Structure of Soil

The structure of a soil may be defined as the manner of arrangement and state of aggregation of soil grains. In a broader sense, consideration of mineralogical composition, electrical properties, orientation and shape of soil grains, nature and properties of soil water and the interaction of soil water and soil grains, also may be included in the study of soil structure, which is typical for transported or sediments soil. The engineering behaviour of soil is influenced by soil structure to varying degrees. Structural composition of sediment soil influences, many of their important engineering properties such as permeability, compressibility and shear strength. Hence, a study of the structure of soil is important. The following types of structure are commonly considered.

1) Single Grained Structure

Single grained structure is characteristic of coarse grained soils, with a particle size greater than 0.02 mm. Gravitational forces predominate the surface forces and hence grain to grain contact results. The deposition may occur in a loose state, with large voids or in a sense state, with less of voids. 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 the 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.

Fig. 1 Single-grained Structure

2) Honeycomb Structure

This structure can occur only in fine-grained soils, especially in silt and rock flour. Due to the relatively smaller size of grains, besides gravitational forces, inter-particle surface forces also play an important role in the process of settling down. When particles approach the lower region of suspension they will be attracted by particles already deposited as well as the neighbouring particles leading to formation of arches. Miniature arches are formed, which bridge over relatively large void spaces. This results in the formation of a honey-comb structure, each cell of a honey-comb being made up of numerous individual soil grains. The structure has a large void space and may carry high loads without a significant volume change. The structure can be broken down by external disturbances.

Fig. 2 Honey-comb Structure

3) Flocculent Structure

This structure is characteristic of fine-grained soils such as clays. In the case of flocculated structure, there will be edge-to-edge and edge-to-face contact between particles. 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. Mutual repulsion of the particles may be eliminated by means of an appropriate chemical; this will result in grains coming closer together to form a ‘floc’. Formation of flocs is ‘flocculation’. But the flocs tend to settle in a honeycomb structure, in which in place of each grain, a floc occurs. Thus, grains grouping around void spaces larger than the grain-size are flocs and flocs grouping around void spaces larger than even the flocs result in the formation of a ‘flocculent’ structure.

Fig. 3 Flocculent Structure

Very fine particles or particles of colloidal size (< 0.001 mm) may be in a flocculated or dispersed state. This type of structure is common in fresh water deposits. In the case of dispersed or oriented structure, the particles will have face to face contact. This type for formation is due to net electrical forces between adjacent soil particles at the time of deposition being repulsive in nature. The flaky particles are oriented edge-to-edge or edge-to-face with respect to one another in the case of a flocculated structure. Flaky particles of clay minerals tend to from a card house structure, when flocculated. When inter-particle repulsive forces are brought back into play either by remoulding or by the transportation process, a more parallel arrangement or reorientation of the particles occurs. This means more face-to-face contacts occur for the flaky particles when these are in a dispersed state. In practice, mixed structures occur, especially in typical marine soils.

Fig. 4 Card House Structure

Fig. 5 Dispersed Structure