The foundation engineers need to classify the site soils for use as a foundation for some important reasons. These reasons are:
1. To be able to use the database of others in predicting foundation performance.
2. To build one's own local database of successes (or any failures).
3. To maintain a permanent record that can be understood by others should problems later develop and outside parties be required to investigate the original design.
4. To be able to contribute to the general body of knowledge in common terminology via journal papers or conference presentations. After all, if one is to partake in the contributions of others, one should be making contributions to the general knowledge base and not be just a "taker."
It is sometimes useful to be able to make a rapid field identification of the site soil for some purpose. This can be done approximately as follows:
1. Differentiate gravel and sand by visual inspection.
2. Differentiate fine sand and silt by placing a spoonful of the soil in a deep jar (or test tube) and shaking it to make a suspension. Sand settles out in 1.5 minutes or less whereas silt may take 5 or more minutes. This test may also be used for clay, which takes usually more than 10 minutes. The relative quantities of materials can be obtained by observing the depths of the several materials in the bottom sediment.
a. Clay lumps are more difficult to crush using the fingers than silt.
b. Moisten a spot on the soil lump and rub your finger across it. If it is smooth it is clay;
if marginally streaked it is clay with silt; if rough it is silt.
c. Form a plastic ball of the soil material and shake it horizontally by jarring your hand. If the material becomes shiny from water coming to the surface it is silt.
4. Differentiate between organic and inorganic soils by visual inspection for organic material or a smell test for wood or plant decay odor.