Compaction of Soil: Definition, Principle and Effect | Soil Engineering

In this article we will discuss about:- 1. Introduction to the Compaction of Soils 2. Definition of Compaction 3. Principle 4. Effect 5. Factors 6. Method.

Introduction to the Compaction of Soils:

Soil is used as a construction material for constructing embankments and subgrades. Embankments are constructed to raise the ground level above the existing level up to the formation level to support buildings, roads, or railways or other structures and also to retain water as in the case of earth dams or canal banks. Subgrades are constructed to provide support to the roads as a base to withstand the traffic loads.

The stability and durability of the embankments and subgrades depends on the improvement of shear strength of the soil as well as the restriction of settlements or deformation within permissible limits. The strength and defor­mation characteristics of embankments and subgrades depend directly on the density of the soil.

The higher the density, the higher is the strength and lesser is the settlement under loads. The seepage of water through embank­ments and subgrades acts to reduce the strength due to its erosive nature. Higher density will result in reduced permeability of the soil, thereby reducing the seepage of water.

Densification of soils during construction of embankments and subgrades is achieved by compaction. Higher density of embankments and subgrades is achieved by compacting the soil by rollers usually in layers, known as lifts.

Definition of Compaction:

Compaction is the artificial and mechanical process of decreasing the volume of the soil rapidly by the expulsion of air voids in the soil resulting in the increase in density.

Densification of soil also occurs naturally due to consolidation of foundation soils by expulsion of pore water due to loads from the structure. This is a rather long-term process compared to compaction.

The difference between compaction and consolidation is given below:

Compaction:

1. Artificial process caused by mechanical means such as rollers.

2. Decrease in volume and increase in the density of soil occurs by expulsion of air from the voids.

3. Compaction occurs in partially saturated soils.

4. Compaction is completed within minutes-and hence is a short-term process.

5. Compaction is effective in well-graded soils containing gravel and sand, and to a less extent in silts and clays.

6. Compaction is caused by short-term dynamic load, which are removed after compaction.

Compaction:

1. Natural process caused by stresses due to foundations or superstructures.

2. Decrease in volume and increase in the density occurs by expulsion of pore water from the voids.

3. Consolidation takes place in fully saturated soils.

4. Consolidation takes several months to years and hence is a long-term process.

5. Consolidation although in principle occurs in all soils but is significant for clayey soils from engineering point of view due to consequent long-term settlements.

6. Consolidation is caused by long-term static loads, which continue to exist after the completion of consolidation.

Principle of Compaction:

The principle of compaction was developed by R. R. Proctor in 1933 during construction of earth dams in California. The objective of compaction is to achieve maximum possible dry density of the compacted soil. The water content used for compaction controls the dry density achieved. Figure 12.1 shows the variation of the dry density with water content.

At low water content, the soil is stiff and the particles offer resistance to come closer, resulting in low dry density. As the water content is increased, water forms a lubricating film around particles causing them to be compacted to a closer state of contact resulting in higher dry density. The dry density increases with increase in the water content until maximum dry density (MDD) is reached.

At this stage, the soil particles come to the closest possible state of contact. On increase of water content beyond optimum moisture content (OMC), the volume of soil does not decrease further by compaction and water starts to occupy additional space causing an increase in the volume of voids and the total volume, and resulting in a decrease in dry density.

The water content at which the dry density is maximum after compaction is known as optimum moisture content or optimum water content. In general, water equal to OMC is added in the field for effective compaction, except in some specific cases. Compactive effort or compaction energy also controls the effectiveness of compaction. Higher the compactive effort, higher will be the dry density achieved for the same soil.

The type of soil and its gradation and plasticity characteristics also influence the degree of compaction achieved. Coarse-grained soils can be compacted to a higher dry density than fine-grained soils. Cohesionless soils can be similarly compacted to a higher dry density than cohesive soils. A well-graded soil is compacted more effectively as compared to a poorly graded soil. Addition of fines to a coarse-grained soil, by an amount just required to fill the existing voids, greatly enhances the dry density.

