Cannot Get 98% Compaction 1

[ansys54]

We are working with a crushed rock from a mine site as a subgrade and the requirement is 98% compaction. The proctor values show 8% optimum moisture and density of 2350 kg/cum. We sat in one area for three minutes with a 1000lb diesel tamper and the density went from 89 to 89.5, a long way from the required 98%. The other tests were at 91. Even a measurement of the subgrade surface with the nuclear densometer showed compaction of 92 and yet you could not make an indent in the surface even by jamming your heel into it.

The crushed mine waste has coarse angular material and very fine powder material but almost nothing in the mid range. The temperatures in Northern Canada are below zero at night and not much higher in the day so we are concerned about watering and then packing.

Is 98% really required or should we be using another parameter to judge the adequacy of the subgrade. Like I said, we packed the same area for 3 minutes and did not move the plate tamper at all and still we could not appreciably increase the density.

We are trying to get a thickened concrete slab installed for the erection of a pre-engineered metal bldg.

I am happy to contact anyone who has information that could help us.

 

[Focht3]

Which compaction standard are you using? Who chose 98%? Why?

At first blush, you probably need to run another moisture-density relationship ("Proctor&quot to make sure that the material hasn't changed. But I suspect that the real problem is that your compactor isn't heavy enough. I would have expected at least a 20 ton vibratory drum roller for the material you have described.

Why the pad compactor? Can you use a larger machine? Can the compaction standard be waived?



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[SirAl]

Hi ansys54

What is the gradation of the material. I would be interested to know % (by dry weight) finer than the following sizes: 75mm, 25mm, 5mm, 2mm, 0.4mm, 0.08mm. Sounds like your product is poorly graded.

2350kg/m3 appears high, particlarly with an OMC of 8%. With that said, I agree with Focht3 that your equipment may be too small to generate the compaction specified ( particularly if the material is quite coarse).

I would be concerned about possible frozen material. You will not be able to compact frozen material (of any type). The same goes for dry materials. Bone dry rock with dry crusher dust will require water for compaction.

As a final comment, forget the heel test.

 

[BigH]

A suspicion that your rock waste is a bit on the coarse side and that it is, in fact, interlocking rather than "adjusting" under your compactive effort. Agree that your plant seems quite light - usually you would use 15 tonne compactors - quite more than 1/2 tonne.

 

[iandig]

Before you get too carried away with comparing the site compaction and the results of the laboratory analysis, have you checked whether or not any material was removed from the lab sample in order to carry out the compaction test. One of the major problems with undertaking laboratory compaction, is that many fill materials have individual particles which are 'too big' to be included in the compaction test. By removing these oversized particles, it will directly effect the optimum moisture content and maximum dry density. Have you also checked the particle density of the rock fill material, in order to confirm the validity of the lab compaction results. Typically, most materials will have an air vod content of between 0% and 5% when at their maximum dry density [MDD], if you find that the calcualted air voids at the MDD is less than 0%, then it questions the lab result, also by looking at the achieved level of air voids in the compacted fill, a further assessment can be made as to the quality of the compaction achieved. The action of compation is to reduce the void content in a material, and by using a combination of percent compaction achieved and air void content, a more balanced assessment can be made. Finally, in most instances that I have come accross, where it has not been possible to achieve the required level of compaction, this relates to the moisture content of the material as much as to insufficient compaction.

 

[DRC1]

If you are using a rockfill you cant use a procter. As iandig pointed out to do a procter, everyting over the #4 sieve comes out. Even if you dont remove a lot of the the material,If the material is large (#4) and angular, the mold provides alot of confinement. 2350 kg/cum is about 146 pcf, which is the density of unreinforced concrete. What is happening is that the material is reaching its maximum density, then shearing and dilating to a looser state. At 92% the density is about 135 pcf. If this is a single story pre engineered building on a concrete slab, I would say that by now you probably have a pretty good idea of when you are at the maximum density. Compact it to that point and forget the procter.

 

[Focht3]

[blue]DRC1[/blue] -


What about the lightweight compactor that's being used? I wouldn't lay this all on the Proctor - the major problem appears to be a mismatch between the fill and the equipment. The contractor is using a 1000lb diesel tamper - my car weighs three times that!

I would expect that a heavier compactor might cause some breakdown of the mine spoil rock material. But not enough to matter.

Still, the question remains:
Who specified the 98% requirement, and why? And how thick will the fill be when the pad is completed?



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[DRC1]

Actually a 1,000 pound tamper is a big tamper. Most large tampers are around 250 pounds and generate around 4,000 pounds of centirugal force at somewhere around 5,000 Hz. A 1,000 pound vibe roller will give you something around the same numbers. The compaction equipment seems more than adequate for the work, I just don't think you will ever get the soil to the density required by the specifications, and again, as you point out why do you want it that dense?

 

[BigH]

Ansys54: By the way - what is the loose thickness of the layer? Could you also give the full gradation by %passing various sieve sizes? Thanks.

 

[Focht3]

[blue]DRC1[/blue] opined,
Actually a 1,000 pound tamper is a big tamper.

It's a big tamper, but it's a small compactor. My point is fairly straightforward: a compactor of that size is unlikely to impart the energy needed to accomplish the required compactive effort - for the material described. Please see my October 21, 2003 post.

