The AASHTO system uses both grain-size distribution and Atterberg limits data to assign a group classification and a group index to the soil. The group. The AASHTO classification system identifies two general classifications: (1) granular materials having 35 percent or less by weight smaller than mm and (2) silt-clay materials having more than 35 percent by weight smaller than mm. AASHTO American Association of State Highway and Transportation Officials – classification method was introduced for selecting soil for the.


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Always round off the GI to nearest whole number.


The typical material of this group is a well-graded mixture of stone fragments or gravels, coarse sand, fine aashto classification of soil, and a non-plastic or sligthly plastic soil binder. This group also includes stone fragments, gravels, coarse sand, volcanic cinders etc, without a well-graded binder of fine material.

Classification of soil - USDA, AASHTO, Unified classification methods - Basic Civil Engineering

The typical material of this group is fine beach sand or fine desert blown sand without silty or clayey fines or with a small amount of non-plastic silt. This group includes also stream-deposited mixtures of poorly graded aashto classification of soil sand and limited amounts of coarse sand and gravel.

The typical material of this group is similar to that described under Group A-4, but it may be highly elastic, as indicated by high liquid limit.


If all have been met, then this is the group classification. If any criterion is not met, move to the right and repeat the process, continuing until all the criteria have been satisfied.

Classification of soil – USDA, AASHTO, Unified classification methods

Do NOT begin at the middle of the chart. For all soils, express the group index as a whole number.

Computed group index values of less than zero should be reported as zero. Then soil was classified depending on texture which was finally developed to triangular classification diagram method.

But Aashto classification of soil technicians found that this method is more suitable for agriculture than Geo technical engineering. Finally, Sir Arthur Casagrande, classified the soil depending on size distribution and consistence of soil particles.

With some development such as considering the mechanical properties of soil to the method of Airfield Classification System by Arthur Casagrande, the British soil classification system was introduced.

In this classification aashto classification of soil believed that the properties of soil are determined by its composition. Here, the percentages of soil compositions such as sand, silt and clay which are obtained from laboratory test is used to determine the textural soil classification.

In this method a textural triangle chart is used to determine the classification. In that chart the corners of triangle indicates the hundred percent of each composition — sand, silt, clay and also the aashto classification of soil textural classes of soil were noted within the triangle with the indication of thick line separation between those classes.

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