Field identification of soils
Soils are generally divided on the basis of their particle size into coarse grained and fine-grained soils.
Coarse-grained soils are of particle size greater than 0.075 mm or 75 micron and particles smaller than this make up fine-grained soils.
Further coarse grains are divided into gravel and sand, and fine grains are divided into silt and clay sized.
To identify the type of soil in the field, first we try to distinguish the soil as coarse grained soil and fine grained soil and we can do so by simply looking at the soil.
If It contains more than 50% of visible particles then the soil is coarse grained soil and if the soil contains less than 50% of visible particles then the soil is fine grained soil. Thus, grain size itself may be adequate to distinguish between gravel and sand but silt and clay cannot be distinguished by this technique. Particles smaller than sand are microscopic and are either silt or clay. ....
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Field identification of soils
Soils are generally divided on the basis of their particle size into coarse grained and fine-grained soils.
Coarse-grained soils are of particle size greater than 0.075 mm or 75 micron and particles smaller than this make up fine-grained soils.
Further coarse grains are divided into gravel and sand, and fine grains are divided into silt and clay sized.
To identify the type of soil in the field, first we try to distinguish the soil as coarse grained soil and fine grained soil and we can do so by simply looking at the soil.
If It contains more than 50% of visible particles then the soil is coarse grained soil and if the soil contains less than 50% of visible particles then the soil is fine grained soil. Thus, grain size itself may be adequate to distinguish between gravel and sand but silt and clay cannot be distinguished by this technique. Particles smaller than sand are microscopic and are either silt or clay. We need to perform a couple of tests to distinguish between them.
The procedures are given briefly hereunder.
i) Gravel from sand:
Individual soil particles larger than 4.75 mm and smaller than 80 mm are called gravel; soil particles ranging in size from 4.75 mm down to 0.075 mm are called sand. Field identification of sand and gravel should also include identification of mineralogical composition, if possible.
ii) Sand from silt:
Fine sand cannot be easily distinguished from silt by simple visual examination. Silt may look a little darker in colour. However, it is possible to differentiate the two by the dispersion test.
Dispersion Test
This test consists of pouring a spoonful of sample in a jar of water. If the material is sand, it will settle down in a minute or two, but if it is silt, it may take 15 minute to 1 hour. In both the cases, nothing may be left in the suspension ultimately.
iii) Silt from clay:
Microscopic examination of the particles is possible only in the laboratory. In the absence of one, a few simple tests are performed.
a) Shaking test (Dilatancy test)
In this test, a part of soil mixed with water to a very soft consistency is shaken after placing in the palm of the hand. If the soil is silt, water will rise quickly to the surface and give it a shiny glistening appearance.
If it is clay, the water cannot move easily and hence it continues to look dark. An estimate of relative proportions of silt and clay in an unknown soil mixture can be made by noting whether the reaction is rapid, slow or nonexistent.
b) Dry strength
The strength of a soil in a dry state is an indication of its cohesion and hence its nature. It can be estimated by crushing a 3 mm size dried fragment between thumb and forefinger. A clay fragment can be broken only with great effort, whereas a silt fragment crushes easily.
c) Rolling test (Toughness test)
A thread is attempted to be made out of a moist soil sample with a diameter of about 3 mm. If the material is silt, it is not possible to make such a thread without disintegration and crumbling. If it is clay, such a thread can be made even to a length of about 30 cm and supported by its own weight when held at ends.
d) Dispersion test
A spoonful of soil is poured in a jar of water. If it is silt, the particles will settle in about 15 minutes to one hour. If it is clay, it will form a suspension which will remain as such for hours, and even days, provided flocculation does not take place.
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