General Types of Soils

Soils which are formed by weathering of rocks may remain in position at the place of region. In that case these are ‘Residual Soils’. These may get transported from the place of origin by various agencies such as wind, water, ice, gravity, etc. In this case these are termed ‘‘Transported soil’’.

1) Residual Soil

Residual soils differ very much from transported soils in their characteristics and engineering behaviour. They are relatively shallow in depth. They are characterized by a gradual transition from soil through partially weathered rocks, fractured and fissured rock, to bedrock. The degree of disintegration may vary greatly throughout a residual soil mass and hence, only a gradual transition into rock is to be expected. An important characteristic of these soils is that the sizes of grains are not definite because of the partially disintegrated condition. The grains may break into smaller grains with the application of a little pressure. Generally, the depth of residual soils varies from 5 to 20 m. Residual soils have not received much attention from geotechnical engineers because these are located primarily in undeveloped areas. In some zones in South India, sedimentary soil deposits range from 8 to 15 m in thickness.

The residual soil profile may be divided into three zones: (i) the upper zone in which there is a high degree of weathering and removal of material; (ii) the intermediate zone in which there is some degree of weathering in the top portion and some deposition in the bottom portion; and (iii) the partially weathered zone where there is the transition from the weathered material to the un-weathered parent rock. Residual soils tend to be more abundant in humid and warm zones where conditions are favourable to chemical weathering of rocks and have sufficient vegetation to keep the products of weathering from being easily transported as sediments. The depth of residual soils depends primarily on climatic conditions and the time of exposure. In some areas, this depth might be considerable. In temperate zones residual soils are commonly stiff and stable. An important characteristic of residual soil is that the sizes of grains are indefinite. For example, when a residual sample is sieved, the amount passing any given sieve size depends greatly on the time and energy expended in shaking, because of the partially disintegrated condition.

2) Transported Soils

Transported soils are soils that are found at locations far removed from their place of formation. The transporting agencies of such soils are glaciers, wind and water. Transported soils may also be referred to as ‘Sedimentary’ soils since the sediments, formed by weathering of rocks, will be transported by agencies such as wind and water to places far away from the place of origin and get deposited when favourable conditions like a decrease of velocity occur. A high degree of alteration of particle shape, size and texture as also sorting of the grains occurs during transportation and deposition. A large range of grain sizes and a high degree of smoothness and fineness of individual grains are the typical characteristics of such soils. Transported soils may be further subdivided, depending upon the transporting agency and the place of deposition, as under:

1) Alluvial Soil

Soils transported by rivers and streams. Alluvial soils are the soils which have been transported and subsequently deposited by flowing water. An alluvial fan is formed when the velocity of a soil laden stream suddenly deceases due to abrupt decrease in gradient. Floodplains are formed on the sides of a stream due to overflowing of flood water. A delta is formed just before a stream reaches the standing water of the sea. Alluvial soil deposits are usually stratified because of fluctuations in velocity of flowing water. The average particle size of alluvial deposits decreases with increasing distance from the source of stream. The delta soils are soil deposits farthest from the source of a stream and usually consist of silt and clay.

Example: Sedimentary clays

2) Aeolian Soil

Soils transported by wind.

Example: loess

3) Glacial Soil

Soils transported by glaciers.

Example: Glacial till

4) Lacustrine Soil

Soils deposited in lake beds.

Example: Lacustrine silts and lacustrine clays

5) Marine Soil

Soils deposited in sea beds.

Example: Marine silts and marine clays

General Types of Soils

The individual size of the constituent parts of even the weathered rock might range from the smallest state (colloidal) to the largest possible size (boulders). This implies that all the weathered constituents of a parent rock cannot be termed soil. According to their grain size, soil particles are classified as cobbles, gravel, sand, silt and clay. Grains having diameters in the range of 4.75 to 76.2 mm are called gravel. If the grains are visible to the naked eye, but are less than about 4.75 mm in size the soil is described as sand. The lower limit of visibility of grains for the naked eyes is about 0.075 mm. Soil grains ranging from 0.075 to 0.002 mm are termed as silt and those that are finer than 0.002 mm as clay. This classification is purely based on size which does not indicate the properties of fine grained materials.

Commonly Used Soil Designations

The following are some commonly used soil designations, their definitions and basic properties.

1) Bentonite

Decomposed volcanic ash containing a high percentage of clay mineral montmorillonite. It exhibits the properties of clay to an extreme degree such as high degree of shrinkage and swelling.

2) Black Cotton Soil

Black soil containing a high percentage of montmorillonite and colloidal material and it exhibits high degree of shrinkage and swelling. The name is derived from the fact that cotton grows well in the black soil.

3) Boulder Clay

Glacial clay containing all sizes of rock fragments from boulders down to finely pulverized clay materials. It is also known as ‘Glacial till’.

4) Caliche

Soil conglomerate of gravel, sand and clay cemented by calcium carbonate.

5) Hard Pan

Densely cemented soil which remains hard when wet. Boulder clays or glacial tills may also be called hard-pan and it is very difficult to penetrate or excavate.

6) Laterite

Deep brown soil of cellular structure, easy to excavate but gets hardened on exposure to air owing to the formation of hydrated iron oxides.

7) Loam

Mixture of sand, silt and clay size particles approximately in equal proportions; sometimes contains organic matter.

8) Loess

Uniform wind-blown yellowish brown silt or silty clay; exhibits cohesion in the dry condition, which is lost on wetting. Loess is a fine-grained, air-borne deposit characterized by a very uniform grain size, and high void ratio. The size of particles ranges between about 0.01 to 0.05 mm. The soil can stand deep vertical cuts because of slight cementation between them. particles It is formed in dry continental regions.

9) Marl

Mixtures of calcareous sands or clays or loam; clay content not more than 75% and lime content not less than 15%.

10) Moorum

Gravel mixed with red clay.

11) Top-soil

Surface material which supports plant life.

12) Varved Clay

Consist of alternating layers of clay and silt of glacial origin, essentially a lacustrine deposit; varve is a term of Swedish origin meaning thin layer. They possess the undesirable properties of both silt and clay.

13) Kaolin, China Clay

They are very pure forms of white clay used in the ceramic industry.

14) Calcareous Soil

It is a soil containing calcium carbonate.

15) Peat

It is a fibrous aggregate of finer fragments of decayed vegetable matter. Peat is very compressible and one should be cautious when using it for supporting foundations of structures.

16) Shale

It is a material in the state of transition from clay to slate. Shale itself is sometimes considered a rock but, when it is exposed to the air or has a chance to take in water it may rapidly decompose.

Organic and Inorganic Soils

Soils in general are further classified as organic or inorganic. Soils of organic origin are chiefly formed either by growth and subsequent decay of plants such as peat or by the accumulation of fragments of the inorganic skeletons or shells of organisms. Hence a soil of organic origin can be either organic or inorganic. The term organic soil ordinarily refers to a transported soil consisting of the products of rock weathering with a more or less conspicuous admixture of decayed vegetable matter. Organic soil contains carbon-based material that is living or was once living. Soil contains many different things that have been deposited over time. Many places around the world do not have adequate soil or soil that needs amendments to become organic and rich. Organic soil also benefits the environment. Non-organic soil media consists of materials that have been manufactured and are free of nutrients and contaminants.