FIELD IDENTIFICATION OF SOILS

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The methods of field identification of soils can conveniently be discussed under the headings of
coarse-grained and fine-grained soil materials.

Coarse-Grained Soil Materials

The coarse-grained soil materials are mineral fragments that may be identified primarily on the
basis of grain size. The different constituents of coarse-grained materials are sand and gravel. As
described in the earlier sections, the size of sand varies from 0.075 mm to 4.75 mm and that of
gravel from 4.75 mm to 80 mm. Sand can further be classified as coarse, medium and fine. The
engineer should have an idea of the relative sizes of the grains in order to identify the various
fractions. 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. The mineral grains can be rounded, subrounded, angular or subangular. The presence of
mica or a weak material such as shale affects the durability or compressibility of the deposit. A
small magnifying glass can be used to identify the small fragments of shale or mica. The properties
of a coarse grained material mass depend also on the uniformity of the sizes of the grains. A
well-graded sand is more stable for a foundation base as compared to a uniform or poorly graded
material.

Fine-Grained Soil Materials

Inorganic Soils: The constituent parts of fine-grained materials are the silt and clay fractions. Since
both these materials are microscopic in size, physical properties other than grain size must be used
as criteria for field identification. The classification tests used in the field for preliminary
identification are

  1. Dry strength test
  2. Shaking test
  3. Plasticity test
  4. Dispersion test

Dry strength: The strength of a soil in a dry state is an indication of its cohesion and hence of 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.
Shaking test: The shaking test 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 soft
consistency is placed in the palm of the hand. The surface of the soil is smoothed out with a knife
and the soil pat is shaken by tapping the back of the hand. If the soil is silt, water will rise quickly
to the surface and give it a shiny glistening appearance. If the pat is deformed either by squeezing
or by stretching, the water will flow back into the soil and leave the surface with a dull
appearance. Since clay soils contain much smaller voids than silts and are much less permeable,
the appearance of the surface of the 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 nonexistent.
Plasticity test: If a sample of moist soil can be manipulated between the palms of the hands and
fingers and rolled into a long thread of about 3 mm diameter, the soil then contains a significant
amount of clay. Silt cannot be rolled into a thread of 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 consists 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 finer
ones. Ordinarily sand particles settle within 30 seconds if the depth of water is about 10 cm. Silt
particles settle in about 1/2 to 240 minutes, whereas particles of clay size remain in suspension for
at least several hours and sometimes several days.

Organic soils: Surface soils and many underlying formations may contain significant amounts of
solid matter derived from organisms. While shell fragments and similar solid matter are found at
some locations, organic material in soil is usually derived from plant or root growth and consists of
almost completely disintegrated matter, such as muck or more fibrous material, such as peat. The
soils with organic matter are weaker and more compressible than soils having the same mineral
composition but lacking in organic matter. The presence of an appreciable quantity of organic
material can usually be recognized by the dark-grey to black color and the odor of decaying
vegetation which it lends to the soil.
Organic silt: It is a fine grained more or less plastic soil containing mineral particles of silt size
and finely divided particles of organic matter. Shells and visible fragments of partly decayed
vegetative matter may also be present.
Organic clay: It is a clay soil which owes some of its significant physical properties to the
presence of finely divided organic matter. Highly organic soil deposits such as peat or muck may be
distinguished by a dark-brown to black color, and by the presence of fibrous particles of vegetable
matter in varying states of decay. The organic odor is a distinguishing characteristic of the soil. The
organic odor can sometimes be distinguished by a slight amount of heat.

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