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. In this method a textural triangle chart is used to determine the classification.

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 classification they believed that the properties of soil are determined by its composition. This method is highly used for agricultural purpose. 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. Finally, Sir Arthur Casagrande, classified the soil depending on size distribution and consistence of soil particles. But Geo technicians found that this method is more suitable for agriculture than Geo technical engineering. Then soil was classified depending on texture which was finally developed to triangular classification diagram method. In early days soil was classified depending on its composition and their weight related to total mass. The soil classification methods were developed step by step with different ideas. 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 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. 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. In Civil Engineering, the study of soil and its composition is called as Geotechnical Engineering. Soil formation starts from weathering of parental rock and they exist as the first layer on the earth surface. Soil is a natural element of the earth and also a mixture of organic, inorganic materials together with gasses and liquids. What is soil and why it should be classified?

Aashto material classification