What is Compaction?

What is Compaction?

Compaction is the compression of soil mass mechanically so that air present in the voids escape and thus improve its engineering properties. 

As a result of compaction, 

• Shear strength increases
• Compressinbility increases
• Permeability decreases

What are the tests on compaction?

There are mainly two tests conducted 

• IS Light Compaction or Standard Compaction test
• IS Heavy Compaction or Modified Proctors test

Features of tests	IS Light Compaction	IS Heavy Compaction
Weigth of hammer	2.60Kg	4.90Kg
Height of freefall	31cm	45cm
Number of layers	3	5
Number of blows per each layer	25	25
Volume	1L	1L

What is Optimum Moisture Content(OMC)?

It is the water content at which maximum dry density is obtained

What happens when compaction energy increases?

The comapction test is repeated for different water contents and in each case dry density is obtained and a graph is plotted between dry density and water content.

As Compaction increases dry density increases, but OMC decreases.It is evident from the graph. 

Comparison when compaction is done in finer soil and coarser soil? 

Finer soil has higher OMC and less maximum dry density. But why?

Finer soils have 

• more surface area and hence more OMC.
• Larger voids and hence lesser dry density

Compaction Equipments

Tampers	Manual compaction
Smooth wheel rollers	Granular or crushed rocks
Pneumatic tyred rollers	For all soils
Sheep foot roller	Clay
Vibratory rollers	Sand
smooth heavy roller	gravel in WBM road