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 weatheing, such as clay minerals— illite, kaolinite and montmorillonite. ‘Clay Mineralogy’ has grown into a very complicated and broad subject.
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 eluviation 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 centimetres 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 eluviation 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 centimetres to more than 30 m. The upper region of this horizon is often oxidised 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. A typical soil profile is shown below
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.
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; and (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 in South India, 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:
1. Coarse-grained soils, with average grain-size greater than 0.075 mm, e.g., gravels and sands.
2. 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.
Further classification according to grain-size and other properties is given in later chapters.
The following are some commonly used soil designations, their definitions and basic properties:
Bentonite-Decomposed volcanic ash containing a high percentage of clay mineral—
montmorillonite. It exhibits high degree of shrinkage and swelling.
Black cotton soil- Black soil containing a high percentage of montmorillonite and colloidal
material; exhibits high degree of shrinkage and swelling. The name is derived from the
fact that cotton grows well in the black soil.
Boulder clay.-Glacial clay containing all sizes of rock fragments from boulders down to
finely pulverised clay materials. It is also known as ‘Glacial till’.
Caliche- Soil conglomerate of gravel, sand and clay cemented by calcium carbonate.
Hard pan- Densely cemented soil which remains hard when wet. Boulder clays or glacial
tills may also be called hard-pan— very difficult to penetrate or excavate.
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.
Loam.-Mixture of sand, silt and clay size particles approximately in equal proportions;
sometimes contains organic matter.
Loess- Uniform wind-blown yellowish brown silt or silty clay; exhibits cohesion in the
dry condition, which is lost on wetting. Near vertical cuts can be made in the dry condition.
Marl-Mixtures of clacareous sands or clays or loam; clay content not more than 75%
and lime content not less than 15%.
Moorum- Gravel mixed with red clay.
Top-soil- Surface material which supports plant life.
Varved clay- Clay and silt of glacial origin, essentially a lacustrine deposit; varve is a term of Swedish origin meaning thin layer. Thicker silt varves of summer alternate with thinner
clay varves of winter.
Structure Of Soils
The ‘structure’ of a soil may be defined as the manner of arrangement and state of aggregation
of soil grains. In a broader sense, consideration of mineralogical composition, electrical properties,
orientation and shape of soil grains, nature and properties of soil water and the interaction
of soil water and soil grains, also may be included in the study of soil structure, which is
typical for transported or sediments soils. Structural composition of sedimented soils influences,
many of their important engineering properties such as permeability, compressibility
and shear strength. Hence, a study of the structure of soils is important.
The following types of structure are commonly studied:
(a) Single-grained structure
(b) Honey-comb structure
(c) Flocculent structure
Single-grained structure is characteristic of coarsegrained soils, with a particle size greater than 0.02
mm. Gravitational forces predominate the surface forces and hence grain to grain contact results. The
deposition may occur in a loose state, with large voids or in a sense state, with less of voids.
This structure can occur only in fine-grained soils, especially in silt and rock flour. Due to the relatively smaller size of grains, besides gravitational forces, inter-particle surface forces also play an important role in the process of settling down. Miniature arches are formed, which bridge over relatively large void spaces. This results in the formation of a honey-comb structure, each cell of a honey-comb being made up of numerous individual soil grains. The structure has a large void
space and may carry high loads without a significant volume change. The structure can be broken down by external disturbances.
This structure is characteristic of fine-grained soils such as clays. Inter-particle forces play a predominant role in the deposition. Mutual repulsion of the particles may be eliminated by means of an appropriate chemical; this will result in grains coming closer together to form a ‘floc’. Formation of flocs is ‘flocculation’. But the flocs tend to settle in a honeycomb structure, in which in place of each grain, a floc occurs. Thus, grains grouping around void spaces larger than the grain-size are flocs and flocs grouping around void spaces larger than even the flocs result in the formation of a ‘flocculent’ structure. Very fine particles or particles of colloidal size (< 0.001 mm) may be in a flocculated or dispersed state. The flaky particles are oriented edge-to-edge or edge-to-face with respect to one another in the case of a flocculated structure. Flaky particles of
clay minerals tend to from a card house structure (Lambe, 1953), when flocculated. This is shown in below fig, When inter-particle repulsive forces are brought back into play either by remoulding or by
the transportation process, a more parallel arrangement or reorientation of the particles occurs, This means more face-to-face contacts occur for the flaky particles when these are in a dispersed state. In practice, mixed structures occurs especially in typical marine soils.