Standard Soil Classification System
Classification Based on Grain Size distribution
The various range of soil particle sizes are encountered in soils is basically very large: In case of boulders with different dimension of over 300 mm down to clay particles that are less than 0.002 mm sizes. Some soil clays which contain particles size less than 0.001 mm in size which behave as colloids, i.e. do not settle in existing water.
In the Standard Soil Classification System (ISSCS), soil mass are classified into different groups according to size, shapes and the groups are further divided into coarse, medium and fine sub-groups categories.
The grain-size distribution range is used on the basis for grouping of soil particles into boulder, cobble, gravel, sand, silt or clay etc.
| Very coarse soil | Boulder size soil | > 300 mm | |
|---|---|---|---|
| Cobble size soil | 80 – 300 mm | ||
| Coarse size soil | Gravel size soil(G) | Coarse sized | 20 – 80 mm |
| Fine soil | 4.75 – 20 mm | ||
| Sand size (S) | Coarse soil | 2 – 4.75 mm | |
| Medium soil | 0.425 – 2 mm | ||
| Fine soil | 0.075 – 0.425 mm | ||
| Fine soil | Silt size (M) | 0.002 – 0.075 mm | |
| Clay size (C) soil | < 0.002 mm |
Gravel, sand, silt, and clay type of soil are represented by group symbols G, S, M, and C respectively in the diagram.
Physical weathering condition produces very coarse and coarse soils. Chemical weathering produce generally fine soils.
Coarse-grained soil : This type soils are those for which , more than 50% of the soil material by weight calculation has particle sizes greater than
0.075 mm. These soils are generally divided into either gravels (G) or sands (S).
According to gradation of soil, they are further classified as well-graded (W) or poorly graded (P) soil sample. If fine soil particle are present, they are classified as containing silt fines (M) or as containing clay fines (C) particles.
For example, the combined symbol SW refers to well-graded sand with no fines.
Both the position and the shape of the grading curve for a soil can aid in establishing its identity and description. Some typical grading curves are shown.
Curve A – a poorly-graded medium SAND sample
Curve B – a well-graded GRAVEL-SAND (i.e. having equal amounts of gravel and sand) sample
Curve C – a gap-graded COBBLES-SAND sample
Curve D – a sandy SILT sample
Curve E – a silty CLAY ( having little amount of sand)sample
Fine-grained soil: These type of soil are those for which more than 50% of the material has soil particle sizes less than 0.075 mm. Clay particles have always a flaky shape to which water exists, so that imparting the property of plasticity index.
Now the plasticity chart , based on the values of liquid limit (WL) and plasticity index (IP), provided in ISSCS to the aid classification. The ‘A’ line in this chart is expressed asby the following equation IP = 0.73 (WL – 20).
Depending on the representation in the chart, fine soils are divided into clays (C), silts (M), or organic soils (O) sections. The organic particle content is now expressed as a percentage of the soil mass of organic soil matter in a given mass of soil sample to the mass of the dry soil sample solids.Three divisions of plasticity properties of soil are also defined as follows.
| Low plasticity state | WL< 35% |
| Intermediate plasticity state | 35% < WL< 50% |
| High plasticity state | WL> 50% |
The ‘A’ line and vertical lines at WL represents equal to 35% and 50% separate portions of the soil into various classes.
For as instance of example, the combined symbol CH refers to clay of high plasticity level.
| Soil classification system using group symbols is as follows:Group Symbol Classification process. Coarse soil : GWWell-graded GRAVEL soil GPPoorly-graded soil GRAVEL soil GMSilty GRAVEL soil GCClayey GRAVEL soil SWWell-graded SANDsoil SPPoorly-graded SAND soil SMSilty SANDSCClayey SAND soil Fine soils: MLSILT of low plasticity level MISILT of intermediate plasticity level MHSILT of high plasticity level CLCLAY of low plasticity level CICLAY of intermediate plasticity level CHCLAY of high plasticity level OLOrganic soil of low plasticity level OIOrganic soil of intermediate plasticity level OHOrganic soil of high plasticity level PtPeat type |
Activity
“Clayey soils” necessarily do not consist of 100% clay size particles. The proportion of clay mineral flakes (< 0.002 mm size) in a fine soil increases its tendency to swell and shrink with changes in water content. This is called the activity of the clayey soil, and it represents the degree of plasticity related to the clay content.
Activity denoted = (PIasticity index) /(% clay particles by weight)
Classification as per activity is as charted below:
| Activity level | Classification stage |
| < 0.75 | Inactive type |
| 0.75 – 1.25 | Normal type |
| > 1.25 | Active type |
Liquidity Index:
In case of fine soils, especially with clay size content in soil , the existing state is totally dependent on the current water content level (w) with respect to the consistency limits (or Atterberg limits) of soil. The liquidity index (LI) provides very quantitative measure of the present state of soil.
Classification as per liquidity index is charted below:
| Liquidity index stage | Classification type |
| > 1 | Liquid state |
| 0.75 – 1.00 | Very soft state |
| 0.50 – 0.75 | Soft state |
| 0.25 – 0. 50 | Medium stiff state |
| 0 – 0.25 | Stiff state |
| < 0 | Semi-solid state |
Visual Classification of soil:
Soil particle possess several physical characteristics which can be obviously used as to identification in this field. A handy sample of soil rubbed through the fingers can yield the following types:
SAND (and coarser) soil particles are visible to the naked eye condition.
SILT soil particles become dusty when dry and easily brushed off hand.
CLAY particles are generally sticky type when wet and hard when dry, and have to be scraped or washed off by hand.