Soil Compaction is a method of stabilizing loose soil by densification, which can be achieved with either static or dynamic load. At constant water content, the process requires a permanent reduction in the amount of air voids.
The most popular and least costly form of soil improvement method is compaction of soil. It improves the soil’s shear strength and thus its stability and bearing capacity power.
Mechanics of Compaction of Soil
When an external force is applied to a structure of soil particles, the particles roll over one another and rearrange to occupy more stable locations, resulting in a denser packing. The following three mechanics are involved in the the process.
Step 1 : Large aggregates are broken down or crushed into smaller pieces.
Step 2 : Dislocation and re-organisation of particles that cause the flaky particles to collapse or reorient.
Step 3 : Bending and rupture to flaky particles.
Volume reduction is generally caused by a combination of these mechanics working together under external pressure for one or more of the following reasons.
- Air compression.
- Air solution in soil water.
- Expulsion of air.
No removal of water from the soil takes place.
Also Read : Compaction and Consolidation Differences
Also Read : Soil Structure and Their Types
Also Read : Bearing Capacity of Soil and Its Importance
Factors Affecting Compaction of Soil
Water Content of Soil :
The soil is stiffer and more resistant to compaction because it has a low water content. The soil particles become lubricated as the water content rises. The soil mass becomes more workable, and the particles begin to pack together more tightly.
If the water content rises, the dry density of the soil rises as well, until the optimum water content is reached. At that point, the air voids have reached a volume that is nearly constant.
The air voids do not decrease when the water content rises, but the total voids (air plus water) do, and the dry density decreases.
By pushing air out of the soil voids, a higher dry density is achieved up to an optimum water content. As the maximum water content is achieved, forcing air out and reducing air voids becomes more difficult.
Regardless of the method of compaction, soils compacted at a water content less than the optimal water content have a flocculated structure.
Soils compacted at a water content higher than the optimum water content have a dispersed structure if large shear strains are induced, and a flocculated structure if the shear strains are minimal.
The water content is so low at point ‘P’ as shown in fig, which is on the dry side of the optimum water content, that the attractive forces outnumber the repulsive forces. As a consequence, the structure becomes flocculated.
The repulsive forces increase as the water content exceeds the optimum value, and the particles become oriented into a dispersed structure.
If the compactive effort is increased, the orientation of the particles also increases, resulting in higher dry densities, as shown by the following upper curve.
Compactive Effort :
The dry density of the soil increases as the compactive effort increases, while the optimum moisture content decreases. It should be noted, however, that maximum dry density does not increase as compaction effort increases.
Finally, a point is reached above which no further increase in dry density can be achieved without increasing compaction effort.
Type of Soil :
The amount of dry density obtained is determined by the form of soil. In the figure, the maximum dry density and optimum content for various soils are shown.
Coarse-grained soils, in general, can be compacted to a greater dry density than fine-grained soils. Even a small amount of fines added to a coarse-grained soil results in a much higher dry-density for the same amount of compactive effort.
The maximum dry density decreases when the quantity of fines is increased beyond what is needed to fill the voids in coarse-grained soils. The dry density of a well-graded sand is much higher than that of a poorly graded soil.
Higher air voids are contained in cohesive soils. As compared to cohesionless soils, these soils achieve a lower overall dry density.
Since these soils need more water than cohesionless soils, the ideal water content is high. Heavy clays with a high plasticity have a low dry density and a high optimum water level.