Okay it’s not about the true meaning of life, but I caught your eye, didn’t I?
A recent post on LinkedIn grabbed my attention. It was about the use of the fall cone, rather than the conventional liquid limit device. The author definitely had some very good points, and I agree fully. The fall cone device is a much more reproducible and is a much less operator dependent means of establishing the liquid limit of a soil.
I keep wondering about the physical meaning of the Atterberg Limits.
Have you ever used a conventional liquid limit device and thought, “Yup! Now there’s a liquid!” Yeah, me neither. One definition of a fluid is that it will assume the shape of the part of the container which it occupies. That’s not really what is really happening in a liquid limit device. It may be the beginning of that behavior, but it isn’t actually that behavior.
No doubt that the need to assess whether a soil acts as a clay or as a granular material is very important, and amongst other things, this is used to place the soil into its class of soils. Atterberg tests may be used to determine the following characteristic behaviors:
- The liquid limit is the point at which the soil changes from freely flowing to a dough-like material.
- The plastic limit is the point at which the soil changes from a paste-like material to a viscous liquid.
Alternatively stated, the plastic limit is the water content at which the material starts showing plasticity, and the liquid limit is the water content at which the material will flow like a liquid.
There is also the shrinkage limit and sticky limit, but those are seldom used.
Even Wikipedia gets it. They say “The liquid limit is conceptually defined as the water content at which the behavior of a clayey soil changes from the plastic state to the liquid state. However, the transition from plastic to liquid behavior is gradual over a range of water contents, and the shear strength of the soil is not actually zero at the liquid limit”. And I have no idea why the definition of plastic limit is where a 3.2 mm diameter thread just begins to fall apart when it is rolled.
Don’t get me wrong. I’d never “throw out the baby with the bath water”, I’m just questioning these definitions of liquid and plastic behaviors, and I encourage you not to take the definitions too literally. They are simply indicator tests for the purpose of classifying soils, which is an incredibly useful thing.
Speaking of liquids… it’s 5 o’clock somewhere!