The determination of maximum dry density and optimum moisture content of the soil is a measure of compaction level of soils. This can be measured by mainly two methods Standard Proctor Compaction Test and Modified Proctor Compaction Test. Both the tests help to determine the optimum moisture content that is required for soil to attain maximum compaction i.e maximum dry density for performing construction.
Need for Determining Optimum Moisture Content (OMC) of the Soil
The soil at the construction site must be stable enough to carry the loads from the structures through footings without undergoing undesirable settlements during the construction process and during the service period.
This function of soil is tested through the site investigation process. The construction site is hence treated and compacted based on the site investigation report. The amount of compaction required for the soil in the respective area varies from site to site.
To determine the amount of compaction required by the soil and the optimum water content for compaction, the compaction tests are conducted on the soil from the site in the laboratory.
Standard Proctor Compaction Test
The standard Proctor Compaction test was developed by R.R Proctor in 1933. Proctor showed that:
- The soil moisture content and the degree of dry density to which the soil is prepared to be compacted maintain a definite relationship.
- The Optimum moisture content (OMC) or Optimum Water Content (OWC) is the moisture content at which the soil attains maximum dry density. This OMC value is with respect to the specific amount of compaction energy applied to the soil.
Scope of Standard Proctor Compaction Test
The scope of the Standard proctor compaction test is to determine the relationship between the moisture content and the density of the soil that is compacted in a mold with a rammer of 2.5kg dropped at a height of 305mm.
Relationship between maximum dry density and optimum moisture content of soil can be obtained from soil compaction curve obtained from Standard Proctor Compaction Test. This relationship helps in determining the optimum water content at which the maximum dry density of soil can be attained through compaction.
Apparatus for Standard Proctor Compaction Test
The apparatus consists of a standard mold of 4 inches in internal diameter. The effective height of this standard mold is 4.6 inches. The maximum capacity of the mold is 1/30 cubic foot. The apparatus is shown in the figure-1 below.
The mold consists of a detachable base plate. The top of the mold consist of two 2-inch height collar which is removable. The soil is added into the mold in three layers, each layer undergoing 25 blows. This compaction is carried out by means of a 5.5 pound rammer falling from a height of 12 inches.
Indian Standard Specification – IS:2720 (Part VII) recommended specification for standard proctor test have some minor modifications and metrifications. The cross-section of the apparatus used as per Indian codes are shown in figure-2. The diameter of the mold is 100mm with a height of 127.3mm. The capacity of the mold is 1000ml.
The rammer used has a mass of 2.6 kg. This undergoes free drop of 310 mm with a face diameter of 50 mm. The soil compaction is carried out in three layers. The height of collar is 60 mm which is removable. The mold is placed over a detachable base plate.
In certain cases, the soil taken for testing may retain on 4.75mm sieve. If this amount is greater than 20%, then a mold of larger internal diameter say 150mm is employed. This mold has a height of 127.3 mm and a capacity of 2250 ml.
Procedure for Standard Proctor Test
The procedure for carrying out the standard Proctor test are as follows.
- Collect the soil sample weighing 3kg. The sample must be 3kg after air drying it. Usually, this soil will be pulverized soil that passes through 4.75mm sieve. If the soil is coarse-grained type, the water is added such that its water content comes to 4%.
- If the soil is fine-grained, water is added to make its water content to 8%. The water content of the sample after addition must be less than the optimum water content.
- The soil after the addition of water is mixed thoroughly and covered with a wet cloth. This sample is kept aside for 15 to 30 minutes for undergoing the maturing process.
- Next, the apparatus is prepared by cleaning the mold thoroughly. The mold has to be dried and greased lightly. The mass of the mold with base plate and without collar is weighed. Let it me (Wm).
- The mold placed over solid base plate is then filled with prepared matured soil to one-third of the height. This layer will take 25 blows with the rammer. The rammer has a free fall height of 310 mm.[Note: If a bigger mold is used, the no: of blows for each layer will be 56 no’s. Here the capacity of the mold will be 2250 ml.]
- The compaction must be done in such a way that the blows are evenly distributed over the surface of each layer.
- Next, the second layer is added. Before adding the second layer the top of the first layer has to be scratched. Now the soil is filled to two-thirds of the height of the mold. This too is compacted with 25 blows.
- Later the third layer is added. It is compacted similarly. The final layer must project outside the mold and into the collar. This amount must not be greater than 6mm.
- The bond between the soil in the mold and the collar is broken by rotating the collar. Next the collar is removed and the top layer of soil is trimmed and levelled to the top layer of mold.
- Next, the mass of the mold with compacted soil and base plate is determined (Wms). Hence the mass of the compacted soil (Ws) is determined as: Ws = Wm -Wms
- The mass of compacted soil and the volume of the mold gives bulk density of the soil. From the bulk density the dry density can be determined for the water content used (w).
- The same procedure from (1-8) is repeated by increasing the water content in the soil by 2 to 3%. Each test will provide a different set of values of water content and dry density of soil. From the values obtained compaction curve is graphed between the dry density and water content.
Calculations for Compaction Curve
1. Weight of Compacted Soil (Ws) in grams.
Ws = Wm -Wms Eq.1
2. Bulk Density
3. Dry Density
Compaction Curve of Soil - Maximum Dry Density and Optimum Water Content
The compaction curve is the curve drawn between the water content (X-axis) and the respective dry density (Y-axis). The observation will be initially an increase of dry density with an increase in the water content. Once it reaches a particular point a decrease of dry density is observed.
The maximum peak point of the soil compaction curve obtained is called the Maximum dry density value. The water content corresponds to this point is called the Optimum water content (O.W.C) or optimum moisture content (O.M.C).
The graph shown in figure-3 is the compaction curve. Initially for a water content lesser than O.M.C the soil is rather stiffer in nature that will have lots of void spaces and porosity. This is the reason for lower dry density attainment.
When the soil particles are lubricated with the increase in the water content, the soil particles will be densely packed resulting in increased density. Now beyond a limit (OMC) the addition of water will not bring a change in dry density or will decrease the dry density.
The graph represents a zero-air void or 100 % saturation line. This is based on the theoretical maximum dry density where it occurs when there is 100 % saturation. As the condition of zero voids in soil is not real and a hypothetical assumption, the soil can never become 100% saturated.
The theoretical maximum dry density can be determined by the equation
G = specific gravity of solids;
Modified or AASHTO Proctor Test
In order to perform construction for heavy loads like airways and highway construction, there comes the need for heavier compaction. The compaction energy provided by the standard proctor test is not sufficient, hence a Modified Proctor test or AASHTO Proctor test was developed.
The modified proctor test was altered and practised by AASHTO and ASTM. The mould used in modified proctor test has a internal diameter of 100mm and height of 127.3mm. This also has a detachable base plate as in the case of standard proctor test.
In order to provide higher compaction energy, the rammer is mechanically operated that has a face diameter of 50mm and the free-falling weight of the rammer is 4.89kg. The height of the drop is 450mm. The sample in the mold is given 25 blows for 5 layers.
The compaction energy provided by the modified proctor test is 4.56 times greater than the standard proctor test.
Also Read: Soil Compaction