Soil Structure and Clay Mineralogy

Soil Structure

Geometric arrangement of soil particles with respect to one another is known as soil structure.
Depending upon the particle size and mode of formation, the following types are found.

Single Grained Structure

Found in coarse grained soils, like gravel, sand.
The major cause for formation is gravitational force. Here the surface forces are negligible.
Under the influence of gravitational forces, the grains will assume a particle to particle contact referred to as single grained structure.

It is possible for fine sands or silts.
Both gravitational force and surface force are responsible.
Such a structure can support loads, only under static conditions.
Under vibrations and shocks, the structure collapses and large deformations take place.

This structure occurs in clays.
Clay particles have a negative charge on surface and a positive charge on edges and flocculated structure occurs when there is an edge-to-face orientation.
A flocculated structure is formed when there is a net attractive force between the particles.
Soils with flocculent structure have a high void ratio and water content and, also have a low compressibility, a high permeability and high shear strength.

A dispersed develops in clays that have been reworked or remolded.
Remoulding converts ‘edge-to-face’ orientation to ‘face- to-face’ orientation.
Dispersed structure is formed when there is a net repulsive force between particles.
Have low shear strength, high compressibility and low permeability.

A composite structure in the form of coarse grained skeleton or clay-matrix is formed when soil contains different types of soil particle

Important clay minerals kaolinite, Illite, montmorillonite and halloysite, are present in clays.
In coarse grained soils, like gravel, sand, rock minerals like quartz, feldspar, mica, etc., are present.

One molecule of kaolinite mineral is made of one silica sheet and one gibbsite sheet.
Various such molecules are joined by hydrogen bonds.
These show less change in volume due to changes in moisture content.
Kaolinite is thus the least active of clay minerals.
Example: China clay

One molecule of Illite is made of two silica sheets and one gibbsite sheet, but in silica sheet, silicon atom is replaced by aluminum atom.
Various such molecules are joined together by ionic bond (potassium ion).
These shows medium swelling and shrinkage properties.
Example: Alluvial soil.

One molecule of montmorrilonite mineral is made of two silica sheets and one gibbsite sheet.
Gibbsite sheet is sandwiched between silica sheets.
Various such molecules are loosely bonded through water.
These soils show high volume changes on moisture variation (i.e., large swelling and large shrinkage).
Example: Black cotton soils, bentonite soil.

Clay particles usually carry a negative charge on their surface.
Because of net negative charge on the surface, the clay particles attract cations, such as potassium, calcium and sodium, from the moisture present in the soil to reach equilibrium.
The layer extending from the clay particle surface to the limit of attraction is known as a diffuse double layer.
The water held in the zone of the diffuse double layer is known as adsorbed water or oriented water.
The plasticity characteristics of clay are due to the presence of adsorbed water.
Clays using non-polar liquid, such as kerosene in place of water, does not show any plasticity characteristics.
The thickness of adsorbed water layer is about 10–15 A°for colloids, but may be up to 200 A° for silts.