The other question that hasn't been addressed is whether the 98% requirement is justified - or is 90% acceptable? That is why I have asked for a description of the fill thickness, compaction standard and specifier. We still don't know whether we are talking 98% of ASTM D698, D1557, some state DOT standard, or a non-U.S. specification. That's a pretty crucial question...

As a general comment (not directed at [blue]DRC1[/blue] or anyone else in particular):


IMHO many (but not all) of the questions posted reveal a lack of understanding of the problem the poster has submitted because the post leaves out key issues needed to answer the question. Many times I find myself trying to redirect the discussion because we aren't "on the same page." Every member of the forum comes from an unique background, and we all bring our biases and prejudices with us. (I specifically include myself in that group - an example of my bias includes my very personal dislike for Bowles' texts.) We sometimes argue points without recognizing that we are not talking about the same problem. My hope is that we will all sharpen our critical thinking skills and ask incisive questions before we provide possible solutions - and not just shoot from the hip. That should help us all to provide more useful suggestions as well -

{[blue]Focht3[/blue] descends from his favorite soap box...}



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[fndn]

See if the moisture content is low. If it is below 6%, add water and let it soak for 15 minutes. Then recompact.

Before retesting with the density gauge, use sand sprinkles to prepare the surface if the gauge is surface one(no rod) or use the untouchable mode-where the test is taken 1/4" above the surface and no sand sprinkles are needed. Also make sure the test duration is 1 minute or more and that the tamper is turned off.

 

[ansys54]

Thank you to everyone.

As an engineer who has been in construction for many years, I agree with DRC1 in that a 1000lb diesel hits hard and deep. We took FOCHT3's advice and requested another proctor, and guess what? The proctor value was not 2350 as originally presented, it was 2140. Right off, we were much closer than was shown. We have now achieved well over the specified 98%.

As to why 98%. In Ontario Canada, it's just the norm around here and frankly, I've even forgotten why we specify it that high. I understand in Manitoba, the norm is 95%. I'll have to sit down with Bowles one night and try to figure out what proctor is reasonable.

Thanks again everyone for your input.

 

[BigH]

I empathize with specified compaction levels. There are two points to remember, though in my view. Firstly, as Focht3 rightly points out - is the compaction level given actually needed? The engineer putting together the specifications has to know this at the time of design/specification preparation because this is when the question needs to be answered. Because the second point is that once the compaction level is specified and the contractor signs on the bottom line - there is a stipulated requirement of the contractor to achieve the level of compaction - whether it is actually needed or not! To lower the level of compaction after the fact can require, in many cases, that the client gives approval. Then, again, is there any legal problems in that relaxation of a perhaps not so lower bidder is now considered upfair to one of the losing bidders. As geotechs we may not believe this is so critical - but now after a few years of working highway construction supervision - these can be the critical points to clients, losing bidders, and, most sadly, non-technical auditors.

Just some things to keep in mind. Looking forward to Game 6 tomorrow, eh?!!!!

 

[Rawand]

I agree with Focht3 and SirAl, and DRC1
Proctor test is not adequate for all types of soil, do u need density or final settlement? I think your material is not compactable, with a heavy compactor u will only smash the material, and will need a new proctor!.
I think 98% is not applicable for your soil, it is more like a (Typical contract requirement) for gravel material u will never be able to get such result, and moisture has no effect on such material.
You will have to address the consultant and discus, shalt he insist on 98% (apart from all our discussion) I think you will have to shoot him.

 

[cphi]

ansys54, you wrote:

....The crushed mine waste has coarse angular material and very fine powder material but almost nothing in the mid range.

My thoughts on a few causes.

The laboratory proctor test may have changed the material gradation from what you describe into a denser gradation and therefore a denser density by fracturing the coarse aggregate into finer sizes. A valid result Proctor test on gap-graded or open-graded material is difficult to do.

The nuclear density gage probably is not determining the density accurately due to the voids in between the coarse angular material and the bottom of the gage. If the nuclear gage uses a rod for insertion into the compacted soil, the method used to make the hole probably resulted in a larger diameter hole and crooked diretion (because of the coarse aggregate gradation)relative to the gage rod, making the test result invalid. If the gage (or the test mode) does not use the insertion rod, the gage "reads" the density only to a very shallow depth below the surface and the voids between the coarse angular material are a greater percentage of the volume of soil tested, leading to a low value and invalid test result. Placing finer aggregate as a filler at the gage test site invalidates the test result in my opinion because the tested material gradation is different than the laboratory tested gradation because it fills the voids and makes the material have a higher density without compaction.

The density test gage is a tool that is used by the geotechnical engineer to assist and confirm visual observation in evaluating the compaction achieved. Observation and testing together, not one or the other only is good geotechnical engineering practice in my opinion.

By your description, it is apparent to me that your visual observation of the compaction operations gave you the opinion something was wrong. Either the compactor equipment was malfunctioning, the lab proctor test was incorrect, the material tested in the lab was different than the material used at the site, or the gage was malfunctioning or determining the density incorrectly as I described above.

I apologize for the late post, I haven't logged on to this forum in awhile. Hope this helps out on your future work. By the way, it is cold here too, 0°F tonight.

 

[Focht3]

Hmmm,

It was 55&[ignore]deg[/ignore]; overnight here in San Antonio; where is home, [blue]cphi[/blue]?



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[cphi]

Focht3,

Milwaukee

 

[Focht3]

Brrrr!




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[BigH]

No!!!! Focht3 It is BEEEEEEEEEEEEEERR!!!!!