The following manual or field tests are useful for preliminary identification of the fine-grained soil before conducting regular laboratory tests such as sieve analysis, sedimentation analysis, or tests on consistency limits.
These manual tests are conducted on soils by passing them through the 425-µm IS sieve.
i. A soil sample of volume 5 cm3 in the form of a soil pat is taken.
ii. Enough water is added to saturate the soil.
iii. The soil pat is placed in the open palm of one hand and shaken horizontally, striking vigorously against the other hand several times. If water appears on the surface of the soil pat, it indicates the presence of silt.
iv. The soil pat is then squeezed with fingers. The disappearance of surface water is another indication of the reaction to the shaking.
v. The reaction to this shaking in terms of appearance and disappearance of water on shaking and squeezing is recorded as rapid or slow.
The test is conducted by determining the consistency near the plastic limit of the given soil sample, as follows:
i. The soil pat used in the dilatancy test is dried by working and molding until it has the consistency of putty.
ii. The soil pat is rolled into a thread of 3-mm diameter and then kneaded back and rerolled into a 3-mm-diameter thread until the moisture content is gradually reduced so that the 3-mm-diameter thread just begins to crumble.
iii. The resistance to molding at this plastic limit water content is called toughness. If high pressure is required to roll the thread between the palms of the hand, the soil is described as having high toughness. Medium toughness is indicated when medium pressure is required, whereas low toughness is indicated by a weak thread that breaks easily and cannot be lumped together when drier than the plastic limit.
iv. Non-plastic soils cannot be rolled into threads of 3-mm diameter at any moisture content.
The soil pat is completely dried and its resistance to crumbling and powdering between fingers is measured. This resistance, called dry strength, is designated low if the dry pat can be easily powdered, medium if considerable finger pressure is required, and high if it cannot be powdered at all.
The presence of high-strength water-soluble cementing materials, such as calcium carbonate or iron oxide, may cause high dry strength.
4. Acid Test:
The acid test is conducted by testing the reaction of the soil to dilute HCl, to detect the presence of calcium carbonate. For soils with a high dry strength, a strong reaction indicates that the strength may be due to the calcium carbonate (acting as a cementing agent) rather than the colloidal clay.
It is a quick test to detect the presence of clay in soils. The test is conducted by cutting a lump of dry or slightly moist soil with a knife. A shiny surface indicates highly plastic clay, whereas a dull surface indicates silt or clay of low plasticity.
A small soil sample is rubbed with fingers. Rough and gritty feel indicates sand, whereas smooth feel indicates silt. Wet clay sticks to the fingers and dries slowly, but silt dries fairly quickly and can be dusted off the hands, leaving only a stain.