Soil Compaction - Part 01

If void ratio of a partially saturated soil is high, when the soil is subjected to external forces, the soil particles will be pushed to fill the void spaces. as a result the soil will be subjected to large deformations.

soil may be caused in civil engineering projects mainly in two different ways,
1. as foundation material
2. as construction material (water retaining dams, embankments etc.)

Soil compaction : soil compaction is the process which dandifies soil through application of mechanical energy, without introducing additives. ( Additives : cement, lime, asphalt, chemicals etc.)

In compacting soil in the field, three parameters can be conveniently and economically controlled to influence the properties of the compacted material. these parameters are,

1). Effect of the water content of the soil at compaction
2). Effect of  the compactive effort
3). Effect of the type of compaction

Note : to measure the compaction we can use the "dry unit weight" of the considered soil.

1). Effect of the water content of the soil at compaction

To investigate the effects of the water content on the compaction of the soil we can do a small testing. we can observe it by compaction of the soil (contain in a mold) by adding different water content and by applying by equal amount of compaction energy ( standard proctor energy). If the dry unit weight of the soil is obtained at different water contents, the plot of the dry unit weight vs moisture content will take a shape similar to the one shown below:

At low moisture contents, the compaction becomes less effective an as a result the dry unit weight is low. when water is is added to the soil during compaction, it acts as a softening agent on the soil particles. the soil particles slip over each other and move in to a densely packed position. as a result the dry unit weight after compaction first increase as the moisture content increases.

However, beyond a certain moisture content any increase in the moisture content tends to reduce the dry unit weight. this is because the water takes up the spaces that would have been occupied by the soil particles.

The moisture content at which the maximum dry unit weight is attained is generally referred as the optimum moisture content. water content less than optimum is known as 'dry of optimum' and water content more than optimum is known as 'wet of optimum'.

The testing procedure used to obtain the maximum dry unit weight and the optimum moisture content is referred to as "Proctor compaction test".

Standard Proctor Compaction test

Volume of the mold = 944 cm^3
Weight of the hammer = 2.5 Kg
Drop length = 300 mm
Layers = 3
Hammer blows = 25
Diameter = 100 mm

First take a look at this video,
https://www.youtube.com/watch?v=tqHNK67IgG4

3 Kg of dry soil is taken for the test. the test is generally carried out at 5 different moisture contents. after compaction if 3rd layer the collar is removed and the top level of the compacted soil should be 3 mm above the top level of the cylinder. soil in the mold is use to determine the moisture content. (samples from top, middle and bottom use for the determination of moisture content)

Knowing the wet density and the moisture content of the soil, the dry density can obtained.





Where,
P dry = dry density
P wet = wet density
w = moisture content

The test is repeated using the same soil after adding some water, for 4 other moisture contents of until the wet density if the soil in the mold is lower than for the previous moisture content. [ consider the wet density it will increase after that it will decrease, if that happen stop the test]

Energy supplied (E1) = mgh/v
                                   = (2.5 * 9.81 * 0.3 * 25 * 3)/ 0.000944     Note: (25 -blows, 3 layers) 
                                   = 584547.14 J/m^3
                                   = 584.6 KJ/m^3

Typical proctor compaction test calculations







  

Modified Proctor Compaction test

With the development of heavy rollers and their use in field compaction, the standard proctor compaction  test was modified to better represent the compaction effort used in the field compaction of soil. procedure same as above but equipments used are different (dimensions and weights)

Volume of the mold = 944 cm^3
Weight of the hammer = 4.5 Kg
Drop length = 450 mm
Layers = 5
Hammer blows = 25
Diameter = 100 mm