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yahoo123 (Bioengineer) (OP)
7 Nov 07 0:17
Can somebody tell me what this value represents? I know that it is the Liquid Limit minus the Plastic limit. But can someone tell me what the significance is?

How does plasticity affect, for example, compressibility, compactibility, strength, cohesiveness, etc..

When and why is high or low plasticity preferable?

Thank you so much. Yes I am totally clueless
JAE (Structural)
7 Nov 07 12:06
The PI has a rough, loose relationship with the potential for being expansive under added moisture content.

As a structural engineer, my geotech buddies in south Texas many times used the PI to relate to me how expansive the clays were.  A PI lower than 20 to 24 was generally a safe area...but higher than that and we would then have to respond to swelling clay conditions.

I'm sure the geotechs on this forum can enlighten better than the above.

PEinc (Geotechnical)
7 Nov 07 14:20
Plasticity index is covered in even the most elementary soil mechanics books.
PEinc (Geotechnical)
7 Nov 07 14:25
The following was copied from a web site:

"The Plasticity Index is simply the numerical difference between the liquid limit and the plastic limit for a particular material and indicates the magnitude of the range of moisture content over which the soil remains plastic.  It is a measure of the cohesive qualities of the binder resulting from the clay content.  Also, it gives some indication of the amount of swelling and shrinkage that will result in the wetting and drying of that fraction tested.  If some soils do not have sufficient mechanical interlock they require amounts of cohesive materials to give a satisfactory performance.  A deficiency of clay binder may cause ravelling of gravel wearing courses during dry weather and excessive permeability."
fattdad (Geotechnical)
7 Nov 07 15:40
The plasticity index is the range of moisture content where a given soil will behave as a plastic material.  How that effect your project is determined by your experience and the type of project.


¡papá gordo ain’t no madre flaca!

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