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In this article, we will discuss the field identification of soil.
Soil classification is the arrangement of soils into different groups such that the 5 as in a particular group have similar behaviour. It is a sort of labelling of soils so different levels. As there is a wide variety of soils covering the earth, it is desirable to systematize or classify the soils into broad groups of similar behaviour.
It is more convenient to study the behaviour of s than that of individual soils.
For a soil classification system to be useful to geotechnical engineers, it should have the following basic requirements:
a. it should have a limited number of groups.
b. It should be based on the engineering properties which are most relevant for the purpose for which the classification has been made.
c. It should be simple and should use terms that are easily understood.
It may be mentioned that soil classification is no substitute for exact analysis based on engineering properties. For the final design of the large structure, the engineering properties should be determined by conducting elaborate tests on undisturbed samples.
2. Field Identification of Soil
Basically, coarse-grained and fine-grained soils are distinguished based on whether the individual soil grains can be seen with the naked eye or not. Thus, grain size itself may be adequate to distinguish between gravel and sand. but silt and clay cannot be distinguished by this technique.
Field identification of soil becomes easier if one understands how to distinguish gravel from sand, sand from silt and silt from clay. The procedures are given briefly hereunder. Field Identification of Soil can help us to study and know the properties of that soil.
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.
This test consists of pouring a spoonful of samples 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 minutes to 1 hour. In both 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 it 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 soil in a dry state is in nature. It can be estimated by crushing the thumb and forefinger. A clay fragment whereas a silt fragment crushes easily.
|Read Also: Unified Soil Classification System|
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.
|Read Also: Soil Classification Systems|
d) Dispersion test
A spoonful of soil is poured into 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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