One-dimensional consolidation settlement

[abusementpark]

The theory of one-dimensional consolidation is only valid for soil profiles that are homogeneous with respect to the coefficient of consolidation (Cv). How is this generally handled in the field where you encounter layered soil profiles with significantly differing values of Cv for each layer?? Is there a widely used approach for approximation or does engineer have their own method??

 

[escrowe]

By treating each layer separately, and summing the results.
Consolidation is essentially dependent on layer thickness, drainage,
maximum past pressure and increase in effective stress.

Your first step is to calculate the average effective stress increase for each layer.
I would consider consolidation tests for each material.

As a short cut, you could use accepted relationships between moisture content, liquid limit, and consolidation.

 

[abusementpark]

OK, I left out an important detail. I want to know how this is generally handled when you are looking to find a rate of consolidation. The theory for RATE of consolidation based on a homogeneous soil profile with a uniform Cv value. How is the generally accounted for / approximation?

I know what you mean about summing the settlement from each layer, but this is for the ultimate settlement number. What if you are interested in the time rate?

 

[BigH]

Well, you could determine for a series of "times" what the consolidation level was in each layer and develop an overall settlement vs time curve. Some layers may reach 100% consolidation and be in secondary consolidation when other layers are still in primary.

 

[fattdad]

I'm still thinking. . . . . It is an interesting question and one that I don't have any practical experience with. I think other's may be in the same boat.

Here is my first thought. Cv varies with permeability. When you determine the (vertical) permeability of a layered system, you use the following:

Kavg=sum(d)/sum(d/k)

What you get is some value that's real close to the lowest k value for the layered system.

By extrapolation, I'd consider just using the lowest value of Cv and the drainage path from the center of the overall layered system (providing two-way drainage).

I'll post if I have a second thought - but it may take inspiration from others. . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[moe333]

Take a look in Navfac. There is a method clearly presented on just that problem. Don't have mine with me right now.

 

[msucog]

here's navfac (now ufc)--i think it's under the soil mechanics section but i'm not 100% positive

http://www.wbdg.org/ccb/browse_cat.php?o=29&c=4

i've fooled around with this on a project or two. i think it ultimately boils down to the engineer's judgement. for example, on a fairly straightforward situation where you run consols, perform regularly spaced borings, etc. and you calculate say 12" at 1000 days...the real number at 1000 days may be say 9" or 15" (or something completely different since settlement could ultimately be caused by outside "rare" occurances--such as a very harsh drought)...and that estimate is for some certain location on a site. move 100 feet away and it could be and likely is different by some degree (lot or little--who knows). hell, you could have 5 consols out of the same 30 foot hole of similar soils, run the analysis 8 different ways and get 10 different answers across a range that will be scattered around some number. the overall scatter may be only 10% or could more than 50% of the total calculated. the more layers the more complex. the more factors (ie. change in water levels) the more complicated and then it throws the whole fantastic time scale off. take in to account "everything" and you've spents years trying to fine tune a total 9" settlement down to 9.131" (for a certain point that is)...and then it actually moves 10"--damn it!

for me around here where the profile is layered and where the drainage path is somewhat obscure: i calculate something "reasonable" based on averages and then i calculate something more "conservative" tilted toward the more dramatic end of the spectrum. i choose a point somewhere in between or even provide a range of what i think it will be and also take in to consideration what really matters to the specific project. if by some chance the actual settlement is less or occurs more quickly, then it's really no loss to the client. if the settlement takes a lot longer or is twice the magnitude, then it creates more problems. i've even seen the settlement essentially stop and then start again due to groundwater fluctuations then stop again (let's hope it doesn't reactivate again). if the cv varies among similar soils, i do the best i can to pick a middle of the road to conservative estimate and go from there. there's no clear answer, scenario, etc...it is all "just dirt" and we never know all the answers, scenarios, etc. no matter how much exploring and testing we do. you're effectively approximating approximations of tests that approximate settlement based on correlations of a range of normalized test data gathered by multiple folks compared to generalized profiles/data and made in to a nice and neat engineering principle...

i'm not sure my babbling helps so i will leave it there

 

[fattdad]

moe333, Can you provide the appropriate NAVFAC publication and page number? I (briefly) looked in DM-7.1, .2 and .3, but didn't see this problem specifically addressed.

msucog, I think as a practical matter, you provide the "correct answer". That said, there is an engineering answer to this problem and I'd be interested in knowing the solution. Never being too humble, I'm still partial to my idea - ha.

f-d

¡papá gordo ain’t no madre flaca!

 

[moe333]

Fattdad, sorry but I'm away from my office for the week and cannot reference the page. However, I recall it is in the section tthat deals with consolidation. Can't remember if it's 7.1 or 7.2.

 

[abusementpark]

Thanks for all the responses. I have found the section that moe333 is talking about. Thanks to msucog for the liknk.

http://www.wbdg.org/ccb/DOD/UFC/ufc_3_220_10n.pdf

It is on pages 260-261 of the pdf file.

This was pretty much what I was looking for. Thanks all.

 

[fattdad]

Basically the DM-7.1 method is the "expanded layer technique", which is a very similar to the permeability transformation.

Thanks for the link and the page reference!

f-d

¡papá gordo ain’t no madre flaca!