Soil Structure & Clay Mineralogy

Soil Structure: 

• It is a geometrical arrangement of soil particles in a soil mass.
• The behavior of soil depends on the soil structure.

Important types of structures: Depending upon the particle size and mode of formation the following are important types:

1. Single grained
2. Honey-comb
3. Flocculated
4. Dispersed
5. Composite Structure’

Single grained Structures: It is present in soils like gravel and sand.If particles are assumed as spheres, the loosest and densest packaging are shown in the figures. 

The void ratio for the loosest state is 0.91, and the void ratio for the densest state is 0.35. when the particles are assumed as perfect spheres.

Honey — Comb Structure:

•  Present in fine sands or SlitS
• under vibrations and shocks, the structure collapses and large deformation takes place.

Flocculated Soil Structure:

• It occurs in clay.
• Formed when there is a net attractive force between particles.
• Edge to face orientation.
• Has high shear strength, low compressible & high permeability.

Dispersed Soil Structure:

• Occurs in remolded clays.
• Face to face orientation
• Formed when there is net repulsive force between particles
• Has low shear strength, high compressibility and low permeability.

>>When soil contains different type of particles, a ‘Composite Structure’ is formed.
Remoulding: Remoulding causes a loss of strength in cohesive soils.

Thixotropy: The phenomenon of regaining of lost strength with the passage of time, with no change in water content, is known as ‘thixotropy’.

Clay Mineralogy:

• In Gravel and sands, rock minerals like quartz, feldspar, Mica etc. are present.
• In Clays, clay minerals are present.
• Important clay minerals are Kaolinite, Illite, Montmorillonite, Halloysite etc.

Kaolinite:

• Stable mineral
• Causes no swelling and no shrinkage
• Present predominantly in china clay.

Illite Mineral:

• exhibits medium swelling and shrinkage

Montmorillonite:

• exhibits large swelling and large shrinkage
• highest percentage is present in Bentonite followed by Black cotton soils.

Halloysite: Behaviour is similar to kaolinite

Specific Surface Area:

1. It is the total surface area of soil particles per unit weight or per unit volume of soil.
2. Finer the soil, higher the specific surface area.
3. Montmorillonite clay mineral has largest specific surface area (about 800m2/gm)