Factors affecting compaction of soil


The various factors which affect the compacted density are as follows 
  1. water content, 
  2. amount and type of compaction, 
  3. method of compaction 
  4. type of soil, and 
  5. addition of admixtures. 

1. Water content 

It has been seen by laboratory experiments that as the water content is increased, the compacted density goes on increasing, till a maximum dry density is achieved after which further addition of water decreases the density. When only a relatively small amount of water is present in soil, it is firmly held by the electrical relay to the surface of soil particles with a high concentration of electrolyte which prevents To diffuse double layer surrounding the particles from developing fully. The double layer depression leads to a low inter-particle repulsion and the particles do not move over one another easily when compactive energy is applied and hence high percentage air voids and density is achieved. The increase in water content results in an expansion of double laver and a reduction in the net attractive forces between particles or in an increased inter-particle repulsion which permits the particles to slide 'more 'easily past one another into a oriented and denser state of packing together, and hence higher density.. After the optimum water content is reached, the air voids approach approximately a constant value as further increase in water content does not cause any appreciable decrease in them, even though more orderly arrangement of particles may exist at higher water contents. The total voids due to water and more air combination go on increasing with increase of water content beyond the optimum and hence the dry density of the soils falls.

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2. Amount of compaction : 

Effect of compactive effort on compaction, obtainted by jodhpur mini-compactor
Effect of compactive effort on compaction, obtainted by jodhpur mini-compactor

The amount of compaction greatly affects the maximum dry density and optimum water content of a given soil. The effect of increasing the compactive energy results in an increase in the maximum dry density and decrease in the optimum water content as shown in above image "Effect of compactive effort on compaction, obtainted by jodhpur mini-compactor" . However, the increase in maximum dry density does not have a linear relationship with increase of compactive effort.

3. Method of compaction 

The density obtained during compaction, for a given soil, greatly depends upon the type of compaction or the manner in which the compactive effort is applied. The various variables in this aspect are 
(1) weight of the compacting equipment, the manner of operation such as dynamic or impact, static, kneading or rolling. and (in time and area of contact between the compacting element and the soil. 

4. Type of soil 

The maximum dry density achieved corresponding to a given compaction energy largely depends upon the type of soil. Well graded coarse-grained soils attain a much higher density and lower optimum water contents then fine grained soils which require more water for lubrication because of the greater specific surface.

compaction curves for a range of soil types
compaction curves for a range of soil types

Dry density water content curves for a range of soil types. In general, coarse grained soils can be compacted to higher dry densities than fine grained soils. 

(i) Compaction curve for cohesion less sands 

typical compaction curve for cohesionless sand
typical compaction curve for cohesionless sand

In the case of cohesion less soils which are devoid of fines, the water content has very little influence on the compacted density. For such soils, the dry density decreases with an in- crease in the water content, in the initial stage of the curve, particularly under a low compactive effect. This is due to the bulking of sands' wherein the capillary tension resists the tendency of soil particles to take a dense state. In other words, the capillary tension developed in the sandy soil is not fully counteracted by the compactive effort and this capillary tension holds the particles in a loose state resisting compaction. It is interesting to note that the same condition, achieves greater density under the same compactive effort. The maximum bulking soil, in air-dried or oven dried con- (and hence the least density) occurs at a water content between 4 to 5%. On further addition of water, the meniscus is destroyed and the soil particles are able to stiffen take a closer packing, resulting in increase in dry density. The density reaches the maxi- mum value when the cohesion less soil is fully saturated. On further addition of water, the dry density again decreases. Also, the maximum density, attained under full saturation condition, is not very much higher than that corresponding to air dried or oven dried condition. Secondly, the attainment of maximum density at full saturation is not due to lubrication action of water but rather it is due to the reduction of effective pressure between soil particles by hydrostatic pressure. Such soils do not display a distinct optimum water content. 

ii, Compaction curve for a fat clay. 

typical compaction curve for fat clay
typical compaction curve for fat clay

A typical compaction curve for fat clay Here also, we see that there is initial decrease of dry density at lower water contents. This is the characteristic feature of black cotton soils, highly swelling clays and some fat clays. The optimum water content for such soils range between 20 to 25%. 

5. Addition of admixtures 

The compaction properties/characteristics of a soil can be modified by a number of admixtures other than soil material. These admixtures have special application in stabilized soil construction. in other Chapter we will describes various types of admixtures, along with their effects on the compaction characteristics of soils.

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