LIQUID LIMIT AND PLASTIC LIMIT
ASTM D4318, BS EN ISO 17892
1. Scope*
1.1 These test methods cover the determination of the liquid
limit, plastic limit, and the plasticity index of soils as defined
in Section 3 on Terminology.
1.2 Two procedures for preparing test specimens are provided
as follows: Specimen Preparation Procedure 1 (Wet
Preparation), as described in 11.1 and Specimen Preparation
Procedure 2 (Dry Preparation), as described in 11.2. The
specimen preparation procedure to be used shall be specified
by the requesting authority. If no specimen preparation procedure
is specified, use the wet preparation procedure. (See
Section 6, Interferences.)
1.3 Two methods for determining the liquid limit are provided
as follows: Liquid Limit Method A (Multipoint Method),
as described in Sections 12 and 13, and Liquid Limit Method B
(One-Point Method), as described in Sections 14 and 15. The
method to be used shall be specified by the requesting
authority. If no method is specified, use Method A. (See
Section 6, Interferences.)
1.4 The plastic limit test, Method for Plastic Limit, is
performed on material prepared for the liquid limit test. When
determining the plastic limit, two procedures for rolling portions
of the test specimen are provided as follows: Plastic Limit
Rolling Procedure 1 (Hand Rolling), and Plastic Limit Rolling
Procedure 2 (Using the Rolling Device).
1.5 The liquid limit and plastic limit of soils (along with the
shrinkage limit) are often collectively referred to as the
Atterberg limits. These limits distinguished the boundaries of
the several consistency states of plastic soils.
1.6 The plasticity index, Method for Plasticity Index, is
calculated using results from liquid limit and plastic limit
testing.
1.7 The methods described herein are performed only on
that portion of a soil that passes the 425-μm (No. 40) sieve.
Therefore, the relative contribution of this portion of the soil to
the properties of the sample as a whole must be considered
when using these methods to evaluate the properties of a soil.
1.8 The values stated in SI units are to be regarded as
standard. The values given in parentheses are mathematical
conversions to inch-pound units, which are provided for
information only and are not considered standard. Reporting of
test results in units other than SI shall not be regarded as
nonconformance with this standard.
1.8.1 The converted inch-pound units use the gravitational
system of units. In this system, the pound (lbf) represents a unit
of force (weight), while the unit for mass is slugs. The
converted slug unit is not given, unless dynamic (F = ma)
calculations are involved.
1.8.2 It is common practice in the engineering/construction
profession to concurrently use pounds to represent both a unit
of mass (lbm) and of force (lbf). This implicitly combines two
separate systems of units; that is, the absolute system and the
gravitational system. It is scientifically undesirable to combine
the use of two separate sets of inch-pound units within a single
standard. As stated, this standard includes the gravitational
system of inch-pound units and does not use/present the slug
unit for mass. However, the use of balances or scales recording
pounds of mass (lbm) or recording density in lbm/ft3 shall not
be regarded as nonconformance with this standard.
12. Procedure for Liquid Limit Method A (Multipoint
Method)
12.1 Thoroughly remix the specimen (soil) in its mixing
dish, and, if necessary, adjust its water content until the
consistency requires about 25 to 35 drops of the cup of the
liquid limit device to close the groove. Using a spatula, place
a portion(s) of the prepared soil in the cup of the liquid limit
device at the point where the cup rests on the base, squeeze it
down, and spread it into the cup to a depth of about 10 mm at
its deepest point, tapering to form an approximately horizontal
surface. Take care to eliminate air bubbles from the soil pat, but
form the pat with as few strokes as possible. Keep the unused
soil in the mixing/storage dish. Cover the dish with a wet towel
(or use other means) to retain the moisture in the soil.
12.2 Form a groove in the soil pat by drawing the tool,
beveled edge forward, through the soil on a line joining the
highest point to the lowest point on the rim of the cup. When
cutting the groove, hold the grooving tool against the surface of
the cup and draw in an arc, maintaining the tool perpendicular
to the surface of the cup throughout its movement. See Fig. 6.
In soils where a groove cannot be made in one stroke without
tearing the soil, cut the groove with several strokes of the
grooving tool. Alternatively, cut the groove to slightly less than
required dimensions with a spatula and use the grooving tool to
bring the groove to final dimensions. Exercise extreme care to
prevent sliding the soil pat relative to the surface of the cup.
12.3 Verify that no crumbs of soil are present on the base or
the underside of the cup.
12.4 Lift and drop the cup by turning the crank at a rate of
1.9 to 2.1 drops per second until the two halves of the soil pat
come in contact at the bottom of the groove along a distance of
13 mm (1⁄2 in.). See Fig. 7 and Fig. 8. The base of the machine
shall not be held with the hand, or hands, while the crank is
turned.
NOTE 10—Use of a scale is recommended to verify that the groove has
closed 13 mm (1⁄2 in.).
12.5 Verify that an air bubble has not caused premature
closing of the groove by observing that both sides of the groove
have flowed together with approximately the same shape. If a
bubble has caused premature closing of the groove, reform the
soil in the cup, adding a small amount of soil to make up for
that lost in the grooving operation and repeat 12.1 – 12.4. If the
soil slides on the surface of the cup, repeat 12.1 – 12.4 at a
higher water content. If, after several trials at successively
higher water contents, the soil pat continues to slide in the cup
or if the number of drops required to close the groove is always
less than 25, record that the liquid limit could not be
determined, and report the soil as nonplastic without performing
the plastic limit test.
12.6 Record the number of drops, N, required to close the
groove.
12.7 Obtain a water content specimen by removing a slice
of soil approximately the width of the spatula, extending from
edge to edge of the soil cake at right angles to the groove and
including that portion of the groove in which the soil flowed
together, place in a container of known mass, and cover.
12.8 Return the soil remaining in the cup to the dish. Wash
and dry the cup and grooving tool and reattach the cup to the
carriage in preparation for the next trial.
12.9 Remix the entire soil specimen in the dish adding
distilled water to increase the water content of the soil and
decrease the number of drops required to close the groove.
Repeat 12.1 – 12.8 for at least two additional trials producing
successively lower numbers of drops to close the groove. One
of the trials shall be for a closure requiring 25 to 35 drops, one
for closure between 20 and 30 drops, and one trial for a closure
requiring 15 to 25 drops.
12.10 Determine the water content, wn, of the soil water
content specimen from each trial, n, in accordance with Test
Methods D2216.
12.10.1 Determination of initial masses (container plus
moist soil) should be performed immediately after completion
of the test. If the test is to be interrupted for more than about
FIG. 6 Example of Grooving Tool Placed in a Properly Grooved Soil Pat
D4318 − 17
Copyright by