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Classification of soil – USDA, AASHTO, Unified classification methods

What is soil and why it should be classified?

Soil is a natural element of the earth and also a mixture of organic, inorganic materials together with gasses and liquids. Soil formation starts from weathering of parental rock and they exist as the first layer on the earth surface. In Civil Engineering, the study of soil and its composition is called as Geotechnical Engineering. Anyone can wonder that why this much of importance is given to soil in Civil Engineering? The reason is simple as the soil is the oldest construction element and any construction is built on soil or built with soil.

Soil should be classified into groups as to know the strength and structure of soil – whether they can resist the load of the construction structure or not and to know the type or ingredient of soil – whether the water content or other composition of soil affect the construction structure in normal condition or during any geo – hazard times. Also, the other advantage of soil classification is, Geo technicians and Engineers can easily understand the property of any soil obtained from unknown place from its laboratory test results.

Soil classification methods

The soil classification methods were developed step by step with different ideas. In early days soil was classified depending on its composition and their weight related to total mass. Then soil was classified depending on texture which was finally developed to triangular classification diagram method. But Geo 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.


USDA Textural soil classification method

This method is highly used for agricultural purpose. In this classification they 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 twelve textural classes of soil were noted within the triangle with the indication of thick line separation between those classes. It’s simply clear that the summation of the percentage of sand, silt and clay should give 100%.


AASHTO Classification method

AASHTO American Association of State Highway and Transportation Officials – classification method was introduced for selecting soil for the construction of highways and road. In this method the classification is done by considering the Atterberg limit and textural soil classification. The classification is done into seven major groups as, A1, A2, A3, A4, A5, A6 and A7. When soil granules of 35% or less than that passes the No.200 sieve, they will belongs to any of the group from A1 to A3 and the soil granules of more than 35% passes through the  200 sieve then they will belongs to any group from A4 to A7. A reference table which includes the ranges of all tests for easy classification is available. If one is needed to classify a soil then the laboratory test results should be applied from left to right on the above mentioned table.


To determine the quality of the soil, a parenthesis written after the groups is used and that parenthesis is – Group Index. Group index is given by the following equation,


In this calculation, when a negative result is obtained, it should be considered as zero and all the results should be rounded off to the nearest whole number. The soils with group index near to zero are considered as good soils.


Unified Classification Method

This is the method which is presently used by geo technical engineers world-wide to classify soil for their construction purpose – other than highway engineering. In this classification the soil type will denoted as a two letter symbol, in which the first letter defines the type of soil and the second letter defines the quality of soil. To classify the soil, the below symbol chart prepared on the basis of laboratory sieve test will be considered mainly.