Explain the phenomenon of identification of soils.

The method of identification of soil can be discussed under the heading  of coarse grained and fine grained soil materials. Gravel, sand and possibly coarse salts fall into coarse grained varieties and silt and clay fall into fine grained categories.

• Course Grained soil materials (gravel vs sand)
• sand vs silt
• fine grained soil materials (silt versus clay)

The coarse grained soil material are mineral fragments that may be identified primarily on the basis of grain size. The different constituent of coarse grained material are: sand and gravel. The size of sand varies from 0.075 mm to 4.75 mm and that of gravel from 4.75 mm 280 mm. The engineers should have an idea on the relative size of the grains in order to identify soil. If the percentage of gravel is greater than sand then the soil is gravel else sand. The description of sand and gravel should include an estimate of the quantity of material in the different ....

The method of identification of soil can be discussed under the heading  of coarse grained and fine grained soil materials. Gravel, sand and possibly coarse salts fall into coarse grained varieties and silt and clay fall into fine grained categories.

• Course Grained soil materials (gravel vs sand)
• sand vs silt
• fine grained soil materials (silt versus clay)

Coarse grained soil materials:

The coarse grained soil material are mineral fragments that may be identified primarily on the basis of grain size. The different constituent of coarse grained material are: sand and gravel. The size of sand varies from 0.075 mm to 4.75 mm and that of gravel from 4.75 mm 280 mm. The engineers should have an idea on the relative size of the grains in order to identify soil. If the percentage of gravel is greater than sand then the soil is gravel else sand. The description of sand and gravel should include an estimate of the quantity of material in the different size ranges as well as  a statement of the shape and mineralogical composition of the grains.

To identify the coarse grained material, soil samples is first spread on the surface. If more than 50% of the particles are visible to the naked eye, the soil is coarse grained. This is the simple test for the identification of coarse grained materials.

Sand versus silt:

Many times it become difficult to distinguish sand from silt by simple visual examination especially for fine sand. Both the material vendre look like dust except that silt maybe slightly darker in colour than sand. However they can be differentiated by means of dispersion test explained later.

Fine grained soil materials:

The different constituent of fine grained material are silt and clay fraction. Since both these material are microscopic in size, physical properties other than grains size must be used as criteria for field identification.

The classification test using field for preliminary identification are:

• Dry strength test,
• Shaking test (dilatancy test),
• plasticity test (rolling or thread test)(toughness test)
• dispersion test

Dry strength test:

The strength of soil in a dry state is an indication of its cohesion and hence its nature. It can be estimated by crushing of 3 mm size of a dried fragment between Thumb and forefinger. A clay fragment can be broken only with a great effort whereas a silt fragment crushes easily.

Shaking or dilatancy test:

Shaking test it is also called as dilatancy test. It helps to distinguish silt from clay since silt is more permeable than clay. In this test, a part of soil mixed with water to a very short consistency is placed in a Palm of the hand. The surface of the soil is smoothed out with a knife and the soil part is shaken by tapping the back of the hand. If the soil is silt, the water will rise quickly to the surface and give it  a shing glistening appearance. If the pat is is then deformed either  by squizzing or by stretching, the water will flow back into the soil and leave the surface with a dull appearance. since the clay soil contain much smaller voids than silt and are much less permeable, the appearance of the surface of pat does not change during the shaking test. An estimate of the relative proportions of silt and clay in an unknown soil mixture can be made by noting whether the reaction is Rapid slow or non existence.

Very fine clean sand gives the clearest and most distinguished reaction whereas  plastic clay has no reaction. Silt shows a moderately quick reaction.

Toughness test (rolling or thread or plasticity test):

If a sample of moist soil can be manipulated between the palm of the hand and fingers and rolled into a long thread of about 3 mm diameter, the soil then contains a significant amount of clay. However Silt cannot be rolled into 3 mm diameter without Severe cracking.

Dispersion test:

This test is useful for making a rough estimate of sand,silt and clay present in a material. The procedure consist in dispersing a small quantity of the soil in water taken in a glass cylinder and allowing the particles to settle. The coarser particles settle first followed by the finer ones. Ordinarily sand particle settle within about 30 seconds if the depth of water is about 10 cm. Silt particle settle in about  0.5 to  240 minutes where as the particles of clay size remain in suspension for at least several hour and sometimes several days.

The time required for some of the particle size to settle through or 10 cm is given below:

The table indicates that colloidal particle finer than 0.0002 mm will not generally settle because of Brownian Movement.