The City College of New York
Department of Civil Engineering
CE 345: Soil Mechanics
Instructor: Dr. George Mylonakis
LAB EXPERIMENT #2:
LIQUID LIMIT AND PLASTIC LIMIT
Introduction
When a cohesive soil is mixed with an excessive amount of water, it will be in a liquid state and flow like a viscous liquid. When the viscous liquid dries gradually due to loss of moisture, it will pass into a plastic state. With further loss of moisture, the soil will pass into a plastic state. With even further reduction of moisture, the soil will pass into a semi-solid and then into a solid state.
The moisture content, w, (%) at which the cohesive soil will pass from a liquid state to a plastic state is called the liquid limit of the soil. Similarly, plastic limit and shrinkage limit can be explained. These limits are called Atterberg limits.
Atterberg Limits
Moisture content increasing
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Solid
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Semisolid
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Plastic
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Liquid
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Shrinkage Limit (SL) Plastic Limit (PL) Liquid Limit (LL)
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Equipment
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Casagrande liquid limit device
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Grooving tool
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Moisture cans
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Porcelain evaporating dish
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Spatula
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Oven
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Balance sensitive up to 0.01g
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Plastic squeeze bottle
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Paper towels
Procedure
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Determine the mass of moisture cans (W1).
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Put 250g of air-dry soil, passed through No. 40 sieve into an evaporating dish. Add water and mix the soil to the form of a uniform paste.
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Place some soil paste into the liquid limit device. Smooth the surface with a spatula such that maximum depth is 8 mm.
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Using the grooving tool, cut a groove along the centerline of the soil pat.
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Turn the crank at the rate of 2 revs. / second. Count the number of blows (N) for the groove in the soil to close through a distance of ½ in. If N = 25-35, collect a moisture sample from the cup to a moisture can and determine the mass (W2).
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If N < 25, place the soil back to the evaporating dish and clean the device. Stir the soil (to dry it up) with spatula. Then redo steps 3, 4 and 5.
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Remove the soil from the cup of LL device and clean it carefully.
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Add more water to the soil paste in the evaporating dish and mix well. Repeat steps 3, 4 and 5 to get N = 20-25. Take a moisture sample from the cup. Clean the LL device.
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Add more water to the soil paste in the evaporating dish and mix well. Repeat steps 3, 4 and 5 to get N = 15-20. Take a moisture sample from the cup. Clean the LL device.
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Put three moisture cans in the oven to dry to constant mass (W3).
Calculation
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Calculate mass of can, W1 (g)
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Calculate mass of can + moist soil, W2 (g)
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Calculate mass of can + dry soil, W3 (g)
4. Determine the moisture content for each of the three trials as
(W2 - W3) x 100
w (%) =
(W3 - W1)
Graph
Plot a semi-log graph between w (%) (arithmetic scale) vs. N (log scale). It should be a straight line, which is called the flow curve. From the straight line, determine the moisture content w (%) corresponding to 25 blows. This is the liquid limit (LL) of the soil. The magnitude of the slope of the flow line is called the flow index (FI).
w1 (%) - w2 (%)
FI =
log N2 - log N1
PLASTIC LIMIT TEST
Introduction
Plastic limit is defined as the moisture content, in percent, at which a cohesive soil will change from a plastic state to a semisolid state. In the lab. the plastic limit is defined as the moisture content (%) at which a thread of soil will just crumble when rolled to a diameter of 1/8 in. (3.18 mm).
Equipment
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Moisture cans
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Porcelain evaporating dish
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Spatula
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Ground glass plate
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Balance sensitive up to 0.01 g
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Plastic squeeze bottle
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Oven
Procedure
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Put 20g of air-dry soil, passed through No. 40 sieve into an evaporating dish.
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Add water and mix the soil thoroughly.
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Determine the mass of moisture cans (W1).
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From the moist soil prepared in step 2, prepare several ellipsoidal-shaped soil masses by squeezing the soil with fingers.
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Take one of the ellipsoidal-shaped soil masses and roll it on a ground glass plate using the palm of the hand. The rolling should be done at the rate of 80 strokes/min. Note that one complete backward and one complete forward motion of the palm constitute a stroke.
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When thread of soil reaches 1/8” in diameter, break it up in to several small pieces and squeeze it to form an ellipsoidal mass again.
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Repeat steps 5 and 6 until the thread crumbles into several pieces when d = 1/8”.
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Collect the small crumbled pieces into the moisture can and put the cover on the can.
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Take the other ellipsoidal soil masses formed in step 4 and repeat steps 5 through 8.
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Determine the mass of moisture can plus wet soil (W2).
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Place moisture can into the oven to dry to constant mass (W3).
Calculations
1. Calculate mass of can , W1 (g)
2. Calculate mass of can + moist soil, W2 (g)
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Calculate mass of can + dry soil, W3 (g)
4. Calculate plastic limit
(W2 - W3) x 100
PL =
W3 - W1
5. Calculate plasticity index, PI = LL – PL.
DATA SHEET: Liquid and Plastic Limit
Description of soil: _____________________
Sample No.: ____________________________
Location: ____________________________
Tested by: ____________________________
Date: ___________________________________
LIQUID LIMIT TEST
Test No.
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1
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2
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3
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Can No.
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Mass of can , W1(g)
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Mass of can + moist soil, W2 (g)
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Mass of can + dry soil, W3 (g)
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(W2 - W3) x 100
w(%) =
W3 - W1
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No. of blows, N
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LL = _____________________
FI = _____________________
PLASTIC LIMIT TEST
Can No.
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Mass of can , W1(g)
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Mass of can + moist soil, W2 (g)
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Mass of can + dry soil, W3 (g)
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PL = (W2 - W3) x 100/ (W3 - W1)
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PI = (LL - PL)
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