After removing particles larger than the No.
40 sieve size, a specimen of soil about 1/2 cube in size as molded to the
consistency of putty.If too dry, water
must be added and if sticky, the specimen should be spread out in a thin
layer and allowed to lose some moisture by evaporation.Then the specimen is rolled out by hand on a
smooth surface or between the palms into a thread about 1/8 inch in
diameter.The thread is then
folded and re-rolled repeatedly.During
this manipulation the moisture content is gradually reduced and
the specimen stiffens, finally loses its plasticity, and crumbles when the
plastic limit is reached.
After the thread
crumbles, the pieces should be lumped together and a slight kneading action
continued until the lump crumbles.
The tougher the thread
near the plastic limit and the stiffer the lump when it finally crumbles, the
more potent is the colloidal clay fraction in the soil.Weakness of the thread at the plastic limit
and quick loss of coherence of the lump below the plastic limit indicate
either inorganic clay of low plasticity, or materials such as
kaolin-type clays and inorganic clays which occur below the A-line.