On Site Identification of Silt & Clay

The distinction between silt & clay cannot be based on particle size because the significant physical properties of the two materials are related only indirectly to the size of particles. Furthermore, since both are microscopic, physical properties other than particle size must be used as criteria for field identification. There are 4 nos of field test that we can easily conduct on the field to identify silt & clay. These are

  • Dry strength
  • Dilatancy or Shaking test
  • Toughness to plasticity
  • Dispersion

Let us discuss each one of them one by one.

Gravel.Sand.Silt & Clay
Gravel.Sand.Silt & Clay

1. Dry Strength Test

The dry strength provides one basis for distinction. A small briquette of the soil is molded and allowed to dry in the air. It is then broken and a small fragment about 1.0 cm in size is pressed between thumb and forefinger. The effort required to break the fragment provides a basis for describing the strength as ver low, low, medium, high or very high. A clay fragment can be broken only with great effort, where as a silt fragment crushes easily.

2. Dilatancy or Shaking Test

Since silts are considerably more permeable than clays, the dilatancy or shaking test may also be used to distinguish between the two materials. In this test a small amount of soil is mixed with water to a very soft consistency in the palm of the hand. The back of the hand is then lightly tapped. If the soil is silty, water rises quickly to its surface and gives it a shiny or glistening appearance. Then if the soil pat is deformed, in some instances by squeezing and in others by stretching, the water flows back into it and leaves the surface with a dull appearance. Usually, the greater the proportion of clay in the sample, the slower the reaction to the test. The reaction is described as rapid, slow or none.

3. Toughness / Plasticity Test

The property of plasticity is characteristic of clays and may be used as the basis for a simple field test. At certain moisture contents a soil that contains appreciable quantities of clay can be deformed and remolded in the hand without disintegration. Thus, if a sample of moist soil can be manipulated between the palms of the hands and fingers and rolled out into a long thread, it unquestionably contains a significant amount of clay. As moisture is lost during continued manipulation, the soil approaches a non-plastic condition and becomes crumbly. Just before the crumbly state is reached, a highly plastic clay can be rolled into a long thread, with a diameter of approximately 3mm, which has sufficient strength to support its own weight. Silt, on the other hand, can seldom be rolled into a thread with a diameter as small as 3mm without severe cracking, and is completely lacking in tensile strength unless small amounts of clay are present. The record of a simple plasticity test should indicate not only whether a plastic thread can be formed, but also the toughness of the thread as it nears the crumbling stage. This condition is described as weak and friable, medium, or tough.

4. Dispersion Test

The fourth procedure, known as the dispersion test, is also useful for distinguishing between silt & clay, and for making a rough estimate of the relative amounts of sand, silt and clay in a material. A small quantity of the soil is dispersed with water in a glass cylinder or test tube and then allowed to settle. The coarser particles fall out first and the finest particles remain in suspension the longest. Ordinarily sand settles in 30 to 60 sec. Materials of silt size settles in 15 to 60 min, whereas that of clay size remains in suspension for at least several hours and usually for several days unless the particles of clay combines in groups or floccules.


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