Soil itself is used as a construction material while constructing engineering structures such as roads, earth dams, landfills etc. It is also used in the foundation of buildings.

But as we know soil is made up of small particles and they have some amount of empty space between them which we call pores or voids. In case of dams, huge amount of water may percolate through these voids out of the dam. While in case of roads or buildings, weight of the running vehicles and weight of the building itself may cause the soil pores to shrink and that may result in undesired settlement of structure. The position and the elevation of the structure changes which increases unnecessary maintenance cost. And sometimes it may even leads to structural failure.

With proper techniques and care we can decrease these pores and densify the soil before constructing any structure on it, so that the settlement of soil after the construction is very less and manageable. The denser the soil the lesser it settles.

Good. But How do we do that? Well that is the process called compaction.

For construction of any structure we need its base, the soil below, to be strong. We want the soil in the base to be incompressible so that it does not settles in high amount with time. We want it to be stable against volume change as water content or any other such factors vary.

To achieve this we employ method of soil compaction. Soil compaction is a crucial part of the construction process. It is the process of pressing the soil by mechanical methods, such as rolling and tamping. By compaction we make soil more tightly packed and solid and provide a strong foundation for structures to be built upon.

As we know soil is composed of solid particles and voids that may contain air and/or water. When soil is loose or not compacted, it contains lots of voids. These voids make soil weak and less stable. It does not provide much support. During compaction, external forces are applied to the soil which causes soil particles to rearrange and come closer. Volume of voids shrink and the air that may be present in the them is expelled out. This decreases soil’s volume which in turn increases its density. Higher the density, lesser is the settlement under the load. And we say the soil is compacted.

Hence compaction is a process that aims to increase the soil's density and decrease its void spaces by applying mechanical energy or force.

We just discussed higher the density, lesser is the settlement, Also higher the density, higher is the strength of the soil. By strength we mean the ability of soil to resist being moved by an applied force and to support a load without getting crushed or deformed.

We may ask how does the increased density increases the soil strength?

During the compaction the applied pressure or compactive effort causes the particles to push against each other more forcefully. This leads to an increase in contact forces between the particles. The higher contact forces create stronger bonds and connections between particles, resulting in a cohesive and stable soil structure. Also because of compaction, particles become interlocked which creates friction and resistance to movement between particles.

This closeness of particles allows the load to be distributed more efficiently across the compacted soil and reduces localized stress concentrations. This uniform load transfer helps prevent excessive deformation under applied loads which is also called failure of the soil. Hence strength of the soil is increased.

In case of water retaining structures like dams, the water that seeps through the walls of an earth dam, reduces the strength of the embankment soil because this flowing water is erosive in nature which erodes soil particles and decreases the soil density.

So by compacting the embankment soil we achieve high density soil embankment. Higher the soil density implies it contains less amount of voids. Consequently permeability of the soil will be reduced and thereby reduced seepage of water.

Great. But how do we do this compaction thing?

Compaction is commonly performed using heavy machinery and equipments, such as vibratory compactors, rollers, or compacting plates.

There are different methods of compacting the soil such as

• Vibration

• Impact

• Kneading

• Pressure

In gravel and sand type of soils we usually choose vibrational equipments to cause re-orientation of the soil particles into a denser configuration. While In silt and clay soils, a sheepsfoot roller is frequently used, to create small zones of intense shearing to bring air out of the soil.

The compaction is usually done in layers which are sometimes called lifts. Each layer compacted before the next one is added.

Good. We know how to compact a soil, but how do we know how much we have compacted it?

The amount of compaction is measured by the Dry Density of the soil. Dry density is the density of soil when soil is in completely dry state which means no water is present in its pores. So the Dry Density of soil is mass of soil solids only divided by its volume.

Dry density is a measure of how densely packed or compacted the soil is. The objective of compaction is to achieve maximum possible Dry Density of the compacted soil. Because higher the soil dry density, less amount of voids it will contain and higher will be its strength.

We have understood that compaction is done to improve engineering properties of the soil.

1. Compaction Increases bearing capacity of soil foundations. Compacted soil can support heavier loads without excessive settlement or deformation.
2. Compaction Increases the shear strength of soil. It Increases the resistance against shear forces thereby reducing the likelihood of slope failure or soil movement.
3. Compaction decreases permeability of soil. It reduces the soil pores, making it less permeable to water. This prevents excessive seepage and improves the performance of water retaining structures.
4. In addition to these compaction also Enhances frost resistance. As the compacted soil has fewer voids, limiting the amount of water that can penetrate and freeze within the soil, thus minimizing frost damage.
5. Compaction also control undesirable volume change in soil.
6. Compaction can prevent the build up of large water pressures that cause soil to liquefy during earthquakes.

We should note few important points about the Compaction:

1. Compaction is an Artificial process caused by mechanical means such as rollers. It doesn’t happen by itself like the other process named consolidation.
2. Compaction Decrease in volume and increase in the density of soil occurs due to expulsion of air from the voids.
3. Compaction is more effective in partially saturated soils than in completely saturated soils because the excess water fills most of the void spaces, making it harder for soil particles to move and rearrange as efficiently. This limits the improvement in soil density.
4. Compaction is a short-term process. it is completed within minutes. We apply force or vibration from machines and the soil is compacted.
5. Compaction is caused by dynamic loads that are short-term. They are removed once the process of compaction is completed.
6. Compaction is more effective in well-graded soils containing gravel and sand while it is less effective in soils containing silts and clays. Because Gravel and sand particles are generally angular/irregular in shape, which contributes to better particle interlocking during compaction. Also Silts and clays, which are finer particles, are more cohesive and plastic. This cohesion makes it more difficult for the particles to move and rearrange during compaction. Also these finer particles become sticky when mixed with water which hinders particle movement during compaction.

It is important to note that while soil compaction provides several benefits but excessive compaction also have negative consequences. Excessive compaction can lead to a significant reduction in soil permeability, making it difficult for water to penetrate the soil. This reduced permeability can result in poor drainage hence causing potential water-logging. The lack of infiltration also leads to increased surface run-off and erosion.

Excessive compaction can adversely affect soil micro-organisms and macro-organisms, disrupting the soil ecosystem. These organisms play essential roles in nutrient cycling, organic matter decomposition and overall soil health.

Also excessive compaction inhibits plant roots penetration through soil and limit the movement of water and air through the soil which negatively impacts soil fertility and aeration.

Therefore, to mitigate potential negative consequences of compaction it is crucial to perform thorough site investigation, carefully consider the soil type, follow appropriate compaction specifications and guidelines based on the project requirements and use of the right equipments for the desired compaction. Sometimes after construction we may have to de-compact the areas around the structure to be landscaped so that vegetations and the ecosystem below can grow.