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Identification of soil is the first and most important step in soil investigation for engineering works. It is the determination of soil-type through which crucial information about the soil properties can be obtained.
Approximate field procedure is a technique by which the type of soil can be determined rapidly in the field. As soon as the soil types are identified, crucial soil properties that control its behavior would be known.
Based on the field identification of the soil, the presumptive bearing capacity of the soil can be guessed by referring to Table-2 of IS:1904 – 1986. Additionally, the result of approximate identification of soil in the field can be used to set an appropriate program for detailed soil investigation.
Soil Identification- Approximate Field Procedure
- Differentiate sand and gravel through visual inspection.
- Fine sand and silt can be differentiated by shaking a spoonful of soil mixed with water in a deep jar(or tube) to make a suspension. Sand would settle in 1.5 minutes, but silt needs 5 minutes or more to settle.
- The above test can be used for clay which requires 10 minutes or more to settle. The relative quantity of materials (sand, silt, and clay) can be specified by observing the depth of the materials sediments at the bottom of the jar.
- Inorganic and organic soils can be differentiated using visual inspection.
- Smell test can be used for wood or plant-decay
- Silt and clay can be differentiated using the following steps:
- Silt lump can be crushed with fingers more easily compared to clay lump.
- Moisten a spot on the soil lump and rub a finger across it; if it is smooth it is clay, if marginally streaked-it is clay with silt, and if rough- it is silt.
Consistencyof soil can be determined by checking the ease with which a soil lump gets moulded.
- Stiff consistency: cannot be moulded with the finger.
- Medium consistency: can be moulded by the fingers on strong pressure and readily indented with a thumbnail.
- Soft consistency: easily moulded with the fingers.