Field identification tests for soils


  • 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.

Soil Identification- Approximate Field Procedure


  1. Wet and Manipulated Strength Tests:- Take a small quantity of the soil specimen in hand moisten it if needed, and work it with fingers and feel it. If the soil is clayey, a soapy touch is felt; if the soil is sandy, a feeling of roughness is experienced and in the cse of silty soils, when the soil is squeezed in between fingers, the moisture comes out. Also clay sticks to the fingers and dries slowly, but silt dries fairly quickly and can be dusted off the fingers leaving only a stain. The test helps to distinguish the predominant soil characteristic, that is, whether it is clayey, sandy or silty.
  2. Thread Test:- Take a specimen of soil about one centimeter cube in size, moisten, if needed, and roll it between the palms of the hands or on a flat, smooth surface into a thread of about 3mm. in diameter. If crumbling does not occur, fold the thread, knead and reroll as before. Repeat the process until the moisture content of the soil has been reduced, by drying during manipulation, to the plastic limit, which is indicated by crumbling which occurs as the soil is being rolled. The characteristic of the thread as it approaches the plastic limit affords the means of identification of the soil.
  3. Dilatancy Test:- After removing particles retaining on IS Sieve 40, prepare a pat of moist soil of a size of 2 cm., cube. Add enough water, if necessary, to make the soil soft but not sticky. Place the pat in the open palm of one hand and shake horizontally striking vigorously against the other hand several times. A positive reaction consists of the appearance of water on the surface of the pat which changes to a livery consistency and become glossy. When the sample is squeezed between the fingers the water and loss disappear from the surface, the pat stiffens and finally it cracks or crumbles. The rapidity of appearance of water during shaking and of its disappearance during squeezing assist in identifying the character of the fines in a soil. Very fine clean sands give the quickest and most distinct reaction whereas a plastic clay has no reaction. Inorganic silts, such as a typical rock flour, show a moderately quick reaction.
  4. Dry Strength Test:– After removing particles retaining on IS Sieve 40, mould a pat of soil to the consistency of putty adding water, if necessary. Allow the pat to dry completely by oven, sun or air drying, and then test its strength by breaking and crumbling between the fingers. This strength is a measure of the character and quantity of the colloidal fraction contained in the soil. The dry strength is characteristic for clays of the CH group. A typical inorganic silt possesses only very slight dry strength. Silty fine sands and silts have about the same slight dry strength, but can be distinguished by the feel when powdering the dried specimen. Fine sand feels gritty where as a typical silt has the smooth feel of flour.
  5. Differentiate sand and gravel through visual inspection.
  6. 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.
  7. 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.
  8. 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.


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