Can replace 57 with 4 and not compact? 3

[kedpe]

I have a contractor asking if he can place a sanitary sewer, backfill w/ 57 stone to a foot over the pipe, then backfill the rest with 4-saying he wouldn't need to compact this. It is in a future parking lot (1-2 years out). Does anyone have experience in this area?

 

[oldestguy]

msucog:

A great couple of posts. It is great for others to say that they don't compact these fills, but when that day comes when they wish it would have been done, the costs to them then will be far more then the added cost of compaction.

I've seen dumb things like this done and some major failures, with the contractor saying he has done this for the last 25 years. My reply is "I bet you don't ever do it again".

 

[msucog]

thanks oldestguy. i may not be an old wise man yet, but i've seen a lot of practical experience in my relatively young career by the time i count all my contracting experience during and after college before i went in to engineering. so, i try to pass along whatever mistakes i've made and seen in an effort to help others avoid such things. i also like to see other's responses in an effort to fine tune my own opinion. a lot of people argue with my stances since they always say that they've seen it work. the problem is that i see it not work quite often since i'm on so many jobsites all the time and then i'm also the one that does the exploratory work when something starts to crack and look ugly. oddly enough, i see uncompacted crushed aggregate as being the apparent culprit more often than poorly compacted soil fill/backfill (that just seems backwards from what it should be--also silly since it's very easy to densify aggregate for the most part). even if i'm completely wrong about densifying aggregate, it's not a lot of extra work and is an expensive investment for the contractor, owner and engineer to require it. my line to people that says something always works is: "well, you're lucky to never have seen it when it's obvious that it has quit working. by the way, how long is your warranty on the work? 1 year? so you don't ever look back after 12 months? huh, imagine that."

i challenge any of you disbelievers to try the method i explain above with using your probe rod to probe up and down in uncompacted #57 stone. again, work the probe rod all the way to the handle and then rod up and down in say 8-12" strokes. as you do, it will gradually back itself out. once you get to the top, you'll have yourself a swanky looking funnel where the aggregate has been rearranged and densified in that one spot. now, do i mean that you'll see 6-12" of settlement out of 3' of uncompacted #57 stone? no...but you dang sure do have the potential for a lot more than 1/2 to 1". i guestimate that you could see 1-2" of settlement per foot of aggregate fill over time in a foundation condition until the aggregate reaches its "happy point". i suppose that under high loads or dynamic loads that those number could easily double. under "no loads", maybe half of my estimate seems reasonable to me. again...just my meager guestimate.

as engineers, we always have safety factors for everything under the sun but wouldn't it be nice to not have to worry about this particular thing so that we can focus on those things that should have safety factors? by the way, when is someone ever going to come up with a safety factor for contractors? (i'd like to see a new thread discussion on that last question)

 

[DarthSoilsGuy]

i'm a little skeptical that the compaction seen with the probe rod can translate to heavy machinery. i think i've done that with a probe rod before i got out of the CMT business.

i'm going to put on my soil mechanics speculation hat now..

it would seem to me that a heavy plate would put a lot of energy over a large area and the whole face/face/corner internal structure, which may not be optimally arranged, would absorb that energy as a total unit. i further speculate that in order for consolidation to occur, the gravel must be allowed to rearrange it's face/face/corner internal structure which is easy to do when it is not under stress (carrying the impact). it would seem to me that the internal structure would become a rigid structure as all the loads pass through the rocks.

With the probe rod, you're applying a greater amount of localized concentrated energy and there is more internal displacement because the neighboring stones are not under pressure and are more free to move and settle into place. You end up with an optimized arrangement of stone.

all right, the speculation hat is off. and i'm ready for the flogging that may come.

msucog have you seen a difference in your probe method after tamping?

 

[TDAA]

“You can milk anything with ni . . . “ Oh, different train of thought, but same concept. You can compact just about anything better than just dumping it into a pile (or trench). It doesn’t matter if it is soil, aggregate, concrete, timber or some pens. It is just a matter of what kind of effort and how much.

Darth referenced compaction to a proctor standard. Perhaps this is not the best for the stone. Would you use the modified Proctor to compare the level of voids there were in a pile of 2x4’s? Doubt it. Would you drive some piece of equipment over them and get them to compact? I hope not. Does this mean that a dumped pile of timber could not be made more compact? No. You just have to stack it properly. Then you would likely weigh the stack, and measure it to determine density of the wood pile.

