Compaction Specification - 5% air voids

[AussieGeoEng]

I am wondering if anyone has experience with using a specification for earthworks based on achieving a minimum air voids percentage?

My theory being that:
1. material placed dry of standard optimum moisture may have satisfactory performance when you proof roll - but will deteriorate over time.
2. material placed dry of optimum will swell under low overburden.
3. material placed dry will collapse under high overburden.

Any thoughts?

 

[BigH]

Maybe the specification is to ensure enough air voids for drainage purposes. I've never run across such a specification. however, I don't believe that 1. is necessarily so. Why do we compact to Modified Proctor maximum dry density - its OMC is dry of the Standard Proctor maximum dry density OMC. As for 2, it might be right for soils that have a swelling potential (fat clays, for example) but it would make no difference for granular materials (low plasticity fines). As for 3, I doubt this as well unless placement was such that led to an unstable skeleton - like, perhaps, way dry of MOC. Just my thoughts. I have always tried to place granular materials dry of optimum - 2 to 3% would not be a worry to me.

 

[dgillette]

Minimum air voids percentage is something I've never seen, and the logic for it eludes me. Seems to me that you could just place your fill wetter and achieve the minimum air voids requirement without achieving good dry unit weight. Is this something you are proposing to do, or something

An awful lot of dams have been built dry of optimum and still hold water, and on a high embankment, it has advantages in limiting post-construction settlement and excess PWP.

I believe BigH is correct on 1, 2, and 3.

DRG

 

[dgillette]

Left my first paragraph short:

Minimum air voids percentage is something I've never seen, and the logic for it eludes me. Seems to me that you could just place your fill wetter and achieve the minimum air voids requirement without achieving good dry unit weight. Just for context, is this something you are proposing to do, or something that is being forced upon you, or what? What is the purpose of the earthwork (dam, highway, landfill liner...)?

 

[Ron]

5% air voids?? In soil? Way out of whack in void ratio.

Sounds more like an asphalt spec.

Have never seen such a specification for soils.

BigH is correct on 1,2, and 3. ( Or at least I agree with him and I also assume that when anyone's opinion is the same as mine, that person is correct by default!!)

 

[BigH]

Whew - just glad there was no 4!

 

[AussieGeoEng]

The 5% comes from UK specification for Earthworks.

http://www.dft.gov.uk/ha/standards/mchw/vol1/pdfs/series_0600.pdf

Primarily talking about medium to high plasticity clay fill for road embankments.

BigH - When modified specified I assume that you also compact to somewhere around 95 to 98% modified?

When you say way dry, how far? A specification is allows for 60-90% of OMC.

Ron - I have attached a standard compaction plot. Looks to me like 5% (or 10%) air voids would be reasonable.

 

[Ron]

A-G-E...yes, but I should have expanded my comment a bit. Some soils can exhibit a valid compaction curve, with a reasonable optimum moisture and not be anywhere near saturation. Although I couldn't explain why, because it didn't make any sense, I've seen saturations as low as 65 to 70 percent in a relatively clean fine sand. I had the lab check the specific gravity (which would have had to be in the 2.20 range to make the saturation calcs work...not practical with a silica sand), re-do points on the compaction curve to see if it tracked, and found no apparent discrepancies....but the odd result.

Is their goal to optimize compaction and saturation on an equal basis?

 

[BigH]

AGE - specifying level of compaction is one that would take into account one's own experience, the "way it has always been done", the nature of the structure, etc. For road bases, we would specify to a minimum 98% modified Proctor MDD (MP MDD). Under a machine foundation, I might actually specify 100% MP MDD. Normal embankment fill, I would specify 95% Standard Proctor - or if modified in the order of 92%.

I've had many discussions with some or my junior engineers years ago - for foundations (under them) I always specify MP MDD, even if I use 95%. They wanted to use 98 or even 100% SP MDD. I won as my name went on the report. The MP test could be called a "hernia test" - but by specifying this (or heavy compaction for the Brits), you are telling the contractor that this IS IMPORTANT; where when you specify SP (standard Proctor) many contractors and testing companies might get a bit lazy.

By very dry of optimum - you will find in the literature that you can have a higher dry density at very low moisture content - it might not be as high as the "real" peak but it can be there. I found it in India with a mix of fly ash and sand - which exhibited "two" peaks depending on the moisture content.

By the way - I try to stay away from moderate to highly plastic clays for embankment fills if at all possible. Have never had to deal with the use of such materials for fills very often . . .

