Factors Affecting Field Compaction of Soil

There are many factors which influence the degree of compaction in the field. Some are compactor dependent and some depend on the soil being compacted. The factors which affect the degree of compaction are given below.

 1. Type of soil

Type of soil has a great influence on its compaction characteristics. Normally, heavy clays, clays & silts offer higher resistance to compaction where as sandy soils and coarse grained or gravelly soils are amenable for easy compaction. The coarse grained soils yield higher densities in comparison to clays. A well graded soil can be compacted to higher density.

2. Compactive Effort / Compactive Energy

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

1. Kneading type equipment
2. Static type equipment
3. Dynamic or impact type equipment
4. Vibratory type equipment

Type of compaction equipment to be used is mainly dependent upon the type of soil to be compacted. The following table can be used as a reference to decide type of equipment for different type of soil.

Type of soil	Suggested Equipment / machinery
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	Sheepfoot roller
Soils in confined zone	Rammer
Sands	Vibratory roller

 

3. Layer Thickness / Thickness of Lift

Degree of compaction is inversely proportional to the layer thickness, i.e. for a given compactive energy, thicker layer will be less compacted as compared to thin layer. The reason is, for thicker soil layer the energy input per unit weight is less. Therefore it is very important to decide the right thickness of each layer to achieve the desired density. Thickness of layer is dependent upon several other factors such as

• Type of soil
• Type of roller
• Weight of roller
• Contact pressure of drum
• So on …..

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

4. Number of Roller Passes

It is obvious that density increases as the no of roller passes increases. But there are two important things we have to remember.

1. After certain number of roller passes, there is no further increase in density
2. More number of roller passes means more cost of project.

So it is very crucial to determine the number of roller passes for a given type of equipment, for a given type of soil at optimum moisture content. Therefore field compaction trial is carried out to economise compaction aspect of earthwork, while achieving desired level of density based on Lab tests (Heavy compaction test, IS:2720-Part-8 and relative density test, IS:2720-Part -14).

5. Moisture Content

Proper control of moisture content in soil is necessary for achieving desired density. Maximum density with minimum compacting effort can be achieved by compaction of soil near its OMC (optimum moisture content). In the field the natural moisture content (NMC) of soil is either less than OMC or above OMC. If NMC of the soil is less than OMC, calculated amount of water should be added to soil with sprinkler attached to water tanker and mixed with soil by motor grader for uniform moisture content. When NMC of the soil is more than OMC, it is required to be dried by aeration to reach up to OMC.

6. Contact Pressure

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

7. Speed of Rolling

Speed of rolling has a very important bearing on the roller output. There are two important things we have to consider.

• First, the greater the speed of rolling, the more length of embankment can be compacted in one day.
• Second, at greater speed there is likely to be insufficient time for the desired deformations to take place and more passes may be required to achieve the required compaction.

So we need to make a balance between these two things. Generally the speed of all rollers is limited to about 5 km/hour. In case of vibratory roller speed was found to be significant factor because its number of a vibration per minute is not related to its forward speed. Therefore, the slower the speed of travel, the more vibrations at a given point and lesser number of pass required to attain a given density.