The liquid and plastic limit tests normally are conducted only on the portion of the soil that passes the No. 40 sieve. A few particles that are large enough to be retained on the No. 40 sieve do not cause serious difficulty. However, it is generally faster to remove these larger particles by hand by kneading the soil between the fingers. If the percentage of particles retained on the No. 40 sieve is higher, these particles must be removed by passing the soil through the No. 40 sieve. Do not oven-dry or subject the sample to any artificial drying before you process or test it.

Soak the sample in water for 24 hr before washing. Then wash it through the No. 40 sieve and collect it in a large evaporating dish or collecting can. Oven-dry the material retained on the sieve, then dry-sieve it through the No. 40 sieve. Combine the portion dry sieved through the No. 40 sieve with the material washed through the sieve; the combined material is used for the tests. Break up soil particles that are lumping together or adhering to aggregate particles and separate them by rubbing them with your hands.

Next, dry the sample to approximately the liquid limit by decanting or blotting the water off, by evaporating it off (taking care to stir the soil frequently during evaporation), or by a combination of both procedures.

After this, place the soil mass in the liquid limit testing device cup, and divide it into sections by a central groove made with the grooving tool (fig. 15-34). The water content at the liquid limit is the water content at which the soil mass makes con-tact for a distance of 1/2 in. when the cup is dropped 25 times (25 blows) for a distance of 1 cm (0.3937 in.) at a rate of two drops per second. First, adjust the machine for this drop distance as follows:

A metric gauge is located on the handle of the grooving tool. The machine has an ADJUSTMENT PLATE and a pair of ADJUSTMENT

Figure 15-34.-Apparatus for determining Atterberg limits.

SCREWS. Manipulate the screws to adjust the height to which the cup is lifted. The point on the cup that comes into contact with the anvil of the machine should be exactly 1 cm above the anvil (the upper surface of the hard-rubber base of the machine). Check the adjustment by turning the crank at a rate of two drops per second. You should hear a slight click when the adjustment is correct.

1. From the prepared test material, take a sample that weighs about 100 g and place a portion in the cup above the spot where the cup rests on the base. Squeeze the sample and spread it with as few strokes of the spatula as possible, taking care to prevent the air bubbles from getting trapped within the mass. With the spatula, level the soil as you trim it to a depth of 1 cm at the point of maximum depth. Divide the soil in the cup by making a groove with the grooving tool along the center line of the cam follower or hook that holds the cup. When you make the groove, hold the cup in your left hand with the hook up-ward, and draw the grooving tool, beveled edge forward, through the material downward away from the hook. With some soils (especially sandy soils and soils containing organic matter), it is not possible to draw the grooving tool through the specimen without tearing the sides of the groove. In such cases, make the groove with a spatula, using the tool only for final shaping. When made correctly, the groove is wedge-shaped in section; it is open at the bottom for a distance equal to the width of the tip of the grooving tool.

2. Attach the cup to the carriage and turn the crank at a rate of two revolutions per second. Count the blows as you continue to turn the crank until the two halves of the soil cake come into con-tact at the bottom of the groove along a distance of about 1/2 in. (fig. 15-35). Record the number of blows required to close the groove in this manner. 

After you record the number of blows, re-move the cup from the testing device. Remix and regroove the sample. Place the cup again in the testing device and repeat the test. If the number of blows on the second test dif-fers from the number on the first by one or less, record both numbers on the data sheet and consider the test finished. If the number of blows on the second test differs by more than one, repeat the test until three successive tests give a reasonably consistent sequence. The average of the three is taken as the number required for the closure.

3. Remove a slice of soil approximately the width of the spatula (say about 10 g), extending from the edge of the soil cake at right angles to

Figure 15-35.-Liquid limit test.

the groove (fig. 15-36). Place this in a moisture content can, weigh it, and record the weight. Oven-dry and record the difference in weights. This is the weight of the water content.

4. Transfer the remaining soil in the cup to the evaporation dish. Wash and dry the cup and grooving tool. Reattach the cup in preparation for the next run.

5. Run at least five tests on each soil, with two closures above, two closures below, and one closure at or near the 25-blow line. An ideal spread is closures at 16, 23.5, 29, and 33 blows. If each testis perfect, the plotted line through all points is shown as a straight line. If some tests are imperfect, the operator can usually get good results by using the three plotted points lying most nearly in a straight line.

To determine the liquid limit, plot a FLOW CURVE on a graph like the one shown in figure 15-37. It is a semilogarithmic graph, in which the vertical coordinates are water content and the horizontal coordinates are number of blows. The flow curve is a straight line plotted as nearly as possible through three or more of the plotted points.

In figure 15-37, the first-run sample was tested three times for an average number of 16 hammer blows. The water content was 47.3 percent. On the graph, 16 and 47.3 are the coordinates of one of the three Xs shown plotted. The second-run sample indicated 24 hammer blows and 46.6 percent water content; these are the coordinates of another of the Xs plotted to the right. Coordinates for the third X are the hammer blows and water content for the third-run sample. The coordinates of the rest of the plotted points are as indicated by the hammer blows and water content for the succeeding runs. The plotted points in the graph may not form a straight line; however, the liquid limit line (or flow curve) is a straight line, passing nearly through the mean of the plotted points (fig. 15-37). The usual recommendation is that five or six trials be made so that the results are more representative.

The liquid limit (LL) is the water content for 25 blows; it is therefore indicated by the point of intersection between the flow curve and the vertical line representing 25 blows. The water con-tent indicated is about 46.4 percent. This, when rounded off to 46, is the liquid limit.

The plastic limit of soil is the lowest water con-tent at which the soil just begins to crumble when rolled into threads 1/8 in. in diameter, at slowly decreasing water content. First, prepare the sample as follows:

If you need only the plastic limit, take a quantity of soil weighing about 15 g from the prepared material in the evaporating dish.

Figure 15-37.-Data sheet, Atterberg limits determination.

dish and thoroughly mix it with distilled water, adding water until the soil mass becomes plas- tic enough to be shaped into a ball easily. Take a portion of the ball weighing about 8 g for the sample.