Consolidation Quiz 2

[Longhorn2]

How much will an embankment of clay fill (CH and CL mix) compacted to between 92% and 95% rel. compaction based on the max. dry unit weight of the ASTM D 698 method settle (consolidate) under its own weight? (in terms of a percentage of embankment height). Ignore foundation soil. Looking for long term total settlement (under centerline of wide embankment). Moisture content at time of compaction is say between optimum and +2%.

In the stormwater management pond industry I have seen requirements of 5% camber but I would like some better feedback.

Thanks!

 

[Focht3]

You should expect a minimum of 1 percent, and 2 percent is more likely - for soils compacted to at least 95% of ASTM D698. You will get more settlement with reduced compaction.

But these 'shrinkage values' are based on "rules of thumb": I suggest that you monitor a few since the soil, weather, etc. all affect the resulting settlement.

 

[jdmm]

I have seen one or two empirical studies of say settlement of compacted fill in trenches but I have not seen any papers that relate long term settlement of clay fill compacted to any degree of density. Usually we use Focht3's approach of the 'rule of thumb'. If it is really important to your design then I would suggest that you compact some of your soil and test it in an oedometer (consolidation test). You would want to do an incremental load test with a long duration of the load to determine the secondary consolidation coefficient. Analysis would involve the insitu stresses from the embankment and a long term consolidation based on the coefficient of secondary consolidation.

You will note that there are not a lot of responses to this question and it is not because it is not a good question.

 

[Focht3]

There are some references from studies of dam embankments...but I can't recall the references right now. You might try posting the question in the 'slope stability engineering' forum - someone may be watching that forum (and not this one) with some suggestions on references.


If I recall the references, I'll post them here -

 

[BigH]

I believe there was a manual on Embankments of Clay Shales (USBR) quite a number of years ago. Focht3's 1 to 2% is in line with what I was "told" years ago. jdmm is right - it is a good question and one that is not really easy to address analytically. In clayey compaction, you are expunging "air", not really water when you compact. Your OMC+2% is on wet side but still probably less than 90% saturation. Kneading/impacting will also impart some higher than ko horizontal stresses too - some of which will dissipate with time; others not. I think, too, that the height of the fill will be a fundamental consideration. A thin fill will, in my view, behave less severelly than say a thick fill (20 to 30m or more).
I am thinking about your query as it is, indeed, intriguing. Maybe with time, if I can get any, will permit me to look at this in more detail.

 

[AZdinak]

NAVFAC 7.02-39 has a table of compacted fill properties. Easy reference and gives quick estimate of settlement as a % of original fill height. Zdinak

 

[Focht3]

BigH:

I saw your note - in another thread - referencing your question about minimum unit weights in an 'embankments' thread. I can't find your posted question.


Point this old dog in the right direction, and I'll try to help -

 

[valfer]

Long horn2,

I'm very interested in your question as it is the red line of my research work.

Though many researchs have been done on this wide subject, you should find interesting points in particular in the proceedings of a conference held in 1978, called "Clay fills" (Proceedings of the conference held at the institution of Civil engineers, London).

I have to say that your question need to be more precise because there can be a confusion between consolidation at constant water content and consolidation due to water content changes. Moreover, the height of the embankment is a key, in particuler in the case of water content changes because an increase of water content can also lead to a swelling behaviour.

P.S : one question for the experts of the site : I asked for informations about use of material containing organic matter in this forum and nobody seems to be interested in answering to me. Was my question inappropriate in this forum or completely irrelevant ????

 

[Focht3]

valfer:

I missed your question on organics. Most of my projects don't encounter much - other than the top 3 to 6 inches of grass mat and the occasional tree root ball. I'm afraid that I won't be of much help.

Did you start a message thread? Point me to it - I'll see if I can help.

 

[BigH]

My thread on unit weights is thread158-43998 .

Valfer: When I see your thread designation, I'll make a note to comment even if nothing more than to say !!
I know how it can be. My first post was on the "real" skinny on percentages of elongated and flaky particles in concrete and asphalt - we have a very tight spec here and the contractor can't/won't meet it. So, what kind of relaxation could we make. Got NO REPLIES !!

 

[valfer]

Focht3 and BigH,

Thanks for your answers. I had never been on the "Eartwork and grading engineering" forum but I'm going to try my question again over there. My thread here was thread274-42324.

 

[Longhorn2]

Thank you for the responses. I am dealing with multiple highway embankments so I tried to leave out specifics on height, anticipated changes in moisture content, etc to get a somewhat generic answer. Thanks again.

 

[dirtman]

Actually, you can expect very little post-construction of compacted embankments that use relatively thin lifts as ordinarily would be the case for engineered fills. We have monitored a number of compacted embankments for years and have found almost no post-construction settlement regardless of soil types. These are for relatively low fills, less than about 50 feet. The most settlement we have measured is about 25 % of that estimated. In other words, if pre-design estimates of settlement were say 2 feet, then we have found no more than 0.5 foot of settlement after construction.

Settlement occurs during construction from compression of air in the voids I think, so that there is little opportunity for post-construction settlement. On soft foundations, this would obviously not be the case.

 

[Focht3]

Hmmm, an interesting comment from dirtman. What do you mean by "relatively thin"? Where are your sites located? Do the embankments hold water most of the year, or are they only wetted infrequently?

Keep in mind that two factors - time and water - affect whether a given fill will experience all of its settlement potential at a given point in time.

And your comments make another important point: variability in performance of compacted fills should always be expected. I was taught to plan for the worst - not pray for the best.

 

[dirtman]

We have measured post construction settlement of many earth dams and have actually found very little post construction settlement.

 

[Focht3]

dirtman:

Together, the U.S. Army Corps of Engineers and the Bureau of Reclamation Division of the U.S. Department of the Interior have probably built more dams than any other organization. Their collective review of their many observations form the basis of my comments. Nothing personal, but their dam experience is "bigger" than yours - particularly since all you have really said so far is that they're wrong, yet you haven't given us anything to use in evaluating your claim!


I am interested in your experience, though. Please describe where the dams were built, the materials used, type of impoundment, embankment height, and when you measured the settlement. Please share with us!

 

[dirtsqueezer]

I would like to point out that instead of consolidating after compaction, some clays do the opposite.
I performed the testing on embankment fill for a retention pond in Burlington and passed them with a minimum 92% in all areas. The following year, I found myself being sent back to the same location. I tested the fill at 82-88% compaction with failure in some areas. It was so bad, I had trouble unsticking my boots from the fill as I walked. My thinking is that if any amount of money is being spent on your operation, it may be prudent to revisit the possibilities of avoiding the use of compacted clay. I enjoy testing compaction that is increacing as time goes by, instead of going the other way...
Best of luck.