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
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.
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.
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.
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
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.