THE COOPER UNION
Albert Nerken School of Engineering
Soil Mechanics Laboratory
Experiment No. 4 - Liquid and Plastic Limits of A Soil
To study the plasticity characteristics of clay and clay soils as affected by variations in the moisture content of the soil.

 

REFERENCES:

1) "Soil Testing for Engineers" by T. W. Lambe; Chapter III.

2) "Engineering Properties of Soils and Their Measurements", 4th ed. by Joseph E. Bowles; Experiment No. 3.

3) A.S.T.M. Standards, 1978, Part 19; Designation D42-366, D424-59.

APPARATUS AND REAGENTS:
 

1) Liquid Limit: Liquid Limit Device, electronic top loading balance sensitive to .01 gm., electric oven, watch glasses, spatulas, evaportating dishes, wash bottle, etc.

2) Plastic Limit: Evaporating dishes, glass plate, electronic top loading balance sensitive to .01 gm., electric oven, spatulas, wash bottle, etc.

EXPERIMENTAL PROCEDURE:
 

A. LIQUID LIMIT :

  1) Take a sample of approximately 160 gm. of air-dry soil passing the U. S. No. 40 sieve.

2) The brass cup of the liquid limit device should fall exactly 1 cm. Check this by inserting the end of the grooving tool, which is a 1 cm. gauge, between the point on the bottom of cup that comes in contact with the base and the base of the device. If the distance is not 1 cm., make necessary adjustments by means of the adjusting screw.

Note: Indicate the type of grooving tool used, on data sheet.

  3) Place the soil in the evaporating dish and thoroughly mix with 10 to 15 ml. of distilled water by alternately stirring kneading, and chopping with a spatula. If the soil exhibits a rather dry consistency, additional water must be mixed into the soil in increments of one or two ml.

4) When the consistency of the soil becomes that of a stiff paste, a trial determination may be made.

5) Remove the cup from the device and with a spatula, transfer a portion of the soil mixture from the evaporating dish to the cup.

6)Trim and level the soil surface, with as few strokes of the spatula as possible , so that this surface will be approximately horizontal when the cup is in its lowest position in the liquid limit device. The maximum thickness of the soil should be 1 cm.

7) Return excess soil to the evaporating dish.

8)Divide the soil in the cup by firmly drawing the grooving tool along the diameter of cup through the cam follover from the lower lip to the edge of the soil. If there is any tendency for the soil to slip along the surface of the cup, the groove should be cut with several strokes, rather than a single stroke, but with no more than six strokes being used. The last stroke should penetrate to the bottom of the cup and scrape the bottom clean.

9) If, at any time during the cutting of the groove, it can be seen that the final groove will be thicker than one cm. at any point, the cutting end of the grooving tool is one cm. thick, and can be used as a gauge for checking groove thickness; the soil must be removed from the cup and steps 5 - 8 repeated.

10) Return the cup and soil to liquid limit device.

11) After ensuring that all striking parts are clean, lift and drop the cup by turning the crank at the rate of two revolutions per second until the two sides of the soil mass come into contact along the bottom of the groove for a distance of 1/2 inch.

12) If the number of shocks required to close the groove is between 45 and 50, the determination is satisfactory.

13) If more than 50 blows are required to close the groove, return the soil to the evaporating dish, add a small increment of water, and mix thoroughly as described in step 3. Repeat steps 6 - 11.

14) If less than 25 blows are required to close the groove, return the soil to the evaporating dish; gently sprinkle from the end of a spatula, several grams of air-dry soil and mix thoroughly as described in step 3. Repeat steps 6 - 11 incl.

15) When a satisfactory determination is completed, record in Table 1, the number of shocks necessary to close the groove.

16) Remove a sample of the soil mass, which includes the closed portion of the groove, by slicing through the soil with the spatula, perpendicularly to the groove, at both ends of the closed portion of the groove.

17) Place this sample of soil in a weighed crystallizing dish and watch glass and weigh the container-and-soil to 0.01 gms. on the electronic top loading balance. Record the weight in Table 1.

18) After weighing the container and soil in step 9, place the watch glass beneath the crystallizing dish and place in the electric oven at 110oC. If any soil adheres to the glass, then place the glass beside the crystallizing dish in the oven. The soil should remain in the oven for at least 12 hours.

19) Wash and dry the cup and grooving tool in preparation for the next trial.

20) Transfer the remaining soil in the brass cup back to the evaporating dish.

