Settlement of Compacted Soil, Wet/Dry of Optimum? 3

[mohanvenn]

Hello:

My question is does the clay soil compacted wet of optimum settle less or more than the clay soil compacted dry of optimum?

For example, a soil has a MDD of say 100 pcf at say 15% OMC.
The soil is compacted in lifts to a in-situ dry density of say 95 pcf (95% Compaction) at 12% mositure (-3% of OMC); and, the same soil is compacted in another area to 95% of MDD at 18% moisture (+3% of OMC). Will these fill soils settle differently (Both, Magnitude and Rate of Settlement)?

I ask this question b'coz, in my practice I came accross a debate with a fellow practioner, who insists that both soils should settle same. He believes that since the void ratio of soil at -3% or +3% of OMC is one and same, they should settle one and the same. The void ratio being the same is true because the degree of saturation of both these soil conditons would be different. The equation; wG = Se would be balanced in both cases only when S is different and; when g and e are same to balance with w.

Having said that, I belive they should settle differently both in magnitude and rate. The starting point (initial void ratio, ei) of the material could be same, but, the ending point (final void ratio, ef) may not be the same and the path (e log p) they take to get a final ef during saturation would different. Also, the path (e log p) they may take after saturation may be different, as there would be some particle rearragment during saturation.

Any thoughts? Any references to read up on this matter?

Thank You.

 

[SirAl]

I would suggest that the soil 'structure' or 'fabric' would be different. The compressive index and coeff. of consolidation would be different resulting in a different response to imposed loads. The wetter soils could be influenced by an increase in pore pressure as well as reduced permeability that could lengthen the duration of consolidation.

 

[BigH]

I would surmise it would depend on the nature of the clay. In fact, if your "dry" clay is expansive, it might actually swell on you as it becomes saturated. As SirAl indicated the texture achieved in each case would also be important - and this depends on the clay minerology. I'll let others go into the Cam-Clay reasoning.

 

[mohanvenn]

I thank both SirAl and BigH for their valuable opinions. One thing is very clear, "dry" and "wet" would have to behave differently (swell or settlement). But, before we go into clam-clay, here is a following question?

Assuming, the following embankments are built on rock (i.e., negligible settlment from foundation soils) when exposed to weather; and, assuming that they will never be subjected to loads (i,e., trafffic or surcharge load); Which one of the embankment swell or settle more with time?

Please give your answer, in the order of decreasing magnitude of settlement, i.e., Case I, Case II, Case IV, Case III (my answer).

Case I: A 30 foot high expansive clay embankment compacted "DRY" of optimum.
Case II: A 30 foot high expansive clay embankment compacted "WET" of optimum.
Case III: A 30 foot high NON-expansive clay embankment compacted "DRY" of optimum.
Case IV: A 30 foot high NON-expansive clay embankment compacted "WET" of optimum.

Thanks!

 

[mohanvenn]

Sorry, I expressed it incorrectly in my previous posting!

My answer is Case IV, Case III, Case II, Case I.

 

[VAD]

I will go along for the ride - Case 1V, CaseIII, Case II, Case1. I think answers to this question are not clear cut. One has to look at the height of embankment to assess the compression, and as well in the case of the swelling soil, the detremination of the swell. Case 1 can be before Case II. In many cases depending on the Clay soil, one would be hard pressed to monitor differences in the field since the effect of weather - additional moisture may not be significant. On the otherhand for the swelling soils shrinkage of the upper layer could result in apparent settlement. This type of question is in my opinion would not attract many answers based on the information provided. The question encompasses unsaturated and sturated soil mechanics and Del Fredlund's book on the subject Unsaturated soil mechanics may be worthwhile reading.

Other questions are possible but as I said I wil go on for the ride.

 

[Focht3]

Well, since I'm pretty sure that I know [blue]MohanVenn[/blue] - and where he is located (,where he works, etc.) - my answer may not be entirely unbiased. {end of disclaimer} [This also had to do with my delay in posting - I wanted others to weigh in first.]

I'm mostly with [blue]VAD[/blue] on this one, although I'm not sure that Del Fredlund's book would do much more than tell one what the important factors would likely be from a soil suction perspective. Embankment height is a key parameter; but so is the embankment geometry. I would expect a 100 meter tall embankment with 3:1 (H:V) slopes to settle differently than one with 4:1 slopes; everything else being equal, I'd expect more settlement with the 3:1 embankments.

I guess one of the key questions in this debate would have to do with the point of reference, and when to begin measuring the settlement -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[mohanvenn]

Thanks Vad! and Thanks Focht3! And, I did read your FAQ158-992, valueble information there.

