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Fat Clay

Fat Clay

Fat Clay

What is the technical definition of fat clay? Is it related to swelling potential? What type of criteria defines how much swelling potential it may have?

RE: Fat Clay

High Index of Plasticity - CH clays.

RE: Fat Clay

Yup - wL > 50.

Please see FAQ731-376  by VPL for tips on how to make the best use of Eng-Tips Fora.

RE: Fat Clay

As big BigH and Focht3 pointed out a fat clay is any clay with a liquid limit over 50% that plots above the A Line, i.e. it is not highly plastic silt.

With respect to the second part of your question, regarding swell potential, that is much more difficult to answer.  In the St. Louis area, where I practice, the traditional approach has been to treat all clays with a liquid limit over 45 as having shrink/swell potential.  Some recent work is indicating that there are some clays with liquid limits as low as about 35 that have some shrink/swell potential, depending on the plastic limit.  

Hope this helps some.

RE: Fat Clay


I agree with all prior comments.  Another aspect of the swell potential of clays is the affinity that they have for water.  One measure of this affinity is the suction (or negative pore water pressure).  When clays dry out, the suction value increases and to potential for swelling also increases.  I agree with GeoPaveTraffic that the liquid limit is not always a direct indicator of swell potential.  If there is a moisture deficit in the clays, there will most likely be swelling due to the introduction of moisture.

RE: Fat Clay

I should have taken more time to read the original post more closely; or perhaps I was distracted by my 10 year old redhead - very persistent, he is.  Anyway, I did not pick up on your question: What type of criteria defines how much swelling potential it (a fat clay) may have?

Correlations between swell potential and other soil properties are tenuous at best - you have to look at each soil individually.  I was involved in remedial work beneath the Catholic Cathedral School in Corpus Christi in about 1992.  We encountered a clayey sand (> 50% retained on the No. 200 sieve) that was dense and dry.  It concerned me, so we ran a swell test - it swelled over 10% during a 48 hour observation period.  Needless to say, we had to re-think our repair options a bit -

Generally speaking, the higher the PI, the greater the swell potential.  The greater the soil density, the greater the swell potential.  And the drier the soil is, the greater the swell potential.  But forget about any sort of "magic" equation to tie it all together.

Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

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