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Pavement placed on some soil layers such as (Subgrade, Subbase and Base) or other layers in between, the capacity of the road mainly depends on the capacity of layers, specially subgrade is the most important one, in building construction there is a quote says "There is nothing more expensive than a deep foundation", this statement is correct for pavements because the load transfer system is almost the same as buildings. In this article mainly focused on Subgrade including the definition, subgrade preparation for construction and subgrade improvement techniques.

Subgrade is natural soil layer under the pavement structure, in any case, its a lowest layer of the pavement. Alongside the road alignment there is a different soil type and different layers which makes Subgrade to behave as a complex soil layer, during construction the top layer of subgrade should be removed about 30-50 cm in depth, because top layer exposed to environment such as rain, frost, wind, flood and contain organic material such as grass and other plants, removing the top part of subgrade is mandatory in all cases, the testing and improvement should be carried on the layer beneath the top part of subgrade which is removed. According to the construction sequence, the subgrade is graded according to proposed alignment; some tips are strongly recommended to decide the proposed alignment should cut or fill because it has a direct effect on the strength of pavement structure:

1- Setting the grade line of subgrade high enough to ensure that the road structure as far as possible from groundwater table, in some cases the groundwater table is close to ground surface, its the matter to consider such cases.

2- Soil type is another case which is very important, some soil type is not stable to carry the road structure, such as expansive soil that should be removed or treated. The soil is the main criteria to decide which part of the alignment should be removed (Cut) or which part should be filled, other criteria which is related to soil type is hardness of soil such as rocky soil which may costly to cut such kind of layer because require a lot of technique, machine, and time.

3-Vertical distance between the cut crest and fill crest is another matter, the slope of the road should confirm the standard requirement, according to experience most of the time the best solution to maintain the economic the cut and fill area should be equal or close to each other, sometimes the soil which is cut down from the part of alignment is a good soil, and it can be used to fill other parts of the alignment which is economically interested, on the other hand, protect the nature because no need to damage another place to bring the soil to fill parts.

4- Environmental consideration is one of the most important parts of highway construction, the road alignment should not go through the environmentally protected area such as a forest, the alignment direction should be based on minimum

Subgrade preparation is similar between all kind of pavements such as surveying, excavation, testing, embankment construction procedure. before placing the embankment on subgrade soil or placing subbase layer, subgrade material should be tested to ensure that the material meets the standard requirement, some series of field or laboratory testing requirements as follows:

1- Particle Size Distribution.

2-Plasticity test such as (Atterberg Limit by Casagrande method or Fall cone test).

3- CBR.

4-Organic Material Determination.

One of the most essential tests which is exceedingly mandatory in all cases is (Particle Size Analysis Test) used to determine the actual gradation of subgrade soil, the gradation should be uniform and the amount of fine soil such as clay and silt shouldn't more than the range of standard, if amount of clay presence in the soil, plasticity test should be carried out to check the plasticity behavior of soil because the plasticity direct related to expansive or swelling ratio of soil, the high plasticity soil indicate higher possibility of expansive soil. CBR in most cases of the highway is required to determine the CBR value, CBR value in some cases is used for design the pavement layer. According to the standard which is used for the project, the soil should be tested in order to determine the percentage of organic material if its presence in some section alongside the alignment, the organic contained soil should be removed entirely and replaced by other types of soil.

Fig-1: Leveling Subgrade by Bulldozer (Image Source)

After all quality checking is complete the subgrade preparing for compaction, the compaction is mainly carried out by rollers such as Steel wheel roller, or sheep foot roller, before start compaction, the soil specimen should be test in laboratory to determine maximum density achieved by compaction, the laboratory compaction can be applied by two methods, first; standard proctor test, second; is modify proctor test, to achieve more possible compaction usually modify proctor was used, this laboratory value later compare with field density value to ensure that the soil is well compacted.

Fig-2: Subgrade Compaction by Sheep Foot Roller (Image Source)

Some methods which may used to improve subgrade soil:

1- Dynamic Compaction.

2- Replacement subgrade soil.

3-Deep mixing.


1- Norbert Dellate, Concrete Pavement Design, Construction and Performance. 1th edition, Taylor&Francis Group, PP 249-251.


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BSc- Geotechnical Engineering\Koya University

MSc- Infrastructure Engineering\ Szechenyi Istvan University

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