PROCEDURE 1 1 Liquid Limit Testing

The liquid limit is defined as the water content at which the soil starts to act as a liquid. To derive liquid limit, the following procedure, described as the Multipoint Method (Method A) in ASTM D4318, is described:

1) Pass the soil through a #40 sieve and use the fraction that passes the sieve.

2) Add distilled water to approximately 50 g of soil until it has the consistency of peanut butter or frosting.

3) Check that the drop height of the cup in the liquid limit device is 1.0 cm (Fig.

4.2), and adjust the apparatus as necessary. Most grooving tools have a tab with a dimension of exactly 1.0 cm that you can use.

4) Spread a flat layer of soil in the cup with the frosting knife (Fig. 4.3).

Fig. 4.2—

Checking the drop height of the cup using the

calibration tab on the grooving tool.

5) Use the grooving tool to cut a groove in the soil (Fig. 4.4).

6) Turn the crank on the liquid limit device at a rate of 2 cranks per second and closely observe the groove. For each crank, the cup will drop from a height of 1.0 cm. Count and record the number of cranks that are required to close the groove over a length of 0.5 in (Fig. 4.5). Most grooving tools have a dimension of 0.5 in.

that you can use.

Fig. 4.3—Spread a flat layer of soil in the liquid limit device cup prior to grooving.

Fig. 4.4—Use the grooving tool to cut a groove in the soil in the liquid limit cup.

7) Clean out the cup and repeat steps 4-6 until successive trials yield consistent results that are within a few cranks of each other, and record the average number of cranks for the soil.

8) Remove the soil from the cup, place it in a moisture container, and obtain its water content using the ASTM D2216 method described in Chapter 2.

The procedure outlined above will provide a data single point corresponding to a single number of cranks and single water content. Liquid limit is defined as the water content at which the groove closes at exactly 25 cranks. Most likely, it will require either more or less than 25 cranks to close the crack for the first test. To derive liquid limit using the multipoint method, the procedure is repeated at three different water contents, and the data are plotted on a semi-log graph of w versus number of cranks. The water content corresponding to 25 cranks (i.e. LL) is derived by interpolation. To obtain two additional points, add either water or soil to the original mixture (depending on w of the first point) and repeat the procedure.

4.5.2. Plastic Limit Test

The plastic limit is defined as the water content at which a 0.125-in. diameter rod of soil begins to crumble. It is measured using the following procedure:

1) Pass some soil through the #40 sieve and use the soil that passes the sieve;

2) Add some distilled water to make little mudballs that would stick to the wall if you threw them (DO NOT throw them).

3) Take a pea-sized mudball and roll it out onto the frosted plate to form a rod with a diameter of 0.125 in. Use the 0.125-in. diameter metal rod as a reference (Fig.

4.6). If the soil crumbles the first time, add more water and repeat.

Fig. 4.5—The groove has closed over a length of 0.5 in.

4) If the rod doesn’t crumble, pick it up and make another mudball in your hands.

As you do this, you will dry the soil.

5) Repeat the process of making a rod, rolling up in your hands with a ball, making a rod, etc., until the soil crumbles while you are making the rod (Fig. 4.7). At this point, the water content of the soil is the PL. Quickly obtain its moist weight and place it in the oven for a moisture content reading in accordance with ASTM D2216 as described in Chapter 2.

Repeat this entire procedure three times, and report an average value for the plastic limit.

4.6. EXPECTED RESULTS

Liquid limit typically ranges anywhere from 20% for silts to over 100% for high-plasticity clays. Plasticity index typically ranges anywhere from near 0% (i.e.; a non-plastic soil) for silts to over 50% for high-non-plasticity clays.

Fig. 4.6 – Rolling the soil to form a 0.125-in.

diameter soil rod without crumbling.

Fig. 4.7—Soil rod crumbles at the plastic limit.

4.7. LIKELY SOURCES OF ERROR

Considering the seemingly archaic and empirical nature of these tests, one will find that the results obtained, particularly when plotting the three data points to obtain LL, are quite reliable. One likely source of error in performing these tests is in obtaining accurate water content measurements for the plastic limit test. Since the volume of soil used for the moisture content measurement is very small, significant moisture loss can occur while obtaining the moist weight of the soil specimen. The best way to minimize this error is to obtain the moist weight of the soil rod as quickly as possible after it crumbles.

4.8. ADDITIONAL CONSIDERATIONS

Plasticity index is a qualitative measure of the swell potential of soil. Clays with high cation exchange capacity, including bentonite, montmorillonite, and smectite, have high swell potentials. General guidelines for swell potential are summarized in Table 4.1.

The method described herein for liquid limit testing, Method A, relies on the use of three or more points and interpolation between points to derive the liquid limit.

However, an alternative One-Point Method (Method B) is also described in ASTM D4318. With this method, one point with a moisture content wⁿ and corresponding number of cranks N is used to calculate LL with the following equation:

121

Finally, ASTM D4318 includes criteria for assessing the acceptability of test results. Assuming that all of the tests are performed by the same laboratory technician, LL and PL for two separate tests of the same material should be within 2.4% and 2.6% of each other, respectively, to be considered acceptable.

Table 4.1—Ranges in LL and PI for typical fine-grained soil.

USCS Soil

Type¹ Common

Mineralogy Swell Potential LL PI

ML, CL Kaolinite Low <50% <25%

4.9. SUGGESTED EXERCISES

1) Measure the liquid limit of the fine-grained soil provided in class using the Liquid Limit Data Sheet at the end of the chapter (additional data sheets can be found on the CD-ROM that accompanies this manual).

2) Measure the plastic limit of the fine-grained soil provided in class using the attached Plastic Limit Data Sheet at the end of the chapter (additional data sheets can be found on the CD-ROM that accompanies this manual).

3) Calculate the plasticity index of the fine-grained soil provided in class.

4) Do these soils possess a high, moderate, or low swell potential?

5) To measure the liquid limit, there are two methods described in ASTM D4318:

Method A and Method B? Which method did you use? Briefly describe the method that you did not use.

LIQUID LIMIT (ASTM D4318)