Residual and Transported 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’’. Residual soils differ very much from transported soils in their characteristics and engineering behaviour. 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.

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;
(iii) the partially weathered zone where there is the transition from the weathered material to the unweathered 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. Residual soils have not received much attention from geotechnical engineers because these are located primarily in undeveloped areas. In some zones, sedimentary soil deposits range from 8 to 15 m in thickness.

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: 

• Alluvial soils: Soils transported by rivers and streams: Sedimentary clays. 
• Aeoline soils: Soils transported by wind: loess.
• Glacial soils: Soils transported by glaciers: Glacial till.
• Lacustrine soils: Soils deposited in lake beds: Lacustrine silts and lacustrine clays.
• Marine soils: Soils deposited in sea beds: Marine silts and marine clays.

Broad classification of soils may be: 

• Coarse-grained soils, with average grain-size greater than 0.075 mm, e.g., gravels and sands. 
• Fine-grained soils, with average grain-size less than 0.075 mm, e.g., silts and clays.

These exhibit different properties and behaviour but certain general conclusions are possible even with this categorisation. For example, fine-grained soils exhibit the property of ‘cohesion’—bonding caused by inter-molecular attraction while coarse-grained soils do not; thus, the former may be said to be cohesive and the latter non-cohesive or cohesionless.