Compacting AASHTO #57
Compacting AASHTO #57
(OP)
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?





RE: Compacting AASHTO #57
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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RE: Compacting AASHTO #57
RE: Compacting AASHTO #57
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.
RE: Compacting AASHTO #57
RE: Compacting AASHTO #57
RE: Compacting AASHTO #57
RE: Compacting AASHTO #57
RE: Compacting AASHTO #57
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.
RE: Compacting AASHTO #57
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.
RE: Compacting AASHTO #57
RE: Compacting AASHTO #57
I don't fully agree with MRM on the "need for compaction." Generally speaking, our local materials have a DR 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...
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: Compacting AASHTO #57
RE: Compacting AASHTO #57
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.
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: Compacting AASHTO #57
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.