Table of Contents

Soil Structure and Clay Mineralogy

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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.

Single grained structure may be loose or dense as shown below.

  • (a) Loosest state
  • (b) Densest state
Single Grained Structure


Honey-comb 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.
Honey-comb Structure

Flocculated Structure

  • 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.


Flocculated Structure


Dispersed Structure

  • 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.

Dispersed Structure

Composite Structure

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


Clay Mineralogy

  • 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.

Kaolinite Mineral

  • 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

Illite Mineral

  • 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.

Montmorillonite Mineral (Also Called ‘Smectite’)

  • 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.


Diffuse Double Layer and Adsorbed Water

  • 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.

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