This is the same concept with any of the soils and aggregate we use in our business. You need to use the proper technique for the material. There is the wheel roll, sheepsfoot, vibrtory, (the dreaded) flooding, and preloading of a site to name a few. It is a matter of finding the proper method for the material. It is about finding a way that will compact the material more than the potential failure catalyst (moisture, vibration, heavy static load, etc).

So, when it comes to the aggregate: Sure, if you dump 5 feet of it into a trench and push/vibrate on the top, you are not going to get much. If you dump it into a Proctor mold and beat the hell out of it, again, you will likely not get much. If you run a relative density, you should see some change from the low to the high (thus it is compactable). And if you place the aggregate in thin lifts, perhaps add a touch of moisture to lubricate things, and move the aggregate so it fills the voids, then you are compacting it. But that is too hard for the contractor that is looking for the easy way out.

It is not that it is “self compacting”, but that it is “easily bridging”. When the moisture, load, and/or vibration causes that bridging action to fail, you then have the problem, which others have posted above.

 

[msucog]

"msucog have you seen a difference in your probe method after tamping?"--absolutely 100%. if it's put it in lifts and compacted (either mechanical hand tamp, big roller, or even repeated blows from trackhoe bucket) the material can't be probed more than a couple inches. if it's 3 or 4 foot put in and the top beat down, you can usually manage to work the probe down past the top foot and hopefully find a place to bury the probe rod. it's just like soil fill in a sense. if they pack the top enough, it's difficult to identify that it sucks down below (if you dig down, you'll find it though).
here's a field trial experiment next time you've got to have a sizable excavation backfilled. try to compact one half of the hole as you come up (say with the trackhoe bucket). let the other side be more or less backdumped. then poke on both side and see if there is a difference. heck, you could probably even reproduce this with #57 stone in 5 gallon buckets. fill up one or two buckets with #57 stone then dump them as fast as you can in to another empty 5 gallon bucket to try and simulate backdumping. then, take another empty 5 gallon bucket and fill it up using a shovel and periodically stick your foot in to tighten it up a little as you backfill. then weigh both and see if there's a difference. do this test a few times and see if the results are consistent. i'll do you one better--i think we've got #57 stone down in our lab and i'll try it to see what the results are and post back the results. it may or may not work but i'm thinking it will work even if the results are scattered due to the size of test samples--maybe i'll be famous and get published with my little experiment

 

[msucog]

i ran down to the lab and tried out my little experiment. i was a little surprised by the numbers but here they are. i used the same 5gal bucket and tared out the scale so the numbers are for #57 stone only.
run #1: quickly dumped 2 buckets of #57 stone in to 1 bucket and leveled off top--68.88lbs per 5 gal bucket
run #2: slowly dumped in stone in about 6 inch lifts and very very little tapped it down (not beat the heck out of or even remotely close to that). leveled off top--78.32lbs per 5 gal bucket
translating that to cubic feet (if i did my conversion right): 68.88lbs/5gal=103.05pcf & 78.32lbs/5gal=117.18pcf
that's a 13.7% increase with very minimal "compaction" (and i literally tapped with about 2-3" blows from the rammer). after i thought about it, that seems reasonable. you'd probably see something like at least 20-35%+ shrinkage out of rock fill with adequate compaction (depending on the gradation of course).
i did only do one test. it seemed enough to prove a point. it would be good for someone to use a big mold and do the same thing except actually compact the stuff and see what the numbers are.

 

[oldestguy]

OK all you experts.

If you can find a copy of the old US Bureau of Reclamation "Earth Manual" take a look at test procedure E-38. Maybe they still use it. Anyhow, that compaction test uses a mold 20 inches in diameter, sixteen inches high. Hammer area is 70 sq. in., weighs 185.7#, 18 inch drop. 22 blows per layer, three layers. Requires samples of 600 to 800 pounds of earth, so you can run several.

Apparently will take gravel up to 3 inches diameter.

So you see there is a lab test that you can use for this material and maybe it will help resolve this for skeptics.

Next question. Where is that testing machine?
Can't find one? OK, just poound the stuff on the job anyhow and don't worry about percent compaction.

Stay with it msucog; you are right.

 

[emmgjld]

oldestguy,

I am pretty sure that one monster compacter is about 60 miles south of me, in the Montrose Colorado office of the former U. S. Bureau of Reclamation. They also broke concrete cylinders which had 6" aggregate, if I remember correctly.

I also agree with msucog.