DHARMIDENTIFICATION AND CLASSIFICATION OF SOILS 93
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 between the two by the ‘Disper-
sion Text’. This test consists of pouring a spoonful of sample 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 one hour.
In both these cases, nothing may be left in the suspension ultimately.
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.
(i)Shaking Test. A part of the material is shaken after placing it in the palm. If it is
silt, water comes to the surface and gives it a shining apearance. If it is kneaded, the moisture
will re-enter the soil and the shine disappears.
If it is clay, the water cannot move easily and hence it continues to look dark. If it is a
mixture of silt and clay, the relative speed with which the shine appears may give a rough
indication of the amount of silt present. This test is also known as ‘dilatancy test’.
(ii)Strength Test. A small briquette of material is prepared and dried. Then one has to
try to break if. It it can be broken easily, the material is silt. If it is clay, it will require effort to
break. Also, one can dust off loose material from the surface of the briquette, if it is silt. When
moist soil is pressed between fingers, clay gives a soapy touch ; it also sticks, dries slowly, and
cannot be dusted off easily.
(iii)Rolling 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 the ends. This is also called the ‘Tough-
ness test’.
(iv)Dispersion Test. A spoonful of soil is poured in 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 for days, provided flocculation does not take place.
A few other miscellaneous identification tests are as follows :Organic Content and Colour
Fresh, wet organic soils usually have a distinctive odour of decomposed organic matter, which
can be easily detected on heating. Another distinctive feature of such soils is the dark colour.
Acid Test
This test, using dilute hydrochloric acid, is primarily for checking the presence of calcium
carbonate. For soils with a high value of dry strength, a strong reaction may indicate the
presence of calcium carbonate as cementing material rather than colloidal clay.
Shine Test
When a lump of dry or slightly moist soil is cut with a knife, a shiny surface is imparted to the
soil if it is a highly plastic clay, while a dull surface may indicate silt or a clay of low plasticity.
Considerable experience is required for field identification of soils. A knowledge of the
geology of the area will be invaluable in this process.