Determining the Plastic Limit and Plasticity Index of Soil - Soil Properties: Testing, Measurement, and Evaluation - page 93

mass into a thread of uniform diameter throughout its length (see
Note 1). The thread shall be further deformed on each stroke so
that its diameter reaches 3.2 mm (1/8 in.), taking no more than
2 min (see Note 2). The amount of hand or finger pressure required
will vary greatly according to the soil being tested; that is, the
required pressure typically increases with increasing plasticity.
Fragile soils of low plasticity are best rolled under the outer edge of
the palm or at the base of the thumb.
Note 1 —A normal rate of rolling for most soils should be 80 to
90 strokes per minute, counting a stroke as one complete mo-
tion of the hand forward and back to the starting position. This
rate of rolling may have to be decreased for very fragile soils.
Note 2 —A 3.2-mm ( 1 ⁄ 8 -in.) diameter rod or tube is useful for fre-
quent comparison with the soil thread to ascertain when the
thread has reached the proper diameter.
(2.2) Rolling Device Method —Attach smooth unglazed paper to
both the top and bottom plates of the plastic limit-rolling device.
Place the soil mass on the bottom plate at the midpoint between the
slide rails. Place the top plate in contact with the soil mass(es).
Simultaneously apply a slight downward force and back and forth
motion to the top plate so that the top plate comes into contact with
the side rails within 2 min (see Notes 1 and 3). During this rolling
process, the end(s) of the soil thread(s) shall not contact the side
rail(s). If this occurs, roll a smaller mass of soil [even if it is less
than that mentioned in (1)].
Note 3 —In most cases, two soil masses (threads) can be rolled
simultaneously in the plastic limit-rolling device.
(3) When the diameter of the thread becomes 3.2 mm, break the
thread into several pieces. Squeeze the pieces together, knead be-
tween the thumb and first finger of each hand, reform into an el-
lipsoidal mass, and reroll. Continue this alternate rolling to a
thread 3.2 mm in diameter, gathering together, kneading and
rerolling, until the thread crumbles under the pressure required for
rolling and the soil can no longer be rolled into a 3.2-mm diameter
thread. It has no significance if the thread breaks into threads of
shorter length. Roll each of these shorter threads to 3.2 mm in
diameter. The only requirement for continuing the test is that they
are able to be reformed into an ellipsoidal mass and rolled out
again. The operator shall at no time attempt to produce failure at
exactly 3.2 mm diameter by allowing the thread to reach 3.2 mm
then reducing the rate of rolling or the hand pressure, or both,
while continuing the rolling without further deformation until the
thread falls apart. It is permissible, however, to reduce the total
amount of deformation for feebly plastic soils by making the initial