The structure of soil is formed by the geometrical arrangement of soil particles with respect to each other. Different soils contain different structures and each structure gives us the properties of particular soil.
Types of Soil Structures
There are six different types of soil structures and they are as follows.
- Single grained structure
- Honey comb structure
- Flocculated structure
- Dispersed structure
- Coarse grained skeleton
- Clay matrix structure
1. Single Grained Soil Structure
Single grained structures are present in cohesion less soils like gravel and sand. The grains of cohesion less soils have less surface force and more gravitational force. So, when we pour some amount of sand or gravel on the ground, the grains will settle using gravitational force rather than surface force.
After attaining the final position, each grain is in contact with the surrounding grains and formed structure is called as single grained structure.
The single grained structure soils are deposited under gravitational forces but here the position of particles after settling also matters. Let us assume that all the particles are spherical in nature then maximum void ratio (e= 0.90) is occurred when they positioned in cubical array manner which is as shown in figure and is called as loosest state of soil.
Similarly, Minimum void ratio (e=0.35) will occur if they positioned in diagonal packing manner as shown in figure which is called densest state.
In general, the particles are not spherical in nature, but we can say that the void ratio of single grained particles is in between 0.35 to 0.90.
To use soil for the civil engineering purpose, the void ratio of the soil should be minimum or in other words dense soil is preferable. Loose soil is very unstable and cannot withstand against vibration and shocks.
2. Honeycomb Soil Structure
Honeycomb structure found in soil contains particle of size 0.02mm to 0.002mm which are generally fine sands or silts. When this type of soils is allowed to settle on the ground, the particles will attract each other and joins one with another and forms a bridge of particles. A large void is also formed between those bridges which makes the soil very loose in nature.
The attraction of particles is due to cohesion between them, but this cohesion is just because of their size but however, these soils are not plastic in nature. In fine sands, when water is added to dry fine sand bulking of sand occurs which is nothing but a structure of honeycomb.
Honeycomb structured soil is limited for static load condition. They cannot resist vibrations and shocks under building and may cause large deformations to the structure.
3. Flocculated Soil Structure
Flocculated structure is present in clay particles which contains larger surface area. These are charged particles which have positive charge on the edges and negative charge on the face of the particle. When there is net attractive force between the particles, then positive charged particles attracted towards negatively charged faces which results the formation of flocculated structure.
Clay present in the marine area is the best example for flocculated structure. Salt present in the marine water acts as electrolyte and reduces the repulsive force between the particles which leads to the formation of flocculent structure. This type of soils has high shear strength.
Because of edge to face orientation void ratio is high in this type soil and water content also optimum but they are light in weight. The compressibility is very low for this type of soils.
4. Dispersed Soil Structure
Dispersed structure also occurs in clay particles when the clay is remolded. Remolding reduces the shear strength of soil which reduces the net attractive forces between the particles. Hence, due to repulsion between them, the edge to face orientation turns into face to face orientation. Finally, dispersed structure of clay will form.
This type of soil is highly compressible and less permeable. The loss of strength during remolding is slowly achieved by the soil with the time. The process of regaining its strength after remolding is called thixotropy.
5. Coarse Grained Skeleton Soil Structure
Coarse grained skeleton, is a structure of soil which is present in composite soils containing both fine and coarse-grained particles. But, it is formed when there is large amount of coarse grained particles than the fine-grained particles.
The coarse-grained particle forms a skeleton like structure and voids between them are filled by fine grained or clayey particles. If it is undisturbed, it will give good results against heavy loads. If disturbed, the strength extensively reduced.
6. Clay Matrix Soil Structure
Clay matrix structure, which also occurs in composite soils, but the amount of clay particles or fine-grained particle is more compared to coarse grained particles. Coarse particles in this soil are separated with each other as they are less in quantity. This type of soil is very stable in nature and it has same properties of an ordinary clay deposit.