Maximum water content at which further reduction in water content does not cause any reduction in the volume of the soil sample. When the water content is reduced below ${w}_{s}$, the particles are so closely packed that volume reduction will not take place and the void space starts getting occupied by air instead of water.
The test for the determination of Shrinkage Limit of Soil is as per IS 2720 Part-6. The test is performed using Evaporating Dish, Spatula, Shrinkage Dish (flat base, 45 mm in diameter and 15 mm in height), Straight Edge (~15 mm in length), Glass cup (50 to 55 mm in diameter and 25 mm in height), Glass plate ( in numbers, 75 X 75 mm and 3 mm thick) and Mercury.
Procedure (Remoulded Soil):
- Take a sample weighing about 100 gram from the thoroughly mixed portion of soil passing through 425 microns IS sieve.
- Place about 30 gram of soil from the soil sample already taken in step 1, in an evaporating dish and thoroughly mixed with distilled water in an amount sufficient to fill the soil voids completely and to make the soil paste enough to be readily worked into the shrinkage dish without entrapping the air bubble.
- Determine the clean empty Shrinkage Dish.
- Determine the Volume of Shrinkage Dish by filling it to overflowing with mercury, removing the excess by pressing the plain glass plate firmly over the top of the shrinkage dish in such a way that the plate is flush with the top of the dish and no air is entrapped.
Weigh the mercury held in the dish and divide it by the unit weight of mercury to obtain the Volume of the Dish. - Coat the inside of the shrinkage dish with a thin layer of grease or vaseline or some other heavy grease.
- Place soil paste equal to 1/3rd of the volume of the dish (approx) and allow it to flow to edges of the dish by tapping the dish on a firm surface.
Add a similar amount of soil paste and repeat the step until the dish is completely filled. Strike off the excess soil using a straight edge. - Weigh the Shrinkage Dish filled with Soil paste immediately and allow the soil pat to dry in the air until its colour changes from dark to light.
- Oven dries the soil pat in the shrinkage dish at $105-110{}^\circ C$ and cool it down in a Desiccator and weigh it immediately after removing from desiccator.
- Fill the glass cup with mercury and remove the excess mercury by pressing the glass plate with prongs firmly over the top of the cup and collect the excess mercury in a suitable container. Place the cup filled with mercury in evaporating dish carefully and then place the oven-dried soil pat on the surface of the mercury in the glass cup by means of the glass plate with same prongs and press the plate firmly over the top of the cup. Weigh the displaced mercury in the evaporating dish and obtain the volume of the soil pat.
Procedure (Undisturbed Soil):
- Take undisturbed Soil sample pats approximately 45 mm in diameter and 15 mm in height. Round off their edges to prevent the entrapment of air during mercury displacement.
- Take the sample in a small dish and air dry it.
- Oven dry the sample at $105-110{}^\circ C$
- Remove the sample from the oven and smoothen the edges of the pat by sandpapering. Brush off the soil dust from the specimen with a soft paintbrush.
- Place it on a clean dish and over dry it again.
- Cool the oven-dry specimen in a desiccator and determine the weight of the oven dry specimen.
- Determine the Volume of the specimen by displacing mercury as done for the remoulded soil (Step 9).
- Determine the Specific Gravity of the soil as per IS 2720-Part 3.
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