A laboratory tests can be used to determine the OMC for a soil under given compaction. The two commonly laboratory compaction test are:
- Standard proctor test (AASHTO Test)
- Modified proctor test (modified AASHTO test)
Standard Proctor Test:
This test was developed by RR Proctor to provide a relationship between the water content and the dry density of soil.
Proctor used a standard model of 4 inches internal diameter and and effective height of 4.6 inches, with a capacity of `1/30` cubic foot. The mould had a detachable base plate, and a removable collar of 2 inches height at it's top. The soil was compacted in the mould in 3 equa....
Show MoreA laboratory tests can be used to determine the OMC for a soil under given compaction. The two commonly laboratory compaction test are:
- Standard proctor test (AASHTO Test)
- Modified proctor test (modified AASHTO test)
Standard Proctor Test:
This test was developed by RR Proctor to provide a relationship between the water content and the dry density of soil.
Proctor used a standard model of 4 inches internal diameter and and effective height of 4.6 inches, with a capacity of `1/30` cubic foot. The mould had a detachable base plate, and a removable collar of 2 inches height at it's top. The soil was compacted in the mould in 3 equal layers, each layer was given 25 blows of 5.5 pounds rammer falling through a height of 12 inches. A curve was obtained between the dry density and the water content.
procedures:
About 3 kg of air-dried pulverised soil passing through 4.75 mm sieve is taken. Water is added to the soil to bring its water content to about 4% if the soil is coarse grained and to about 8% if the soil is fine grained. The water content should be much less than the expected optimum water content. The soil is mixed throughly and cover ed without cloth and left for measuring for about 15 to 30 minutes.
The mould is any cleaned, dried and greased lightly. The mass of the empty mould with the base plate but without collar, is taken. The cooler is then fitted to the mould. The mould is placed on a solid base and filled with fully matured soils to about one third of its height. The soil is compacted by 25 blows of the rammer, with a free fall of 310 mm. The blows are evenly distributed over the surface. The soil surface is scratched with spatula before the second layer is placed and compacted. The mould is filled to about two third height with the soil and compacted again by 25 blows. Likewise, the third layer is placed and compacted. The third layer should project above the top of the mould into the Collar by not more than 6 mm.
The collar is rotated to break the bond between the soil in the mould and that in Collar. The Collar is then removed, and the soil is trimmed off flush with the top of the mould. The mass of the mould ,base plate, and the compacted soil is taken, and thus the mass of the compacted soil is determined. The bulk density of the soil is computed from the mass of the compacted soil and the volume of the mould.
Representative soil samples are taken from the bottom, middle and top of the mould for determining the water content. The dry density is computed from the bulk density and the water content.
Bulk mass density,`rho=M/V`
Where,M=mass of compacted soil
V=Volume of the mould
Thus, dry density is:
`gamma_d=rho/(1+w )`
Where, `w=`water content.
The soil removed from the mould is broken with hand. More water is added to the soil so as to increase the water content by 2 to 3%. It is throughly mixed and allowed to mature. The test is repeated and the dry density and the water content are determined.
A graph is plotted between dry density and water content to obtain the compaction curve as shown in the figure. It is observed that the dry density initially increases with increase in water content till the maximum density is attained. With further increase in water content, the dry density decreases. The water content corresponding to the maximum dry density is known as optimum water content OEC or the optimum moisture content OMC.
At water content lower than the optimum, the soil is rather stiff and has a lot of void spaces and, therefore, the dry density is low. As the water content is increased l, the soil particles get lubricated and slip over each other and move into densely packed position and the dry density is increased. However, at water content more than the optimum, the additional water reduces the dry density.
For a given water content, theoretical maximum density is obtained corresponding to the condition when there are no air voids (i.e, degree of saturation is equal to hundred percent). The theoretical maximum dry density is known as saturated dry density. In this condition, the soil will become saturated by reduction in air voids to zero with no change in water content.
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