There are many factors that affect the degree of compaction in the area. Some depend on the compactor and some depend on the soil to be compacted. The factors affecting the degree of compaction are given below.

1. Soil Type

The type of soil has a great influence on its compaction characteristics. Generally, heavy soils, clays and silts offer higher resistance to compaction, while sandy soils and coarse-grained or gravelly soils are liable to be easier to compaction. Coarse-grained soil gives a higher density than clay. A well graded soil can be compacted to a high density.

2. Joint Effort / Active Energy

The term compactive effort or compact energy simply means the type of equipment or machinery used for condensation. The greater the condensation, the greater the condensation. The equipment used for soil compaction can be broadly classified into the following categories:

  1. kneading type equipment
  2. stationary equipment
  3. dynamic or impact type equipment
  4. vibratory type equipment

The type of compaction equipment used depends primarily on the type of soil to be compacted. The following table can be used as a reference to decide the type of equipment for different types of soil.

soil type Suggested Equipment/Machinery
crushed rock, gravel sand smooth wheel roller
gravel, sand rubber tire roller
sand, gravel, silt soil, clay Pneumatic Tire Roller
clay, clay sheepfoot roller
limited area soil Rolling pin
Sand vibratory roller

3. Layer Thickness / Lift Thickness

The degree of compaction is inversely proportional to the thickness of the layer, i.e. for a given compaction energy, the thicker layer will be less compressible than the thinner layer. This is because the energy input per unit load is less for a thicker layer of soil. Therefore it is very important to decide the correct thickness of each layer to get the desired density. The thickness of the layer depends on many other factors such as

  • soil type
  • Roller Type
  • Roller weight
  • drum contact pressure
  • Soon …..

Usually 200 to 300 mm layer thickness is used in the field to achieve homogeneous compaction.

4. Number of Roller Passes

It is clear that the density increases as the number of roller passes increases. But we have to remember two important things.

  1. After a certain number of roller passes, there is no further increase in density
  2. Higher number of roller passes means higher cost of the project.

It is therefore very important to determine the number of roller passes at the optimum moisture content for a given type of soil. Therefore, to reduce the compaction aspect of soil work, while achieving the desired level of density based on laboratory tests (Heavy Compaction Test, IS: 2720-Part-8 and Relative Density Test, IS: 2720-Part-14). Field compaction test is carried out for

5. Moisture Content

Proper control of the moisture content in the soil is essential to achieve the desired density. Maximum density with minimum compaction effort can be achieved by compaction of soil near its OMC (optimal moisture content). The natural soil moisture (NMC) in the field is either less than OMC or more than OMC. If the NMC of the soil is less than the OMC, the calculated amount of water should be added to the soil with sprinklers attached to the water tanker and mixed into the soil by motor grader for uniform moisture content. When the NMC of the soil is higher than the OMC, it is necessary to dry it by aeration to reach the OMC.

6. Contact pressure

The contact pressure depends on the weight of the roller wheel and the contact area. In the case of a pneumatic roller, the tire inflation pressure also determines the contact pressure in addition to the wheel load. A higher contact pressure increases the dry density and lowers the optimum moisture content.

7. Rolling Speed

Rolling speed has a very important effect on roller output. There are two important things we have to consider.

  • Firstly, the higher the rolling speed, the greater the length of the embankment can be covered in a day.
  • Second, at higher speeds there is likely to be insufficient time for the desired deformation to occur and more passes may be required to achieve the required compaction.

So we need to strike a balance between these two things. Generally the speed of all rollers is limited to around 5 km/h. In the case of vibratory roller speed was found to be a significant factor as its number of vibrations per minute is not related to its forward speed. Therefore, the slower the speed of travel, the more vibration at a given point and the less number of passes needed to achieve a given density.

Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.