SOIL STRUCTURE / SOIL FABRIC

The term soil structure in general, refers to the arrangement or state of aggregation of particles in a soil mass. But deeper understanding of soil structure demands consideration of mineralogical 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 influenced by soil structure to varying degrees.

Types of Soil Structure or Soil Fabric

Following are the types of soil structure which have been recognized in various soil deposits

1. Single grained structure – in case of coarse grained soil deposits (sand)
2. Honeycomb structure – in the case of silt deposits
3. Flocculated structure – in the case of clay deposits
4. Dispersed structure – in the case of clay deposits
5. Coarse grained skeleton structure – in the case of composite soils
6. Cohesive matrix structure – in the case of composite soils

1. Single Grained Structure

This type of structure will be found in the case of coarse grained soil deposits (see Fig-1). When such soils settle out of suspension in water, the particles settle independently of each other. The major force causing their deposition is gravitational and surface forces are too small to produce any effect. There will be particle-to-particle contact in the deposit. The void ratio attained depends on the relative size of grains.

2. Honeycomb Structure

This type of structure is associated with silt deposits. When silt particles settle out of suspension, in addition to gravitational forces the surface forces also play a significant role. When particles approach the lower region of suspension they will be attracted by particles deposited as well as the neighboring particles leading to the formation of arches. The combination of a number of arches leads to the honeycomb structure as shown in Fig-2. As the deposit has high void ratio, when disturbed as in pile driving, there will be large reduction in volume due to breakdown of structure.

3. Flocculated Structure

These are the two types of structures found in clay deposits. In the case of flocculated structure there will be edge to edge and edge to face contact between the particles (see Fig-3). This type of formation is due to the net electrical forces between the adjacent particles at the time of deposition being attractive in nature. The concentration of dissolved minerals in water leads to formation of flocculated structure with very high void ratio as in the case of marine deposits.

4. Dispersed Structure

In the case of dispersed or oriented structure, the particles will have face to face contact (see Fig-4). This type of formation is due to net electrical forces between adjacent soil particles at the time of deposition being repulsive in nature. This type of structure is common in fresh water deposits.

Clay with flocculated structure will have relatively high void ratio. Remoulding of such soils or application of pressure as in compaction leads to slippage of particles resulting in dispersed structure with decrease in void ratio. Consolidation also tends to reorient the particles to form dispersed structure with decrease in volume.

5. Coarse Grained Skeleton Structure

The coarse grained skeleton structure can be found in the case of composite soils in which the coarse grained fraction is greater in proportion compared to fine grained fraction. The coarse grained particles forms the skeleton with particle to particle contact and the voids between the particles will be occupied by the fine grained particles.

6. Cohesive Matrix Structure

The cohesive matrix structure can be found in composite soils in which the fine-grained fraction is more in proportion compared to coarse grained fraction. In this case the coarse grained particles will be embedded in fine grained fraction and will be prevented from having particle to particle contact. This type of structure is relatively more compressible compared to the more stable coarse grained skeleton structure.