For the compaction of a given soil, the sample of soil is compacted in the laboratory applying standard compaction energy at different water contents. The dry density of the compacted soil at each of the water con­tent is determined and a graph is plotted with the water content on the x-axis and the dry density on the y-axis.

The water content corresponding to maximum dry density is determined, which gives optimum water content. For the compaction of soil in the field, water equal to OMC, or less (dry of OMC) or more (wet of OMC) water is used depending on the objective of compaction and type of construction. Same compaction energy per unit volume of soil, as used in the laboratory compaction test, is used to compact the soil in the field.

The following are some of the objectives of compaction:

i. Increase the shear strength of soil.

ii. Decrease the undesirable settlement of structures.

iii. Control undesirable volume change.

iv. Decrease permeability of soil.

v. Increase the bearing capacity of foundations.

vi. Increase the stability of slopes.

Effect of Compaction on Engineering Properties of the Soil:

Compaction improves the strength and deformation characteristics of the soil, improving their stability and durability. Lambe (1958) found that the properties of soil after compaction depend on the soil structure, which, in turn, is influenced by the type of soil, amount of water relative to OMC, and the compaction energy applied.

The effect of compaction is discussed on the following soil properties:

1. Soil Structure:

Soil compacted at the water content less than OMC (dry of optimum) will have flocculent structure with edge-to-face particle arrangement, irrespective of method of compaction. The structure of soils compacted at water content greater than OMC (wet of optimum) depends on the magnitude of the shear strain. Soils compacted wet of optimum, which undergo relatively small shear strain during compaction, will have flocculent structure. Soils compacted wet of optimum, which undergo large shear strains during compaction, usually have a dispersed structure with face-to-face (oriented) particle arrangement.

The degree of orientation of soil particles increases gradually with increase in water content and the soil still possesses a flocculated structure up to the OMC. The orientation of particles increases more rapidly with increase in water content for soils compacted wet of optimum.

Increase of compaction energy increases the orientation of soil particles even at the same water content.

2. Shear Strength:

Soils compacted dry of optimum have more shear strength than those compacted wet of optimum. The cohesion and friction angle are both higher for soils compacted dry of optimum. Thus, the Mohr-Coulomb strength envelope is steeper for soils compacted dry of optimum and also lies above that of soils com­pacted wet of optimum. However, the difference in shear strength of soils compacted dry and wet of optimum decreases when the compacted soils are fully saturated.

It may be noted that soils with a flocculent structure possess more shear strength. This is because the attrac­tive forces are predominant in flocculent structure and also because the soil offers higher resistance to deforma­tion due to particle interference in edge-to-face particle arrangement existing in flocculent structure.

On the other hand, repulsive forces are predominant in soils with dispersive structure resulting in lower shear strength. The particle interference and hence the resistance to deformation is also less in dispersed struc­ture, which has relatively oriented particle arrangement.

Saturation of compacted soils increases the repulsive forces, causing a decrease in shear strength.

3. Pore Water Pressure:

As the water content is less for soils compacted dry of optimum, there is zero or negligible pore water pressure (due to discrete and local pockets of saturation). Soils compacted wet of optimum show higher pore water pressure, which reduces the effective stress and frictional component of shear strength.

4. Stress-Strain Relationship:

Soils compacted dry of optimum possess a steeper stress-strain relationship compared to those compacted wet of optimum. Consequently, the deformation and settlement are less for soils compacted dry of optimum, and show relatively sudden and brittle failure. Soils compacted wet of optimum show large strains and settlements and the failure is gradual and plastic.

5. Compressibility:

Soils compacted dry of optimum are less compressible due to their flocculent structure and greater particle interference and resistance to deformation. Soils compacted wet of optimum are initially less compressible at low stresses due to their dispersed structure and predominance of repulsive forces.