I have a foot in my mouth! I did it again, misquoted myself! Yes CASE I can Settle MORE than CASE II. In general, i believe "DRY" soils settle more than "WET" soils...... I am glad I not on a witness stand. The lawyers would have had a field day with me.....

I am there with you Focht3, Fredlunds'book wouldn't do much. However, the simple defenition can be gathered that settlement occurs due to explusion of pore AIR, pore WATER and Particle Rearrangement.

He is My logic....

Air can be expelled out of the soil "very quickly" and with a lot "Less intensity of load". Now, both are unsaturated soils, wet or dry of optimum. Even though both soils have equal volume of voids (Vv) they would have unequal Volume of Air (Va) and Volume of Water (Vw). "Dry" soil has more Va, when compared to the "wet" soils. Therefore, when the soils (especially non-expansive soils) are loaded during exposure to weather, or due to surcharge loads, or increase in load due to water saturation, they should settle more. Not to mention, the particle rearragement that may happen.

Any comments/objections to the above?

Now, for expansive soils, there is the swelling issue. Here
for now, I leave it with the thought, height of the embankment or fill is a factor, so is swelling pressure and so is the preconsolidation stress induced during compaction.

Oh! Wellcome to the ride VAD!






I beleive dry soils

 

[GeoPaveTraffic]

Good question, here is my two cents worth.

Material compacted dry of optimum will be stiffer than material compacted wet of optimum. This will result in the dryer material having less elastic settlement at the same load. It will also result in a higher effective preconsolidation pressure, which will make the consolidation settlements less.

Therefore, my vote from least to most is Case I, III, II, IV.

 

[iandig]

Just to add my own comments to this, and based on research, talks and published papers I have come accross, there are three distinct zones around the curve of a compaction, which can result in long-term movement of compacted fill material. The following comments really relate only to cohesive soil, and assume that the compactive effort has been correctly selected for the long-term conditions of the structure.
If the material is compacted 'too wet', then there is a risk of consolidation.
If it is 'too dry', there are two further zones the material can fall within. If the compaction level provides values below the 100% compaction, then there is a risk of collapse settlement, particulary under conditions of water inundation. The second zone when looking at 'dry' material, is where the compaction is greater than 100%, in this instance there is a risk of swell. The BRE has undertaken a whole series of reseach studies into this, and concentrated on deep fill, and the effect of long-term conditions. In the main they looked at controlled and uncontrolled cohesive fill, and the level of compaction used in the lab was the 2.5kg standard proctor. In the UK we tend to use the 2.5kg for road construction and the 4.5kg (modified proctor) for fill under buildings. Current reccomendations for buildings is that the compaction should be >100% of 2.5kg to 95% of 4.5kg, with less than 5% air voids. Roads just tend to require >95% of 2.5kg, but this is changing.
Anyway, back to the original question, will two samples at the same percentage compaction move the same amount, if one is wet and one is dry. In my opinion, the answer is NO, the process of settlement for each condition will be driven by different parameters, and as such it is unrealistic to expect the identical level of settlement.

 

[mohanvenn]

Been Busy with work to review the postings!
One thing is very clear, based on the postings, everybody seems to agree they would settle differently.

Anway, I see there are two new postings, Thanks to both!

One appears to be definitive, claiming that dry soils settle less than wet soils.

To the second, could I get some references for any research publications.

I disagree with the GeoPaveTraffic. He is the following logic....

We can easily see this from a series of simple one dimensional consolidation tests (ASTM D 2435) on a non-expansive soil, non-collapsable soil (PI of 15) using remolded samples, molded at same the dry density with -3% and +3%; and, the tests to be run "Without Saturating", because in field the soils may not completely get saturated"; before application of loads (i.e., an ASTM NON-conformance).

For the tests, we can except instantaneous large volume changes due to explusion of air with every load increment. Now, for the case of -3% (the soil with larger volume of air), quicker expulsion of air in the inital stages could be expected. However, the rate may slow-down with higher loads as the mositure is lower.

Due to this quicker instaneous changes-in-volume, the e-log p curves would be different. A steeper e-log p for dryer soils would be expected. The slopes of the e log-p curve could be steeper, especially in the initial portion, thus resulting in larger Cr or Cc values and result in larger settlement.

The only thing is, Are there any objections in running the tests "WITHOUT" saturating the sample? Second, do you object to the logic?

My colleague suggests that, if larger loads are placed on these soils such that it completes secondary consolidation (i.e., taking the soils to 0.4 of initial void ratio; or complete explusion of all the air and water), in both cases, the soils would more or less achieve similar final void ratio (assuming the particle rearragement or particle compression is one and the same). However, in the field this is impracticle, as most field conditions would never see such large loads.

