Compacting AASHTO #57 2

[PennsylvaniaPE]

Is it realistic to specify a compacted density for AASHTO #57 stone? I've been told that using this material is like "dumping marbles" into the excavation and that additional compaction effort accomplishes little. I've also been told that the significant voids in this material prevent a meaningful compaction test using the nuclear probe. Is this true?

 

[Focht3]

We don't use that specification around here - could you kindly point to an on-line PennDOT or AASHTO spec for the material?

If the material is a well rounded, washed gravel then the comments you have received are pretty much on target. The good news is, that if the material is well confined, you don't need additional compaction - it's sufficiently dense as placed for most engineering use.

I designed a local custom home that rests on about 7 feet of 1.5 inch "washed river gravel", which is more than 10 feet below ground surface. (It's a long story of "why" - don't ask!) It has been complete and occupied for several years - no movement so far. And I don't expect any -



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[BigH]

Can you give more details on AASHTO 57 stone? I'm not used that term before although I have seen it - it's not in the AASHTO books I have. I know in local experience I had in Canada that contractors used single sized crushed stone in sewer backfills since they didn't "have" to compact it - and as Focht3 says, in a trench it is confined and is great for almost all purposes - do have to worry, from time to time, on its behaviour as a french drain - in some cases you don't want that.

 

[dirtdoctor]

I do not believe it is referred to as 'AASHTO 57 stone', however, #57 stone as defined by quarries, state agencies, etc. is an aggregate blend of size 5, 6, & 7 stone. This material cannot be 'compacted' in a true sense, but can be properly oriented with compaction equipment. This is particularly important when using stone as a replacement material in undercut footing excavations where sidewalls are soft. I have used 57 stone in this application where sidewalls are as soft as 0.25 tsf at total stone depths of 10 feet. As the stone is placed, we use a trackhoe bucket to beat on the stone and press it into the sidewalls - helps to reduce the potential for future stone loss into footing sidewalls, and subsequent loss of contact between poured footings and in-place stone.

As far as testing w/ a nuclear gauge, it is impossible. I have never seen a proper means of doing this, but have often seen specifiers state that it should be compacted to 95% of Proctor values. We simply state in our reports that it should be properly oriented per the directions of the geotechnical engineer.

 

[PEinc]

According to PennDOT's 408 Specifications, AASHTO 57 coarse aggregate stone has 100% passing 1 1/2", 95-100% passing 1", 25-60% passing 1/2", 0-10% passing #4 screen, 0-5% passing #8 screen. PennDOT Section 210 - Subgrade specs say that when material is too coarse (more than 20% retained on the 3/4" sieve) to use these (previously indicated compaction) methods, compaction will be determined based on nonmovement of material under compaction equipment specified in Section 210.3(a).

 

[BigH]

Thanks on the info on #57 stone! Is there an actual spec somewhere for this and the other sizes. Dirtdoctor said made up of #5, 6, 7 stone - what is that. Ontario is so much easier - Granular A (a base course from quarry crush), Granular B Types 1 and 2, Granular C, SSM . . .

 

[PEinc]

I believe the Pennsylvania Department of Transportation's Standard Highway Specifications, Publication 408, are available on line. AASHTO 57 stone was listed in the section on coarse aggregates. The gradations are also given for AASHTO No. 1, 3, 5, 67, 7, 8, and 10 coarse aggregate.

 

[cvg]

look in the AASHTO specifications for all of these ...

 

[cphi]

PennsylvanniaPE:

Open graded stone described in your post and the other gradations described in this thread needs the additional compaction. Try this demonstration for yourself. Fill up an empty 6 in. diameter and 12 in. height concrete cylinder mold with No. 57 stone using a shovel to simulate "dumping". Then level the stone with the top of the mold so no pieces project above the mold. Then hit the entire perimeter face of the mold repeatedly (without tipping the mold over) with a 2x4 or concrete form board steel stake. This will simulate vibratory compaction. The top of the No. 57 stone will be about 1 inch below the top of the mold, about 8% settlement.

On projects where I recommend the use of open graded aggregate, I recommend the compaction method described by PEinc in the above post, but I add "determined visually by a geotechnical engineer." Hope this helps.

 

[cphi]

Focht3 and dirtdoctor:

The voids between the open gradation aggregate (Focht3's 1.5 inch washed river gravel and especially dirtdoctors' #57 stone) can allow the adjacent soil below and around the sides to erode into the voids with water flow or extrude into the voids from pressure until the so-called "filter-cake develops." A graded filter design calculation is applicable here.

Placing a geotextile to serve as an envelop around the open graded aggregate to maintain separation between the open voids and underlying and surrounding soils will reduce the chance of settlement of the open graded aggregate and settlement of the adjacent ground surface. Cheap insurance.

 

[MRM]

I agree with cphi. Just like any other bulky granular material-poorly graded gravels (i.e. #57 stone) have minimum density and a maximum density values. It definitely needs to be compacted if it is to be used below anything you do not want to move much! I'd venture to say that there is even another potential settlement component involved with gravels beyond sands-and that is the potential for particle breakage with time. This only reinforces the idea of applying some level of compaction effort to the gravel as you are placing the material.

 

[Focht3]

[blue]cphi[/blue] makes a very good point; I insist on the use of a geotextile separator in my practice, too. But I was mute on that point since the original post only asked about compaction of the material, and didn't indicate what the subsurface conditions were like. But it's an important point.

I don't fully agree with [blue]MRM[/blue] on the "need for compaction." Generally speaking, our local materials have a D_R of about 85 to 90 percent when it's dumped from a 40 ton (long bed) dump truck. Even after they are reworked, the density is about the same - with no vibratory compactor used at all. (I do recommend a compactor be used with visual observation, but no density tests. It keeps the contractor honest.) Settlement just isn't an issue around here. And our gravels are mostly chert - siliceous materials. Since the bearing pressure will need to approach or exceed 100 ksf to begin grain breakage/crushing, I just don't see a problem for most structures...



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[MRM]

No compaction, Focht3? Don't you know I do construction observation on an hourly basis?!! I've got a wife, a dog, and 3 cats here to feed!

 

[Focht3]

Hey, [blue]MRM[/blue] - don't bust my chops!

I said,

(I do recommend a compactor be used with visual observation, but no density tests. It keeps the contractor honest.)

While I recommend mechanical compaction, it frequently isn't necessary because of the "self compacting" nature of the material. And there isn't a good way to test it - hence the use of visual observation (by my personnel) only. The main reason that we're present is to be sure the right material is used...

This kind of fill probably represents less than 1% of the fill used beneath building foundations in this area. We mostly use crushed limestone base, with some pit run material. Both of these materials require compaction testing, visual observation, etc.

P.S. I prefer cats as door stops, myself. A wife, three kids and an old mutt dog are enough for me...



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[dirtsqueezer]

PennsylvaniaPE,

You probably have enough information already, but if you're going to go with site observation only, I have seen useful specs that call out a specific method for compacition, which can be confirmed by your observer. For what it sounds like your aggregate size is, I believe what I saw called out in the plans was 4 passes with a vibratory roller. Just wanted to pass on that it's not unheard of to specify a method in the contract in the absence of a compacion percentage.