However, when the stresses are increased further to overcome the repulsive forces, such soils show high compressibility resulting in large deformation. The face-to-face particle arrangement in dispersed structure of such soils also offers less resistance to deformation and increases the compression.

6. Shrinkage:

Shrinkage is the decrease in the volume of soil due to the evaporation of water. Soil compacted dry of optimum undergoes less shrinkage due to random particle arrangement and particle interference that offers more resistance to deformation. Shrinkage is more for soils compacted wet of optimum due to dispersed structure and lesser particle interference and resistance to deformation.

7. Swelling:

A clay soil compacted dry of optimum has more water deficiency and large void ratio and hence imbibes more water resulting in larger swelling, compared to the soil at the same dry density compacted wet of optimum.

8. Permeability:

Soils compacted at low water content possess low dry density and large void ratio and hence are more permeable. With increase in water content dry of optimum, the dry density increases and void ratio decreases causing a decrease in permeability.

Thus, permeability of soils compacted dry of optimum decreases with increase in water content. Permeability is minimum at or slightly above the OMC. With further increase in water content, permeability slightly increases due to decrease in dry density. However, permeability of soils compacted wet of optimum is always much less than those compacted dry of optimum.

Factors Affecting Compaction:

The MDD achievable by the compaction depends on the following factors:

1. Water content.

2. Type of soil and its gradation.

3. Gradation of Soil

4. Compaction energy.

5. Method of compaction.

1. Effect of Water Content:

Increase of water content used for compaction increases the dry density initially until the dry density reaches its maximum. After reaching MDD, further increase in the water content decreases the dry density.

2. Type of Soil:

The type of soil used for compaction primarily decides MDD achievable by the compaction. Figure 12.8 shows the compaction curves for different types of soil.

Coarse-grained soils can be compacted to a higher dry density than fine-grained soils. Cohesive soils usually have high air voids content. The void ratio of cohesive soils increases with increase in plasticity index. Thus, coarse­-grained soils have higher MDD and lower OMC than fine-grained soil. The MDD decreases and OMC increases for low plastic silt, high plastic silt, and high plastic clay.

3. Gradation of Soil:

For a given soil, a well-graded soil has higher MDD and lower OMC then a poorly graded soil. This is because a well-graded soil contains particles of all sizes and the finer size particles fill the void space between the coarser particles resulting in lower air voids and higher MDD.

Addition of small amount of fines to a coarse-grained soil increases its MDD for the same reason. However, when the amount of fines added is more than that needed to fill the voids of coarse-grained soil, the MDD again decreases.

4. Compaction Energy

The compaction energy applied to the soil during compaction has a significant influence on the MDD. In general, the higher the compaction energy or compactive effort, the higher will be the MDD and lower will be the OMC. This is the reason why the subgrades of airfield pavements are compacted using heavy compaction. Thus, the com­paction curve of a modified Proctor test, which uses more compactive effort on the soil, is above and to the left of that of standard Proctor test or IS light compaction test as shown in Fig. 12.9.

The increase in dry density due to the increase in compactive effort is more at water content less than OMC (dry of optimum) than that on the wet of optimum.

It may be noted that the increase in compactive effort does not go on increasing the MDD indefinitely. When compactive effort is increased in equal increment, the increment in MDD becomes smaller and smaller with each increment of compactive effort. Finally, a stage is reached where further increase of compactive effort does not bring any significant increase in MDD.

Care should be taken to see that the compactive effort does not cause a stress on the soil particles beyond their crushing strength, in which case the higher compactive effort crushes the individual particles, causing a reduction in MDD in some soils.

The line joining the peak points of different compaction curves of the soil compacted with different compactive effort is known as line of optimums and is roughly parallel to the ZAVL.

Method of Compaction:

Compaction of soils in the field can be done by a variety of compaction equipment.

The following are the different actions or effects of various compaction equipment on soils:

1. Static compaction – smooth wheel rollers.

2. Kneading compaction – sheep’s foot rollers.

3. Vibration compaction – vibratory rollers.

4. Tamping – tampers.