Determining the Plastic Limit and Plasticity Index of Soil - Soil Properties: Testing, Measurement, and Evaluation - page 95

NUMERICAL
EXAMPLE

A laboratory test was conducted according to the procedure previously
described. The following data were obtained:

Can No.: A-4 A-5

Mass of can plus moist soil, M cws

20.47 g 21.27 g

Mass of can plus oven-dried soil, M cs

18.93 g 19.57 g

Mass of can, M c 11.56 g 11.68 g

These data are indicated on the form shown on page 97. (At the
end of the chapter, two blank copies of this form are included for the
reader's use.)

Inasmuch as sample water content calculations have been given in
Chapters 4 and 7, none is given here. The two computed water contents
are 20.9% and 21.5% (see form on page 97). Because the difference
between these two water contents (0.6%) is less than the acceptable
range listed in Table 8-1 for single-operator precision, the plastic limit
is the average of the two water contents, or 21.2%.

Table of Precision Estimates A [1]

Table 8-1

Standard Acceptable Range

Material and Type Index Deviation B of Two Results B

Single-operator precison:
Liquid Limit 0.8 2.4

Plastic Limit 0.9 2.6

Multilaboratory precision:
Liquid Limit 3.5 9.9
Plastic Limit 3.7 10.6
A The figures given in Column 2 are the standard deviations that have been found to
be appropriate for the test results described in Column 1. The figures given in Column 3
are the limits that should not be exceeded by the difference between the two properly
conducted tests.
B These numbers represent, respectively, the (1S) and (D2S) limits as described in
Practice C 670.

The plasticity index is determined by subtracting the plastic limit
(21.2%, as just determined) from the liquid limit (46.4%, as determined
in the section “Numerical Example” in Chapter 7). The value of the plas-
ticity index in this example is therefore 46.4 - 21.2, or 25.2%.

If either the liquid or the plastic limit cannot be determined, the plas-
ticity index cannot be computed and should be reported as “NP”
(indicating nonplastic). The plasticity index should also be reported as
“NP” if the plastic limit turns out to be greater than or equal to the
liquid limit. For very sandy soil, the plastic limit test should be
performed before the liquid limit test; if it (the plastic limit) cannot be
determined, both the liquid and the plastic limit should be reported
as “NP.”

SUMMARY