 

[dodgeyc]

I've been told that Engineered fill that has DDOMC outside of 0-5% air voids is at risk of collapse compression on inundation.

 

[fattdad]

Your reference to "air voids" is what I would call my reference to "saturation." I prefer the latter and have specified a minimum level of saturation on earthwork projects. Bearing in mind there are a few engineering characteristics in compacted fill that are prominently affected by compaction moisture content - namely hydraulic permeability.

When specifying placement criteria for compacted clay liners, for example, compaction moisture content is critical as when you are less than about 90 percent saturation the permeabilty is affected as much as two orders of magnitude.

I just don't think research shows that your conditions 1 through 3 are typically true.

Not to digress too much, please recognize that optimum moisture content for 100 percent compaction is not the same as optimum moisture content for 95 percent compaction. Just like there is a line of optimums that parallels the zero air voids curve (i.e., as referenced above, the optimum moisture content for modified proctor and standard proctor line up with the ZAV). This whole topic of material properties and optimum moisture content looses focus when your placement criteria accepts 95 percent proctor. This is true whether you are considering modified or standard proctor.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

All - AussieGeoEng is correct in that %Air Voids is a British thing - rather than use % Saturation, they use % Air Voids - see attached - from Bell's Ground Engineer's Reference Book, article written by Parsons of TRRL.

 

[AussieGeoEng]

f-d, I realise that the optimum varies for each compaction level. I like to think of the SP OMC as a good reference. I have heard of other moistures - even machine optimum. Which would be the optimum moisture for achieving compaction with a certain type of machine and compaction procedure.

BigH - in your post on 4 Apr 11 you say "way it has always been done". I agree to a certain extent. As you also say, most of the time we try and stay away from moderate to high plastic clays. Sometimes this cannot be done. In that case I don't know that experience that is gained for low plastic clays can be applied to moderate to high plastic. At least not without a lot of careful attention.

I also think it is one of those things that, to tell the difference between a good and a bad compaction specification could take upwards of 10 years. And then to nail down that the poor performance (i.e. ride quality on a highway) is due to the compaction specification would be a very costly and time consuming process.

 

[BigH]

I think that you have, perhaps, misinterpreted my "the way it has always been done." What I was trying to say is that the selection of what percentage of MDD (whichever standard you use) is based on 1) personal experience and bias, 2) the way it has always been done (company experience - say for 20 years tank pad fills of the same material have been placed at X% MDD and they have behaved properly) - so why change?; 3) requirements of a particular agency - say ODOT;

You are correct in that highly plastic clays are a different animal and I indicated that I tried not to use clays at all in fills - although I know that there are times that it must be done. For HP clays, one needs to know the effects of swelling and shrinkage due to moisture migration - both from the initial compaction moisture content and 'final' - but what seasonal changes might occur. I understand in California they use 85% compaction levels in many fills for this reason when dealing with such materials. One also might wish to use lime or other additives to reduce the level of plasticity - so when one deals with moderately to highly plastic clays there is really a solid need for treating the compaction level and equipment with a higher level of care.

 

[fattdad]

Level of compaction may help control compression of highly plastic clays, but in the long term will have little effect on the effective friction angle. Work by Stark (University of Illinois) and Wright (University of Texas) shows that the strength of highly-plastic clay will return to the "fully softened" shear strength over time. This is best represented as the normally-consolidated shear strength and best determined from a sample at the liquidity index (i.e., a wet paste), placed into the shear device (i.e., DDS or torsional ring shear), consolidated to test pressure and sheared. FSS is the peak strength obtained from the normally consolidated sample.

Yeah, compaction is great, but if the topic is plastic clays, be careful about the long-term performance!

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

All: I was packing up my Canadian Geotechnical Journals when I ran across V44#2 (February 2007) which contained an article "Earthwork Compaction Evaluation Using Soil Air Voids" by Robert Mokwa and Stefan Fridleifsson. What was interesting is that the concept was proposed in the US back in the 1940s but never gained traction. I've only just glanced at the article but it seems rather interesting and I might suggest that y'all give it a look-see. They discuss how it is much less time consuming than doing Proctors when fill materials change.

 

[AussieGeoEng]

f-d & BigH, Thanks for the last two posts.

f-d do you have any references to the work you talked about?

I will have some more reading to do.

 

[BigH]

AussieGeoEng - there were some caveats in the paper - and the paper uses 10% air voids as the "basis" - not the 5% that you had given. To their study, the air voids method (AVM) should not be used on poor graded granular soils and it definitely not suited for plastic clayey soils. Try to get a copy of the article.