Factors affecting Compaction of soil

Compaction of the soil is the process where the dry density of soil is increased by reducing air content or air voids present in the soil. This process is an essential part of the construction of any structure, as it strengthens the soil. Many factors influence the degree with which the soil is compacted. The various factors which affect the compaction of soil are as follows:

Moisture content

To achieve the desired density of the soil, the moisture content of that soil has to be controlled properly. If the water content is low, it leads to soil being stiff which will resist compaction. When the water content is increased, lubrication takes place between the soil particles and the soil becomes more workable.

Dry density of the soil increases with an increase in the water content till the optimum water content is reached. Adding more water at this stage, will reduce the dry density. The amount of water content added to the soil till it reaches its maximum dry density is called the moisture content of the soil.

Types of soil

The soil type influences the compaction of that soil to a great extent. The coarse grained soils can be compacted to higher dry density than the fine grained soils. The maximum dry density decreases if the quantity of fines is increased to an amount more than that required to fill voids in the coarse grained soils. Hence it’s safe to say a well graded soil obtains a much higher dry density than a poorly graded soil.

Cohesive soils such as heavy clays, clays & silts provide higher resistance to compaction as they attain lower maximum dry density. Cohesionless soils such as sandy soils and coarse grained or gravelly soils are susceptible for easy compaction.

Amount of compaction

The optimum water content required for compaction decreases with an increase in the compaction effort. This effect of increase in compaction is significant only until the water content reaches its optimum level. After that level, the volume of air voids becomes almost constant and the effect of increased compaction is not significant. It should be noted that the maximum dry density does not go on increasing with an increase in the compactive effort.

The word ‘compactive effort’ means the effort taken by the equipment used for compaction of the soil. Following are the type of equipments used for different types of soil:

• Crushed rock, gravelly sand              : Smooth wheel roller
• Gravels, sand                                        : Rubber tyred roller
• Sands, gravel, silty soil, clayey soils : Pneumatic tyred roller
• Silty soil, Clayey soil                            : Sheep-foot roller
• Soils in confined zone                          : Rammer
• Sands                                                       : Vibratory roller

 Contact Pressure

Contact pressure is the pressure between the soil and wheels of the equipment used for compaction. This pressure depends on the weight of the roller wheel and the contact area. A higher contact pressure increases the dry density and lowers the optimum moisture content.

Speed of Rolling

The speed of rolling is the speed with which the soil is compacted is a significant factor. There are two important things to be considered. The first thing is, the higher the speed of rolling, the more length of embankment can be compacted in a day. Secondly, at higher speed of rolling there is likely a chance of insufficient time required for the deformations to take place and hence more passes may be required to achieve the desired compaction.

The slower the speed of travel, the more vibrations at a given point and lesser number of pass required to attain a given density.