Principle Soil Types

In Civil Engineering, soil is defined as an unconsolidated agglomerate of minerals with or without organic matter and found at or near the surface of earth crust, with which and upon which Civil Engineering structures are built. 

Compared to rocks, soils are easy to excavate and generally disintegrate when agitated in water. Soil mass is a particulate material consisting of  solid particles with voids filled with air or water or both.
Soils can be classified in various categories based upon their method of formation and and minerals they contain. Principle types of soil are briefly described below:
Loam is a mixture of sand, silt and clay with or without organic matter. according to the properties of their ingredients, there are sandy loams, silty loams, clayey loams. Loams are good for plant growth.
It is a fine grained rock flour of light grey to pink colour as found in borrow pits. In rivers especially close to deltas, there are deep deposits of organic silt sometimes 30 meters or more thick, they often have unpleasant smells due to the presence of methane gas in these organic silt deposits. Organic silt is dark in river and light grey when dry. Organic silt is plastic whereas inorganic silts are generally non-plastic.

Mud is a shining and pasty mixture of soils and organic materials.
It is cemented clay, sand and gravel mixture. The cementing material in caliche is calcium carbonate, desposited through evaporation.

Marl is a clay containing calcareous material and they are more homogeneous than caliache.
Hardpan is a highly cemented compressed mixture of sand, clay, gravel and boulders. They are generally found on top of the rock ledge.

Peat is a partly decayed plant and/or animal remains. It has fibrous texture. A totally decayed plant material is called Muck. These soils are also known as organic soils.
Fine grained (particle size < 0.002 mm), cohesive plastic soils are called clays. Many local varieties of clay are found. Some of these types are described below:
  • Boulder Clays: Calcareous clay containing wide range of particle sizes varying from boulders (particle size > 300 mm) to very fine rock flour (particle size<0.002 mm).
  • Varved Clays: Clays with alternating thin partings of very fine sand and/or silt.
  • Bentonite: Clays with main constituent mineral of  montmorillonite and they are formed due to chemical weathering of volcanic ashes.
It is a fine grained weak sedimentary rock, composed primarily of hardened clay in very thin layers and separates easily when exposed to air. It swells by absorbing considerable amount of water when come in contact with it. Thus it is a fissile rock with laminated structure formed by consolidation of rock. It is not considered as a good foundation soil.
Black Cotton Soil
Black cotton soils are very fertile soils. They are highly compressible clays of dark to black colour. They are not good as road or construction foundation. Black cotton soils are expansive clays with high potential for shrinking or swelling as a result of changing moisture content. Due to intensive shrink-swell processes, surface cracks resulting in openings during dry seasons.

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