Atterberg and shrinkage limits liquid & plastic limits

Objective: To determine the range of water content which exhibits consistency of a given soil sample at liquid limit, plastic limit and shrinkage limit.

Theory:

Liquid limit (LL): It is the water content at which a 2 mm wide groove in a soil pat will close for a distance of 12.5mm when dropped 25 times in a standard liquid limit device (casagrade apparatus).

Plastic limit (PL): It is the water content at which a thread of soil just begins to crack and crumble when rolled to a diameter of 3mm.

Shrinkage limit (SL): It is the maximum water content at which no change in volume of the soil mass occurs when the water content is further reduced.

Plasticity index (PI): The plasticity index (PI) is defined as;

PI=LL–PL

Liquidity index (LI): This index is defined as;

LI = (w – PL) / (LL – PL)

—Apparatus

—Liquid limit device and grooving tool—Shrinkage limit set consisting of shrinkage dish, glass plate with prongs, mercury and evaporating dish—Large glass plate for plastic limit—Distilled water—Oven——Balance of 0.01g accuracy—Dessicator—Water can—Spatula, cotton waste, duster and grease Procedure:

—Liquid Limit—Take about 120g of dry soil passing 425µ IS sieve and mix it thoroughly with distilled water using spatula until the soil mass becomes homogenous paste.

—Adjust the liquid limit device with the aid of the gauge on the grooving tool to obtain the fall of cup equal to 1cm above the base. Turn the handle and practice to obtain a speed of 2 blows per second.

—Place the soil paste in the cup, level up to a depth of 1cm at the point, which comes in contact with the base. Divide this paste by drawing the grooving tool through the sample along the symmetrical axis of the cup, holding the tool perpendicular to the cup at the point of contact.

—Turn the handle at the rate of two revolutions per seconds and count the blows necessary to close the groove in the soil for a distance of 12mm.

—Take the sample of the soil paste from near the closed groove and keep in the water can to determine the water content.

—Repeat the above process three or four times by adding some distilled water each time. Adjust the amount of water by visual judgment so that two readings are above 25 and two below 25 blows. However, the blows should not be lower than 15 nor greater than 35.

— Preserve this soil for shrinkage limit test.

—Draw the flow curve, and determine the liquid limit.

—Plastic limit—Take about 20g of soil passing through the sieve of size 425Pm and place it on a glass plate. Mix it thoroughly with distilled water until it is plastic enough to be rolled.

—

—Roll the soil between the hand and the glass plate so as to form a thread of 3mm diameter.

—Knead the soil together and roll again until a 3mm diameter thread shows sign of crumbling.

—Take some of the crumbling material obtained in step 3 for water content determination. This defines the plastic limit.

—Repeat steps 2 to 4 three times so as to obtain average plastic limit.

—Shrinkage Limit

—Place about 30g of the soil fraction passing 425Pm IS sieve in a porcelain dish and thoroughly mix it with distilled water. The water should be added to make the soil slightly flowing.

——Note: The amount of required water could be about the liquid limit in low plastic (friable) soils; otherwise it could be about 1.1 to 1.2 times the liquid limit in medium to high plastic soils.

—Apply a thin coat of grease to the inside of the shrinkage dish and measure its empty weight.

—Place the soil paste in the shrinkage dish, simultaneously tapping it so that it fills completely the dish without entrapping any air bubbles. Weigh the dish with soil paste inside.

—Keep the dish in the oven set at 105 to 1100C for 24 hours. Take the dry weight of the soil pat.

——Determine the volume of the dry soil pat by mercury displacement method *

—Determine the volume of the dish by filling it with mercury.

——Enter the observation in the record sheet and compute the shrinkage limit

—*****Important: Every precaution should be taken while handling mercury (e.g. your finger nails should be trimmed). For detailed safety requirements, you are advised to refer to the web page http://www.ilpi.com/safety/mercury.html

—References:

—IS 2720 (Part 5): Determination of liquid and plastic limit

——IS 2720(Part 6): Determination of shrinkage factors

—Thank’s for watching

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