The soils we see in the nature can be deposited having different structures depending upon their constituting particles size and their mode of formation.
Soil structure is the geometrical arrangement of the solid parts of the soil, which are soil particles, with respect to one another. Soil structure significantly influences the movement of air and water in the soil. And we know that the amount and movement of water inside the soil affects the engineering properties of soil.
Therefore, it is important for us to understand the structure of soil deposits. Let’s discuss some of them.
Single Grained Structure:
This kind of structure is found in coarse grained soils. Grains, which are cohesionless and make up soils like gravel and sand, form this kind of structure.
These grains are large enough and gravity is the dominant force acting on them. When these particles get deposited they acquire an equilibrium position by each particle being in contact with other surrounding particles. Soil structure so formed is called single grained structure.
The arrangement of these soil particles in their deposition gives the soil loose or dense structure.
Closely the particles are packed together leaving very less void space between them, denser is the structure. As volume of the voids is less, lower will be its void ratio and lower will be its permeability. Dense soils have higher shear strength and low compressibility which makes them more preferable for engineering purposes (for construction).
While soil particles which are loosely packed, leaving large void spaces between them, make up loose soil structure. Loose soil structure has large volume of voids, hence they have high void ratio and high permeability but they are unstable. When loose soils subjected to vibrations their particles move and acquire a more stable position and form a denser state.
Honey comb structure.
Sometimes smaller soil particles of silt sized when depositing join with one another and form a bridge like structure.They contain large voids between those bridges and make the soil very loose in nature. Such a formed structure is called honey comb structure.
As soil in honey-comb structure is loose they can support load only under static condition. When they are subjected to vibration or shock the structure collapses, large deformation takes place and soil achieves relatively denser state.
Clay particles are very small, flaky in shape and they have large surface area and because of that their surface forces are more prominent in them than gravitational force. These particles have negative charge on the surface and positive charge on the edges. They combine each other by joining negative surface of the particle to the positive edge of the other. This way they form a soil structure which is called flocculated structure.
We can see soils with flocculated structure have large amount of voids, so their void ratio is high. These soils are less sensitive to vibrations as they formed a strong electrical bond.
When clay soils are remoulded, their flocculated structure changes and particles change their orientation from edge-to-face orientation to face-to-face orientation. This kind of formed soil structure is called dispersed soil structure.
These soils have relatively lower volume of voids consequently low void ratio.
When both coarse-grained and fine-grained particles are present in the soil they make up two kinds of soil structure.
Coarse grained skeleton
and Cohesive Matrix Structure
In coarse-grained skeleton, coarse-grained particles are in large amount than fine-grained particles and coarse particles remain in direct contact with other coarse particles forming a framework or a skeleton.
The space between these large grains is occupied by the fine particles.
These soils are stable and less compressible. They can take heavy loads without much deformation.
Cohesive matrix Structure:
On the other hand in cohesive matrix structure fine particles, that are predominantly clay particles, are in very huge quantity than coarse-grained particles. Coarse grains appear embedded in fine grains and there is no direct particle to particle contact between them.
Their behaviour is similar to that of an ordinary clay deposit.