Relation of Cohesion and moisture content

[rocdoc90]

I am having a heated discussion with a colleague of mine about the relationship of cohesion and moisture content of a clay soil. His argument is that magnitude of cohesion is independent of soil moisture content and I disagree.
Am I incorrect in understanding that the cohesion of a soil will vary as the moisture content varies?

 

[RWF7437]

Ask your colleague;
If the moisture content is zero, what is the cohesion ?
If the moisture content exceeds the saturation point what is the cohesion ?

good luck

 

[fattdad]

I guess you have to define "cohesion". From the Mohr envelope perspective (along with general practice) the tests on cohesive soils are performed in a saturated condition - so there is no issue of the moisture content. Cohesion is the intercept of the failure envelope when the average deviator stress is zero.

Now if you are really talking about undrained shear strength at varying moisture contents - yes the shear strength can vary with moisture content. Take a mud soup and let it dry and the shear strength will be greater. Rewet it and the strength will lessen. This is not "cohesion" this is undrained shear strength, which can vary with moisture content.

One of the big problems with subgrade failure is when rutted sites become dry and the subgrade "passes" a proofroll. With an ensuing pavement job, there may (will likely) be premature failure as the subgrade rewets and the traffic loads begin.

Now, if you properly compact a subgrade (i.e., start with some engineered strength at a favorble moisture content), there may be some increased dry strength, but upon rewetting, the original strength will remain intact. Similarly, if you properly compact a fill and it becomes saturated, it will unlikely go to mush.

Just one man's opinion.

f-d

¡papá gordo ain’t no madre flaca!

 

[RWF7437]

Does it matter if one calls it cohesion or undrained shear strength? Can you control the moisture content of a subgrade or backfill material once it has been placed and compacted ? Would you want to bet your license on it ?

 

[fattdad]

I don't refute RWF7437 questions, I'm just clarifying what the original poster meant by cohesion. To me if the original post related to undrained shear strength then that would mean something else.

I fully acknowledge that the moisture content of a subgrade or compacted structural fill cannot be modified after the project is completed and you should likely anticipate drying and wetting cycles during the year. This is one reason that pavement design (not that this is the subject of this thread) is based on the soaked CBR value.

I note that so far your contribution to this thread has been five questions. Just trying to answer from my perspective.

I don't "bet" my license and am not sure what you mean by that.

f-d

¡papá gordo ain’t no madre flaca!

 

[RWF7437]

fattdad,

The questions were not intended for you but were meant to be asked by rocdoc90 of himself, and/or his colleague. Questions are an excellent way to get at an understanding of almost anything. At least I believe that. To the extent they do that they are a "contribution".

Each time you, or I, sign and seal plans, reports, etc. we risk our license to practice. We are betting that our work is complete and correct enough that our project will not fail and we won't be sued.