Explain effect of compaction on various properties of soils.

The the effect of compaction on various soil properties can be discussed below:

• Soil structures 
• Permeability 
• Swelling 
• Pore water pressure 
• Shrinkage
• Compressibility 
• Stress-strain relationship 
• Shear strength 

The water content at which the soil is compacted plays an important role in the engineering properties of the soil. Soil compacted at a water content less than the optimum water content generally have a flocculated structure regardless of the method of compaction. Soils compacted at a water content more than the optimum water content usually have a dispersion structure if the compaction induces large strain and the flocculated structure if the shear strains are relatively small.

The the effect of compaction on various soil properties can be discussed below:

• Soil structures 
• Permeability 
• Swelling 
• Pore water pressure 
• Shrinkage
• Compressibility 
• Stress-strain relationship 
• Shear strength 

Soil structures:

The water content at which the soil is compacted plays an important role in the engineering properties of the soil. Soil compacted at a water content less than the optimum water content generally have a flocculated structure regardless of the method of compaction. Soils compacted at a water content more than the optimum water content usually have a dispersion structure if the compaction induces large strain and the flocculated structure if the shear strains are relatively small.

In  figure at point A on the dry side of the optimum, the water content is so low that the attractive forces are more predominant than the repulsive forces. This result in flocculated structure. As the water content is increased beyond the optimum, the repulsive forces increases and the particles get oriented into the dispersed structure. If the compactive effort is increased, there is a corresponding increase in the orientation of the particles and higher dry densities are obtained.

Permeability:

The permeability of a soil depends upon the size of voids. The permeability of soil decreases with increase in water content on the dry side of the the optimum water content. The minimum permeability occurs at or slightly above the optimum water content. After that stage, the permeability slightly increases, but it always remains much less than that on the dry side of the optimum. If the compactive effort is increased, the permeability of the soil decreases due to increased dry density and better orientation of particles. 

Swelling:

A soil sample compacted dry of optimum water content has high water deficiency and more random orientation and hence exert greater swelling pressure and swell to higher water content than the sample of the same density of obtained from wet side compaction.

Pore water pressure:

A soil sample compacted dry of the optimum has low water content. Thus, the pore water pressure developed for the soil compacted dry of the optimum is therefore less than that of the sample compacted wet of the optimum.

Shrinkage:

For the same density, soil sample compacted dry of optimum shrink appreciable less than the sample compacted wet of optimum. This is so because the soil particles having dispersed structure have nearly parallel orientation and can pack more efficiently.

Compressibility:

Saturated sample of clay, compacted wet side of optimum is more compressible than another sample of the same soil, having the same void ratio, but compacted dry of the optimum, when the applied pressure is in low pressure range. This is so because the sample compacted dry of optimum has flocculated structure and requires extra pressure to cause parallel orientation of particles. However, in the high pressure range, a sample compactor dry of optimum is more compressible than the one compacted wet of the optimum.

Stress-Strain relationship:

For a given soil, a sample compacted dry side of the optimum has a steeper stress-strain curve and hence, has a higher modulus of elasticity than the one which is compacted wet of optimum ,at the same density. Soil compacted dry of optimum have brittle failure while soil compacted wet of optimum, having dispersed structure, continue to increase in strength even at higher strains.

Shear strength:

The strength of compacted clay depends upon:

• dry density
• moulding water content
• soil structure
• method of compaction
• strain used to define strength
• drainage condition and
• the type of soil 

In general, at low strains, strength of cohesive soil compacted dry of optimum is higher than those compacted wet of optimum. Figure below shows the failure envelope of two samples of the same soil, one  compacted dry of optimum and other compacted wet of the optimum but both compacted at the same density. However, of higher strains, the flocculated structure of the same compacted on the dry side is broken giving rise to ultimate strength for both the samples. The manner of compaction also influences the strength of soil sample compacted wet of the optimum. It is interesting to note that the clay cores in earth dams are usually compacted wet of optimum to tolerate largest settlements without cracking.