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.
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.
Place some soil paste into the liquid limit device. Smooth the surface with a spatula such that maximum depth is 8 mm.
Using the grooving tool, cut a groove along the centerline of the soil pat.
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).
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.
Remove the soil from the cup of LL device and clean it carefully.
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.
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.
Put three moisture cans in the oven to dry to constant mass (W3).
4. Determine the moisture content for each of the three trials as
(W2 - W3) x 100
w (%) =
(W3 - W1)
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 (%)
log N2 - log N1
PLASTIC LIMIT TEST
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).
Porcelain evaporating dish
Ground glass plate
Balance sensitive up to 0.01 g
Plastic squeeze bottle
Put 20g of air-dry soil, passed through No. 40 sieve into an evaporating dish.
Add water and mix the soil thoroughly.
Determine the mass of moisture cans (W1).
From the moist soil prepared in step 2, prepare several ellipsoidal-shaped soil masses by squeezing the soil with fingers.
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.
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.
Repeat steps 5 and 6 until the thread crumbles into several pieces when d = 1/8”.
Collect the small crumbled pieces into the moisture can and put the cover on the can.
Take the other ellipsoidal soil masses formed in step 4 and repeat steps 5 through 8.
Determine the mass of moisture can plus wet soil (W2).
Place moisture can into the oven to dry to constant mass (W3).