Soil Formation

Soil Formation

Soil is formed by the process of 'Weathering' of rocks, that is, disintegration and decomposition of rocks and minerals at or near the earth's surface through the actions of natural or mechanical and chemical agents into smaller and smaller grains. The factors of weathering may be atmospheric, such as changes in temperature and pressure; erosion and transportation by wind, water and glaciers; chemical action such as crystal growth, oxidation, hydration, carbonation and leaching by water, especially rainwater, with time. Obviously, soils formed by mechanical weathering (that is, disintegration of rocks by the action of wind, water and glaciers) bear a similarity in certain properties to the minerals in the parent rock, since chemical changes which could destroy their identity do not take place.

It is to be noted that 95% of the earth's crust consists of igneous rocks, and only the remaining 5% consists of sedimentary and metamorphic rocks. However, sedimentary rocks are present on 80% of the earth's surface area. Feldspars are the minerals abundantly present (60%) in igneous rocks. Amphiboles and pyroxenes, quartz and micas come next in that order. Rocks are altered more by the process of chemical weathering than by mechanical weathering. In chemical weathering some minerals disappear partially or fully, and new compounds are formed.

The intensity of weathering depends upon the presence of water and temperature and the dissolved materials in water. Carbonic acid and oxygen are the most effective dissolved materials found in water which cause the weathering of rocks. Chemical weathering has the maximum intensity in humid and tropical climates.

'Leaching' is the process whereby water-soluble parts in the soil such as Calcium Carbonate, are dissolved and washed out from the soil by rainfall or percolating subsurface  water. 'Laterite' soil, in which certain areas of Kerala abound, is formed by leaching. Harder minerals will be more resistant to weathering action, for example, Quartz present in igneous rocks. But, prolonged chemical action may affect even such relatively stable minerals,

resulting in the formation of secondary products of weathering, such as clay minerals-illite, kaolinite and montmorillonite. 'Clay Mineralogy' has grown into a very complicated and broad subject (Ref: 'Clay Mineralogy' by R.E. Grim).

1 Residual soils

To remain at the original place

·        In Hong Kong areas, the top layer of rock is decomposed into residual soils due to the warm climate and abundant rainfall .

·        Engineering properties of residual soils are different with those of transported soils

The knowledge of "classical" geotechnical engineering is mostly based on behavior of transported soils. The understanding of residual soils is insufficient in general

2 Transported soils

To be moved and deposited to other places.

The particle sizes of transported soils are selected by the transportation agents such as streams, wind, etc.

Interstratifications of silts and clays.

The transported soils can be categorized based on the mode of transportation and deposition (six types).

·        (1) Glacial soils: formed by transportation and deposition of glaciers.

·        (2) Alluvial soils: transported by running water and deposited along streams.

·        (3) Lacustrine soils: formed by deposition in quiet lakes (e.g. soils in Taipei basin).

·        (4) Marine soils: formed by deposition in the seas (Hong Kong).

·        (5) Aeolian soils: transported and deposited by the wind (e.g. soils in the loess plateau, China).

·        (6) Colluvial soils: formed by movement of soil from its original place by gravity, such as during landslide (Hong Kong). (from Das, 1998)

1.5            Soil Profile

A deposit of soil material, resulting from one or more of the geological processes described earlier, is subjected to further physical and chemical changes which are brought about by the climate and other factors prevalent subsequently. Vegetation starts to develop and rainfall begins the processes of leaching and eluviations of the surface of the soil material.

Gradually, with the passage of geological time profound changes take place in the character of the soil. These changes bring about the development of 'soil profile'. Thus, the soil profile is a natural succession of zones or strata below the ground surface and represents the alterations in the original soil material which have been brought about by weathering processes. It may extend to different depths at different places and each stratum may have varying thickness.

Generally, three distinct strata or horizons occur in a natural soil-profile; this number may increase to five or more in soils which are very old or in which the weathering processes have been unusually intense. From top to bottom these horizons are designated as the A-horizon, the B-horizon and the C-horizon. The A-horizon is rich in humus and organic plant residue. This is usually eluviated and leached; that is, the ultrafine colloidal material and the soluble mineral salts are washed out of this horizon by percolating water. It is dark in colour and its thickness may range from a few centimeters to half a metre. This horizon often exhibits many undesirable engineering characteristics and is of value only to agricultural soil scientists.

The B-horizon is sometimes referred to as the zone of accumulation. The material which has migrated from the A-horizon by leaching and eluviations gets deposited in this zone. There is a distinct difference of colour between this zone and the dark top soil of the A-horizon. This soil is very much chemically active at the surface and contains unstable fine-grained material. Thus, this is important in highway and airfield construction work and light structures such as single storey residential buildings, in which the foundations are located near the ground surface. The thickness of B-horizon may range from 0.50 to 0.75 m. The material in the C-horizon is in the same physical and chemical state as it was first deposited by water, wind or ice in the geological cycle. The thickness of this horizon may range from a few centimeters to more than 30 m. The upper region of this horizon is often oxidized to a considerable extent. It is from this horizon that the bulk of the material is often borrowed for the construction of large soil structures such as earth dams. Each of these horizons may consist of sub-horizons with distinctive physical and chemical characteristics and may be designated as A1, A2, B1, B2, etc. The transition between horizons and sub-horizons may not be sharp but gradual. At a certain place, one or more horizons may be missing in the soil profile for special reasons.

The morphology or form of a soil is expressed by a complete description of the texture, structure, colour and other characteristics of the various horizons, and by their thicknesses and depths in the soil profile. For these and other details the reader may refer ''Soil Engineering' by M.G. Spangler.