21) Add a small increment of water, possibly one or two ml., to increase the fluidity sufficiently so that the number of shocks necessary to close the groove will be between 35 and 45.

22) Repeat steps 5 to 11 and 16 to 20.

23) Repeat steps 21 and 22 three additional times; once, to alter the consistency of the soil so that the number of shocks necessary to close the groove, will be between 25 and 35, and, once again to alter the consistency so that the shocks necessary for closure are between 15 and 25 and once again to alter the consistency so that the shocks necessary for closure are between 10 and 15.

24) Weigh each container containing the dry soil samples to .01 gm on the electric top loading balance.

25) Record these weights in Table 1.

COMPUTATIONS AND GRAPH:
 

1) Compute the moisture contents of the samples.

2) Plot on semi-log paper the moisture content on arithmetic scale vs. the corresponding number of shocks on the log scale. This is known as the Flow Curve.

3) Record the value of the liquid limit on the data sheet as the moisture content corresponding to 25 shocks.

4) Compute the flow index, which is the slope of the curve of step 2, and record in Table 3.

 

B. PLASTIC LIMIT :

  1) Take a sample of approximately 75 gm. of soil passing the U. S. No. 40 sieve, and divide the sample into three approximately equal portions.

2) Place one of these portions in an evaporating dish and mix thoroughly with distilled water so that the mass may be easily shaped into a plastic ball without any of the soil sticking to the fingers excessively when squeezed.

3) Samples of soil taken from the material being prepared for the second or third trials of the liquid limit test are at suitable moisture contents for a plastic limit determination.

4) Take a portion of the ball weighing approximately 7 or 8 gm. and squeeze into a ellipsoidal-shape mass, about three inches long.

5) Place this mass on a glass plate and roll it under the fingers with just sufficient pressure to form a thread of a uniform diameter.

6) The rate of rolling shall be between 80 and 90 strokes per minute considering a stroke as one complete motion of the hand forward and back to the starting position again.

7) When the diameter of the thread becomes 1/8-inch, break the thread into six or eight pieces. squeeze the pieces together to form a roughly ellipsoidal mass, using the thumbs and forefingers of both hands, and reroll. Continue this alternate rolling to 1/8-inch diameter thread,gathering together, kneading and rerolling, until the thread crumbles under the pressure required for rolling and the soil can no longer be rolled into a thread.

8) The crumbling may occur when the thread has a diameter of greater than 1/8-inch, provided the soil has previously been rolled into a 1/8-inch diameter thread.

9) The manner in which the crumbling develops will vary with different soil types. Some soil types disintegrate in numerous small aggregations of particles; others form an outside tubular layer that begins splitting longitudinally at both ends. The splitting progresses toward the middle, and finally, the thread falls apart in many small platy particles. Frequently lateral and spiral-like cracking may also develop accompanying the splitting. Heavy, highly plastic, clays require considerable pressure to deform the plastic limit; usually these threads break into barrel-shaped segments about 1/4 to 3/8 inches long. At no time should the operator attempt to produce failure at exactly 1/8-inch diameter by allowing the thread to reach 1/8-inch, then reducing the rate of rolling or the hand pressure, or both, and continuing the rolling without further deformation until the thread falls apart. It is permissible, however, to reduce the total amount of deformation for slightly plastic soils such as clayey silts, by making the initial diameter of the ellipsoidal-shaped mass nearer to the required 1/8-inch final diameter.

10) Place the portion of crumbled soil in a weighted crystallizing dish and watch glass.

11) Repeat steps 4 to 10 on another portion of the original soil sample of step No. 2, and add the crumbled material to that of step 9.

12) Weigh the watch glasses and soil to 0.01 gm on the electric top loading balance and record the weight in Table 2. Place container and soil in an electric oven at 110oC and dry for at least 12 hours.

13) Remove the container and soil, cool in a dessicator, weigh to 0.01gm and record the weight in Table 2.

14) Repeat steps 1-12 twice more as check determinations.

COMPUTATIONS:
  1) Compute the plastic limit values and the average value of the plastic limit. Record all values in Table 3.

2) Compute the plastic index using Formula 1 and record the value in Table 3.

3) Compute the liquidity index using Formula 3 and record the value in Table 3. Use the water content from Table 1, Data Sheet (A), Experiment No. 3, for a value of wn.

4) Compute the Activity using Formula 4 and record the value in Table 3. Use the grain size distribution curve from Experiment No. 3 for any percent finer needed.

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