In general, the term soil structure refers to the arrangement or condition of aggregation of particles in the soil mass. But a deeper understanding of soil structure requires consideration of the mineral composition, shape and orientation of soil particles; The nature and properties of soil water, and the forces of interaction between soil particles and soil water. The engineering behavior of soils is affected to varying degrees by soil composition.
Following are the types of soil structure which have been identified in different soil deposits:
- Single grained structure – in case of coarse grained soil deposits (sands)
- Hive structure – in case of siltation
- Flocculated structure – in case of soil deposition
- Scattered structure – in case of soil deposits
- Coarse grained skeletal structure – in case of mixed soils
- Connective matrix composition – in case of mixed soils
1. single granular structure
This type of structure will be found in the case of coarse grained soil deposits (see Fig.-1). When such clay comes out of suspension in water, the particles settle independently of each other. The main reason for their deposition is gravitational force and surface forces are too small to produce any effect. The deposit will have particle-particle contact. The obtained void ratio depends on the relative size of the grains.
2. honeycomb structure
This type of structure is associated with silt deposits. When silt particles come out of suspension, in addition to the gravitational force, surface forces also play an important role. When the particles reach the lower region of the suspension they are attracted to the deposited particles as well as the surrounding particles which leads to the formation of arches. The combination of several arches forms a honeycomb structure as shown in Figure-2. Since the deposit has a high void ratio, when the pile is disturbed such as driving, there will be a large reduction in volume due to the breakdown of the structure.
3. flocculated structure
These are two types of formations found in clay deposits. In the case of flowing structure will be edge to edge And face to face Contact between particles (see fig-3). This type of formation is due to the attractive in nature of the net electric forces between adjacent particles at the time of deposition. The concentration of dissolved minerals in water creates a flocculated structure with a very high void ratio as is the case with marine deposits.
4. scattered structure
In the case of a scattered or oriented structure, the particles will have a face-to-face interaction (Fig. 4). This type of formation is due to the net electric forces being repulsive in nature between the surrounding soil particles at the time of deposition. This type of structure is common in freshwater deposits.
Soils with a clump structure will have a relatively high void ratio. The application of pressure, such as soil remolding or compaction, results in slippage of the particles resulting in a dispersed structure with a reduction in the void ratio. Consolidation also re-orientates the particles to form a dispersed structure with a decrease in volume.
5. coarse-grained skeletal structure
The coarse-grained skeletal structure can be found in the case of mixed soils in which the coarse-grained fraction is in greater proportion than the fine-grained fraction. The coarse-grained particles form the skeleton with particle-to-particle contact and the spaces between the particles will be occupied by the fine-grained particles.
6. cohesive matrix structure
Cohesive matrix composition can be found in mixed soils in which the fine-grained fraction is proportionately higher than the coarse-grained fraction. In this case coarse-grained particles will be embedded in the fine-grained fraction and particle-to-particle contact will be prevented. This type of structure is relatively more compact than the more stable coarse-grained skeletal structure.