Datasheet No.1
Revised 2009
MOHAWK COLLEGE OF APPLIED ARTS AND TECHNOLOGY
BUILDING AND CONSTRUCTION SCIENCES DEPARTMENT
Atterberg Limits: Liquid and Plastic Limits
INTRODUCTION
Both the liquid and the plastic limits tests will be carried out in one laboratory
session. The tests will be carried out in accord with A.S.T.M. designation D423 for
the liquid limit, and D424 for the plastic limit.
APPARATUS (LIQUID LIMIT TEST):
The apparatus for the liquid limit test consists of:
a) an evaporating dish
d) a grooving tool
b) a spatula
e) moisture cans
c) the liquid limit device
f) a balance sensitive to 0.01 g
PROCEDURE (LIQUID LIMIT TEST)
1. By means of the gage on the handle of the grooving tool, check the adjustment of
the liquid limit device to assure that the height of the drop of the cup is exactly
one centimetre. The flat end on the back of the grooving tool is exactly one
centimetre. Before proceeding further, obtain a sample of about 15 to 20 g of the
sample provided and set aside for the plastic limit tests. See step 1 of the Plastic
Limit procedure below.
2. Place about 100 g of the soil sample in the evaporating dish and mix it with a small
amount of distilled water. The mixing operation should consist of alternately and
repeatedly stirring, kneading and chopping with the spatula. Keep adding water,
slowly, mixing each time until sufficient water has been mixed with the soil to
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produce a consistency that will require 30 to 40 drops of the cup to cause closure
of the groove (the forming of the groove is described in the next section). This
must be determined by trial and error and will vary greatly with the soil being
tested. In placing the material in the liquid limit device, the surface of the
material should be horizontal and flush with the front edge of the cup. Use as few
strokes of the spatula as possible in placing the material in the cup and take care
to prevent entrapping air bubbles within the soil. The maximum thickness of the
soil in the cup should be 1 cm.
3. Divide the soil in the cup with the grooving tool, using as few strokes as possible to
make a clean groove through it. Turn the crank on the liquid limit device at a rate
of about 2 revolutions per second and record the number of drops required to
produce a closure of the groove approximately 13 mm (½ inch) long.
4. Remove a slice of soil approximately the width of the spatula and extending from
edge to edge of the soil cake at right angles to the groove, place in a tared
moisture can, weigh immediately and dry in the oven. Clean the liquid limit cup
after each trial.
5. It is necessary to attain 4 points with the number of drops between 15 and 40
according to the following table:
Trial:
1
2
3
4
# of Drops, N:
30 - 40
25 - 29
20 - 24
15 - 19
The tests shall always proceed from the drier to the wetter condition of the soil.
Avoid having two consecutive trials with less than a 3 drop difference. Take care
not to add too much water (its easier to add water than remove it!). If you do
overshoot one range, place it in the appropriate trial and come back the the one
missed afterwards. For example, if N for the 3rd trial is 18, record the results as
Trial 4, work the material in the evaporating dish without adding water and try
again until you get an N between 20 and 24 (Trial 3).
6. Calculate the water content of the soil expressed as a percentage of the oven
dried mass for each of the 4 trials. Plot a flow curve on the semi-logarithmic graph
provided on the website representing the relationship between the water content
and the corresponding number of drops of the cup. The flow curve is a best fit
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straight line drawn as nearly as possible to the four plotted points. (Best fit
means that the total of the distances from all points plotted to the line is
minimized.)
7. The liquid limit, LL, of the soil is the water content which corresponds to the
intersection of the flow curve with the 25 drops ordinate reported to the nearest
integer. This can also be done in Excel using a scatter plot, fitting a trendline
using a logarithmic model to find the equation (w = a + b[log(N)] ), then
substituting a value of N = 25 into this equation and manipulating to solve for w,
the Liquid Limit.
APPARATUS (PLASTIC LIMIT TEST)
a) an evaporating dish
b) a spatula
c) a ground glass plate
d) moisture content containers
e) a balance sensitive to 0.01 g
PROCEDURE (PLASTIC LIMIT TEST)
1. Separate the material set aside in step 1 of the liquid limit test procedure into
two halves. Squeeze and form one of the halves into an egg shaped form and roll it
between the fingers and the ground glass plate. Sufficient pressure should be
used to roll a thread of uniform diameter. The rate of rolling shall be between 80
and 90 strokes per minute, counting a stroke as one complete motion of the hand
forward and back.
2. When the diameter of the thread becomes 3.2 mm (1/8 in.) break the thread into
6 or 8 pieces and reform into an egg shaped mass. Continue this alternate rolling
to a thread 3.2 mm in diameter, gathering together, kneading and re-rolling until
the thread crumbles under the pressure required for rolling and the soil can no
longer be rolled into a thread.
3. Gather the portions of the crumbled roll together and place in a moisture can.
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Weigh the container and the moist soil immediately and dry in the oven. Repeat
with other half.
4. Calculate the moisture content of the specimen as a percentage of the oven dry
mass. The resulting moisture content is the plastic limit, PL, of the soil, reported
to the nearest integer. If the two trials agree within a couple of percentages
report the PL as the average of the two trials. If the two trials do not have
adequate agreement then one or the other should be selected and the other
rejected. If one of the attempts is rejected an explanation must be provided.
Sometimes it is useful to compare the trial PL values to the LL obtained from the
flow curve.
5. Calculate the plasticity index, PI = LL – PL
SUPPLEMENTARY DISCUSSION (ONE-POINT LIQUID LIMIT)
The liquid limit is now often obtained from a one-point determination using the
relationship:
LL = wN  (
N 0.12
)
25
where
LL = the one-point liquid limit
wN = the water content at N drops
N = the number of drops to produce closure
This relationship has proved relatively accurate for values of N between 20 and
30. When using this method, the number of blows is checked by re-mixing the soil
after the first determination and carrying out a second trial at the same moisture
content. If the number of blows is the same, the moisture content of the sample
is then determined. For the purposes of this lab, however, perform the calculation
for each of the trial number of drops for comparison only.
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REPORT
Include in the report:
a) a labelled sketch of (i) the liquid limit device and (ii) the grooving tool.
b) a plot of the flow curve on the logarithmic graph (download from web or plot
using Excel over the range of 10 to 50 for N)
c) the liquid limit obtained from the flow curve.
d) the plastic limit (average of the two tests).
e) tabulate the values of the liquid limit based on each of the one point tests
made, and compare with the more accurate value determined from the flow
curve.
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