Shouldn't we re-visit the application ASTM D 2435 for compacted soils that may not get saturated in the field?

 

[GeoPaveTraffic]

MohanVenn,

I follow your logic; however, I disagree with some of your conclusions. Summarizing, you state that since the soil compacted dry of optimum has less water that there would be more settlement at a given load therefore Cc and Cr would be greater. What you are not discussing is Pc. The soil compacted dry of optimum would have a much greater Pc point than the soil compacted wet of optimum. Therefore at all pressures up to and somewhat above the Pc point for the "dry" soil the total settlement would be less.

I agree with your penultimate point that if you ran the test far enough the amount of settlement of both samples would be approximately the same.

With respect to your last point about running consolidation tests in an unsaturated condition. All tests should be ran under conditions that make the most logical sense based on the field conditions that are expected. Just two important points to remember. First the test would not be in accordance with ASTM, not necessarily a big problem, just make sure you and everyone involved knows why. Second remember that most of the theories used in soil mechanics are based on fully saturated conditions and if the soil is not saturated the theories may not apply.

 

[BigH]

Isn't Cc and Cr a material property for saturated soils? The water content shows you where you are on this curve if saturated; or on a similar family of curves for non-saturated soil at varying degrees of saturation. With additional vertical loads due to overlying new fill, the underlying soils would respond in kind to the change in saturation caused by the change of stresses from the overlying fill. Another factor that might/should be considered is the residual horizontal stresses in the fills as they are placed. I'm still trying to digest it but Malcolm D Bolton's book "A Guide to Soil Mechanics" (McMillian Education Limited) ISBN 81 7371 245 8, Section 10.3, pages 343 to 350 (in my Indian edition) clearly discusses the strength and deformation characteristics of compacted clayey fill. I would strongly suggest to those interested to get a copy and read it. Try yours Focht3!

 

[Ron]

Why are you worried about settlements in compacted material. That's one of the reasons to compact, wet or dry of optimum, is to mitigate later settlements. Unless your compacted layer is awfully thick, your post-compacted settlement should be very small, either wet or dry of optimum.

The void ratio can be the same whether compacted wet or dry of optimum. Voids are voids...whether filled with water or air. This is the parameter that will control additional consolidation.

 

[BigH]

Sorry . . . If expansive clays are compacted dry of optimum, they will swell with positive changes in moisture content - i.e., more of the voids being filled with water. While this will not be "settlement" - it is volume change, negative settlement if you wish.

 

[mohanvenn]

Friends:

The answer to the question is found in the Paper, "Performance of Compacted Clay" published in May 1959 in the "Journal of Soil Mechanics and Foundation Engineering", by William T. Lambe. Partial information is also listed in the Lambe and Wittman's book.

The paper also addresses the differences in fundemental properties such as particle arrangement, permeability, compressibility, shear strength,etc. in relation to "wet and dry" side.

On the subject at hand, compacted Clay is:

"More compressible on the wetside in low pressure ranges, more compressible on the dryside in high pressure ranges."

However, the paper also makes the following statement on what should be done dry or wet?; which confused me.

"A low clay dam on soft foudation soil should be compacted wet. A high highway embankment on good foundation should be compacted dry".

It is unclear, if "Lambe's quote" intended to cover expansive nature of the soil.

Nonetheless, good reading.

 

[GeoPaveTraffic]

mohanvenn, I'm not sure what Lambe had in mind in the quote, however, typically dams are compacted wet of optimum to maintain a more plastic material. This helps prevent crack which can lead to piping of the dam.

 

[mohanvenn]

I understand the part about the clay "dam". As paper indicates "dry side more permeable".

By the way, Prof. Lambe also states that "which is better dry-side or wet-side compaction, depends on the Soil and Problem involved."

But, my confusion was more with high higway embankment on a good foundation. If dry-side is more compressible in HIGH pressure ranges, Why compact a HIGH embankment on the dry side? The reasons I can think of are, higher pc values, rapid settlement.

 

[GeoPaveTraffic]

My guess is that even with a HIGH highway embankment the pressures may not exceed the Pc at any point in the embankment. Also, even if the pressures exceed Pc in the lower part of the embankment, the settlement is likely to occur during construction and therefor not impact the final embankment.

 

[BigH]

In highway embankments - unless your embankment is very very high, you worry more about the settlement of the foundation than you would of the embankment internally. Yes, it can be up to 1 to 2% of the height of embankment but small compared to the foundation.