It is then broken and a small fragment about 1.0 cm in size is pressed between thumb and forefinger. The effort required to break the fragment provides a basis for describing the strength as ver low, low, medium, high or very high. A clay fragment can be broken only with great effort, where as a silt fragment crushes easily.

2. Dilatancy or Shaking Test

Since silts are considerably more permeable than clays, the dilatancy or shaking test may also be used to distinguish between the two materials. In this test a small amount of soil is mixed with water to a very soft consistency in the palm of the hand. The back of the hand is then lightly tapped. If the soil is silty, water rises quickly to its surface and gives it a shiny or glistening appearance. Then if the soil pat is deformed, in some instances by squeezing and in others by stretching, the water flows back into it and leaves the surface with a dull appearance. Usually, the greater the proportion of clay in the sample, the slower the reaction to the test. The reaction is described as rapid, slow or none.

3. Toughness / Plasticity Test

The property of plasticity is characteristic of clays and may be used as the basis for a simple field test. At certain moisture contents a soil that contains appreciable quantities of clay can be deformed and remolded in the hand without disintegration. Thus, if a sample of moist soil can be manipulated between the palms of the hands and fingers and rolled out into a long thread, it unquestionably contains a significant amount of clay. As moisture is lost during continued manipulation, the soil approaches a non-plastic condition and becomes crumbly. Just before the crumbly state is reached, a highly plastic clay can be rolled into a long thread, with a diameter of approximately 3mm, which has sufficient strength to support its own weight. Silt, on the other hand, can seldom be rolled into a thread with a diameter as small as 3mm without severe cracking, and is completely lacking in tensile strength unless small amounts of clay are present. The record of a simple plasticity test should indicate not only whether a plastic thread can be formed, but also the toughness of the thread as it nears the crumbling stage. This condition is described as weak and friable, medium, or tough.

4. Dispersion Test

The fourth procedure, known as the dispersion test, is also useful for distinguishing between silt & clay, and for making a rough estimate of the relative amounts of sand, silt and clay in a material. A small quantity of the soil is dispersed with water in a glass cylinder or test tube and then allowed to settle. The coarser particles fall out first and the finest particles remain in suspension the longest. Ordinarily sand settles in 30 to 60 sec. Materials of silt size settles in 15 to 60 min, whereas that of clay size remains in suspension for at least several hours and usually for several days unless the particles of clay combines in groups or flocules.

After conducting these tests we can use the table given below to classify the soil as silt or clay.

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