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
1. Liquid limit device and grooving tool
2. Shrinkage limit set consisting of shrinkage dish, glass plate with
prongs, mercury and
evaporating dish
3. Large glass plate for plastic limit
4. Distilled water
5. Oven
6. Balance of 0.01g accuracy
7. Dessicator
8. Water can
9. Spatula, cotton waste, duster and grease
Procedure
Liquid Limit
1. Take about 120g of dry soil passing 425Pm IS sieve and mix it
thoroughly with
distilled water using spatula until the soil mass becomes homogenous
paste.
2. 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.
3. 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.
4. 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.
5. Take the sample of the soil paste from near the closed groove and keep
in the water
can to determine the water content.
6. 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.
7. Preserve this soil for shrinkage limit test.
8. Draw the flow curve, and determine the liquid limit.
Plastic limit
1. 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.
2. Roll the soil between the hand and the glass plate so as to form a
thread of 3mm
diameter.
3. Knead the soil together and roll again until a 3mm diameter thread
shows sign of
crumbling.
4. Take some of the crumbling material obtained in step 3 for water
content
determination. This defines the plastic limit.
5. Repeat steps 2 to 4 three times so as to obtain average plastic
limit.
Shrinkage Limit
1. 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.
2. Apply a thin coat of grease to the inside of the shrinkage dish and
measure its empty
weight.
3. 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.
4. Keep the dish in the oven set at 105 to 1100
C for 24 hours. Take the dry weight of the
soil pat.
5. Determine the volume of the dry soil pat by mercury displacement
method *
6. Determine the volume of the dish by filling it with
mercury.
7. 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
Calculation
1. Liquid limit: A graph is drawn between the Number of blows (on log
scale) that are
required to close the 2mm wide groove in soil for a distance of 12.5mm
and the
corresponding moisture content of the soil (on linear scale). The
relationship is linear
and referred to as flow curve. From this, the water content corresponding
to the 25
blows is noted and reported as the liquid limit.
2. Plastic limit: Water content of the soil when it is crumbled when 3mm
threads were
rolled.
3. Alternatively, shrinkage limit (SL) can also be calculated using the
following
formula:
