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AASHTO classification of soil

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Classifying the soil considered a significantly important in road engineering as it will help the Engineer in understanding the soil behavior under the loads, change in moisture and the traffic volume. AASHTO classification is one of the most used soil classification system in road projects. 

This system is used mainly to evaluate the soil used in road construction especially materials used for road Embankment, Subgrade, subbase. The AASHTO classification system doesn't relies on the simple visual description, it relies mainly on lab testing such as sieve analysis and atterberge limits to classify the soil. 

AASHTO classification system used mainly to classify soil according to their expected performance as a road construction materials. In road engineering, the road subgrade support the road pavementayers above it. If the road Subgrade is weak or highly compressible, etc this will result in road settlement, cracks formation, Formation of rutting and low performance of Pavement. 

AASHTO assist Engineers in answering an important question, does the material suitable for the use in road construction or do we need to improve it or replace it. 

To classify soil according to AASHTO system we need to find the following information:

1. Percentage of materials passing sieve #10. 

2.Percentage of materials passing sieve #40.  

3.Percentage of materials passing sieve #200.

4. Liquid limit (LL).

5. Plasticity index (PI).

Sieve #200 cobsidered significantly important as it separates the fine and coarse particles. Soil that have high percentage of materials passing sieve #200 contain usually high amounts of silt and clay which reduces the material quality and makes it unsuitable in road construction. 

Soil groups according to AASHTO:

1.AASHTO classify the soil into seven geoups: A-1، A-2، A-3، A-4، A-5، A-6، A-7 and these groups classified into two general groups: granular material and silt-clay materials. 

1-Granular materials:

Soil classified as granular materials if the percentage of passing sieve #200 is equal or less than 35%. This type of soil will consist of gravel or sand or a mix of them. In general, granular soil considered as a good material for the road construction because it posses a high strength, good drainage characteristics and less compressibilty. 

Grangular soil includes the following groups:

-A-1: usually soil with A-1 classification is a well graded soil consist of gravel and sand. Considered as excellent to good materials for road Subgrade. A-1 soil often strong, stabe and drain well. 

-A-3: the soil in this group consist mainly of fine sand with low or no plasticity index. It may have a good drainage characteristics but it may become problematic if it loose and un-compacted. 

-A-2: granular soil that contain a considerable amount of silt and clay. The quality of this soil group depends on the amount of fine materials. It is subdivided into various groups such as A-2-4، A-2-5، A-2-6، A-2-7. 

2-Silt-Clay Materials:

According to AASHTO Silt and clay materials is material with more than 35% passing sieve #200. This material is weaker than granular soil when used as Subgrade materials especially when it contains plastic clay or absorb water. 

This soil includes the following groups:

-A-4: it is usually silty soil with low plasticity. It can loose strength when get moist and it is sensitive to moisture change. 

-A-5: it is elastic silty soil with higher liquid limit. It can be more compressible than A-4

-A-6: clay soil with moderate plasticity. A-6 shrink and swell with moisture change and it has a bad performance as Subgrade material. 

-A-7: clay soil with high plasticity, this soil considered as unsuitable material for Subgrade because it has a high compressibilty, low resistance and high volume change and it subdivided into A-7-5 & A-7-6

GROUP INDEX

In addition to soil group, Group index is a numerical index that indicates the quality of the soil material. Lower group index indicates better quality soil for example A-2-4(0) is having a better than A-7-6(20)

Group index depends on the percentage of material passing sieve #200, liquid limits and plastic index.

 

GI = (F − 35)[0.2 + 0.005(LL − 40)] + 0.01(F − 15)(PI − 10)

Where:


F = percentage passing the No. 200 sieve

LL = liquid limit

PI = plasticity index

If the value of GI is in negative then it is considered as zero and the final value of GI is rounded to nearest whole number. 

General Quality of AASHTO Soil Groups

The quality of soil as Subgrade materials decreases from A-1 to A-7. A-1 is the best soil as material for subgrade while A-7 is the poorest soil. 

A simple order of expected subgrade quality is:

A-1: Excellent to good

A-2: Good to fair

A-3: Good to fair, depending on compaction and drainage

A-4: Fair to poor

A-5: Poor

A-6: Poor

A-7: Very poor

This classification is considered as a general guide only and selection of materials should consider compaction, moisture, settlement, CBR, drainage and project requirements. 

Importance in Highway Engineering

The AASHTO classification considered a very useful as it provides a quick engineering judgment of the material suitably and quality of being used as a Subgrade material. Engineer can determine the suitability of soil through the classification of material into the correct group and determining the soil GI. 

Common soil improvement methods include:

-Compaction control

-Lime stabilization

-Cement stabilization

-Replacement with better material

-Geotextile or geogrid reinforcement

-Drainage improvement


Advantages of the AASHTO System: this systems is practical and simple and it is using common laboratory test. Furthermore it provides a clear picture of the soil quality as Subgrade material. 

Another advantage is the group index which can distinguish between two different soil of same group, for example if we have two different soil classified as A-6 material, for this case material with lower group index is better than the other material. 

Limitations of AASHTO system: the AASHTO classification system doesn't provide data needed for the design. It doesn't provide the soil strength, bearing capacity, settlement, permeability or soil behavior under compaction. Therefore, it should not be used alone in design. In important projects, Engineers should usw other tests such as compaction tests, CBR tests, direct shear tests, consolidation tests, permeability tests, and field density tests. 


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