When it comes to construction and civil engineering, ensuring the stability and strength of the soil is paramount. One of the most effective ways to achieve this is through soil compaction, as discussed under soil stabilization methods. Compaction of soil is the process by which the solid particles are packed more closely together, usually by mechanical means, thereby increasing the dry density of the soil. The moisture content of the soil plays a key role in soil compaction, with each soil sample having a particular optimum moisture content at which it achieves its maximum dry density. The Proctor Compaction Test examines the relationship between the moisture content of a soil sample and its density upon compaction. The Proctor Compaction Test is named after R. Proctor who developed the relationship.
NOTE : The Proctor Compaction Test described here is only applicable to cohesive soils. For cohesionless highly permeable soils such as clean gravels, uniformly graded and coarse clean sands, the Proctor Compaction Test might indicate meaningless values of moisture contents.
Applicable Terms
Density – Concentration of particles per unit volume.
Moisture Content – Amount of water, expressed as a percentage, of the dry mass of material.
Maximum Dry Density (MDD) – the greatest mass of material that can be packed in a unit volume. In soil mechanics, this can be determined by compacting the soil through a range of moisture contents.
Optimum Moisture Content (OMC) – the moisture content at which the Maximum Dry Density (MDD) is achieved.
Proctor Test Specifications
The dry density that can be achieved for a particular soil depends on the degree of compaction applied and the amount of water present in the soil. To simulate different degrees of compaction in the laboratory, light manual compaction can be achieved using a 2.5 kg rammer whereas heavy manual compaction is achieved using a 4.5 kg rammer – with a greater drop on thinner layers of soil. The choice between light and heavy compaction depends on the expected load on the soil in its final application. Heavily compacted soils are generally denser and can support greater loads, making them ideal for high-stress environments. Furthermore, different layers on a flexible pavement will often require different compaction efforts, with layers close to the riding surface requiring to be of superior quality, thus a greater compaction effort.
British Standards (BS) and American Association of State Highway and Transportation Officials (AASHTO)
BS and AASHTO utilize different sizes of moulds and different compaction efforts as highlighted below:
| Specification | British Standard (BS 1377 -4) | AASHTO (T99 & T180) | ||
|---|---|---|---|---|
| Mould Size | 105 mm dia by 115.5 mm high – 1L | 152 mm dia by 127 mm high – CBR Mould – 2.3L | 101.6 mm dia by 116.4 mm high – 0.95L | 152.4 mm dia by 116.4 mm high – CBR Mould – 2.1L |
| Light Compaction – 2.5 Kg rammer in 3 layers | 27 blows from 300mm | 62 blows from 300mm | 25 blows at 305mm | 56 blows from 305mm |
| Heavy Compaction – 4.5 Kg Rammer in 5 layers. | 27 blows from 450mm | 62 blows from 450mm | 25 blows at 457mm | 56 blows from 457mm |
Proctor Test – BS vs AASHTO
The light compaction using AASHTO (T99) is commonly referred to as the Standard Test whereas the heavy compaction (T180), is referred to as the Modified AASHTO.
NOTE: The soil sample to be used in the proctor compaction test should contain particles all of which pass the 20mm sieve (refer to Grading for an understanding of sieve analysis). However, in case there is a limited amount of particles upto 37.5 mm sieve, equivalent tests are carried out using the larger California Bearing Ratio (CBR) mould, as mentioned in the table above. If more than 30% of material is retained on a 20 mm test sieve the material is too coarse to be tested. A third type of proctor test, which is not the subject of today's discussion, makes use of a vibrating hammer and can be used for this material.
Specifications for compaction by rammer in the CBR mould are based on the same compactive effort per unit volume of soil as in the 1L compaction mould. The variable effects of side wall friction might result in differences between the densities achieved in the two moulds. For a series of tests on a particular soil, one size of mould should be used consistently.
General Procedure
- Obtain about 6kg of air-dried soil passing 20mm sieve.
- Weigh the mould with it’s base plate and record its mass (M1).
- Put the soil in a tray and mix it thoroughly with water. The water added for each stage of the test should be such that a range of moisture contents is obtained which includes the optimum moisture content.
- Place the mould on a concrete base and fix the collar.
- Compact the soil in accordance with the specified method, that is, BS or AASHTO. Make sure to use the proper mould and rammer as described in the table above.
- Distribute the blows uniformly over the surface of each layer – the last layer should not be more than 6mm over the mould.
- After the last layer, remove the collar, trim the excess soil over the mould and weigh it with its base plate and soil (M2).
- Place the mould in a tray, remove the soil and take a small portion for moisture content.
- Break the specimen, rub it through the 20mm test sieve and mix it with the remainder of the material in the tray, and more water and mix it thoroughly.
NOTE : For particles susceptible to crushing, prepare separate samples for compaction testing, discarding each compacted sample.
- Repeat steps (5) to (8) at least four times, with appropriate increments of water contents.
Calculations
For any soil sample, the moisture content, w, is given as:
The bulk density of a soil sample, 
, defined as the mass per unit volume, including the air spaces and moisture content within the soil, is given as:
The dry density of the soil, 
, is given as:
Presentation of Results and Interpretation
The Maximum Dry Density (MDD) at Optimum Moisture Content (OMC) is determined by plotting the dry densities from a series of determinations as ordinates (y-axis) against the corresponding moisture contents as abscissae (x-axis). The maximum point of a curve that best fits the plotted points is the maximum dry density (MDD), and the corresponding point on the x-axis is the maximum moisture content (OMC).
Additionally, on the same graph, plot the curves corresponding to 0%, 5% and 10% air voids, calculated from the equation:
Where:
= dry density of soil
= density of water, assumed to be 1
= % of air voids
= specific gravity of the soil
=moisture content of the soil
The output of the Proctor Compaction Test has been presented in the following Microsoft Excel and PDF templates:
Significance of Proctor Compaction Test in Engineering Applications
The Proctor compaction test is a fundamental procedure in geotechnical engineering that determines the optimal moisture content at which a soil type will achieve its maximum dry density. The Proctor Compaction test aids engineers in designing embankments, earthworks and pavement layers as well as issuing guiding specifications for quality control on these works. For instance, the Standard Specifications for Road and Bridge Construction in Kenya guides the compaction of earthworks as a percentage of the Maximum Dry Density and Optimum Moisture Content. For example, a section of the specification may state that ‘The moisture content during compaction should be between 75% and 105% of the Optimum Moisture Content (T99). The layer should be compacted to at least 95% MDD (AASHTO T99).’ Ideally, the compaction of soil material in the field should be done as close as possible to the Optimum Moisture Content, to ensure that the material achieves the maximum density and therefore stability and strength. In the field, upon the compaction of the said layer, various methods can be used to determine the field density of the material, which then can be compared to the specifications to determine whether the said layer has met the prescribed engineering standards and specifications.
