COMPACTION OF SOILS

INTRODUCTION

Soil natural found material from weathering processes of different rocks, soil in construction industry can be used as construction material or natural foundation for different structure erected on it.

Soil compaction is most popular and important construction process that is must be performed when working with road layers, embankments and structural beds.

Compaction of a road layer using steel roller

Compaction of soil is the mechanical process on soil, that is applied to increase density of soil by reducing void spaces between soil particles, compaction as mechanical process can occur either naturally or artificially, naturally this can be seen in soil formation process.

There are three things that need be present so that compaction can occur, which are water (lubricant), air voids and compactive effort.

Water as the lubricant reduce friction resistance (shear strength) between soil particles, that allow soil particles to be driven or moved into air voids during compaction.

Air voids, soil in its natural state is composed by soil solids, and void spaces or pores; these pores can be filled with either air or water, sometimes both of them, void spaces filled with air is known as Air voids.

Compactive effort this is energy required or applied to drive or move soil particles from their original locations into air voids, in compaction soil particles (solids) replaces air and not water.

FACTORS AFFECTING SOIL COMPACTION

1. Moisture content

Moisture content as the amount of water in the soil, which can be calculated by dividing mass of water in the soil by mass of soil solids.

Curve showing relationship between water and dry density.

Compaction increases density of soil by packaging soil solids (particles) into air voids, in order fro the soil solids to move, water as lubricant is required to reduce frictional resistance between particles.

With a certain amount of water soil solids are able to be moved into air voids and  density of that soil increases.

Optimum density is reached at a point when any further increase in amount of water will decrease air void spaces that could be replaced with soil solids on compaction.

2. Compactive effort

This is the energy supplied to soil during compaction, it is energy required to move lubricated soil particles into air voids; this reduced air content of soil into a certain percent.

When using or applying higher effort to compact soil, more soil particles will be moved into air voids at a certain amount of water compared to when using lower effort on the same type of soil.

The curve shows relationship between dry density and water with different compaction efforts

Soil layers are compacted with different compactive efforts from compaction equipments due to difference in speed of rolling, contact pressure and thickness of soil to be compacted.

3. Soil type

Different types of soils behaves differently on compaction, their behaviours could be the result of difference in mineral composition of soil or specific surface area of soil.

Soils having higher specific surface areas require higher amount of water for their compaction to reach desired optimum density compared to soils with lower specific surface area.

Coarser soils have higher specific surface area while finer soils have lower specific surface areas.

Some soils with clay minerals such as montmorillorite which are formed by weak bonds, these soils have weaknesses when meet water and external forces such as compaction efforts, temperature changes.

NECESSITY OF SOIL COMPACTION

Soil compaction as the process of packaging soil particles closer to increase its density is performed for the following reasons or advantages.

1. To increase shear strength of soil

As soil particles are packed or moved more closer to each other with less or no air voids, the friction force between particles increases, which results to soil with higher shear strength (higher bearing capacity).

This reduces high chance of having future settlement of soil due to soil compactness and stiffness produced during compaction.

2. It decreases seepage of water through the soil.

Compaction reduces if not eliminating all air voids between soil particles that could allow passage of water through soil.

This reduction in air voids reduced seepage (quantity and rate of water passing through the soil), as the soil is sensitive and weak to water, through this some properties of soil when comes contact with water such as expansion and shrinkage will be reduced if not eliminated.

CONCLUSION

Adequate compaction of soil as the natural foundation or road layer should be done, so as to have stable and strong foundation soil or load layer.

Adequate compaction results are dependent to amount of water mixed with soil and type and rate of compactive effort applied.