Can replace 57 with 4 and not compact?
Can replace 57 with 4 and not compact?
(OP)
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?





RE: Can replace 57 with 4 and not compact?
RE: Can replace 57 with 4 and not compact?
another problem with large screened stone deposits is that you have to consider what happens to any water (or wastewater) that may fill these voids. you really have to look at the site and look for potential issues and places for water to get in and out. in most cases this is not an issue, but it has been in forensic case studies too.
Lastly, are there any other utilities going next to, under, or crossing, EVER. if so, go ahead and count on some undermining of the parking lot.
RE: Can replace 57 with 4 and not compact?
To say nothing of the long-term problems, as hinted to above. Imagine cutting a future trench trough this mistake and maintaining the trench sides and the support under adjacent pavement, sidewalks, etc.
Sorry to sound so negative. I don't recommend or allow this sort of material any thicker than a leveling layer or underslab drainage layer, 6 inches thick MAXIMUM.
RE: Can replace 57 with 4 and not compact?
RE: Can replace 57 with 4 and not compact?
Sieve % passing
2 in (50mm) 100
1 in (25mm) 70-100
3/4 in (19 mm) 50-90
No. 4 (4.75 mm) 30-60
No. 40 7-30
No. 200 0-13
RE: Can replace 57 with 4 and not compact?
clean stone is used as backfill in freezing weather when it would not be prudent to use granular material.
RE: Can replace 57 with 4 and not compact?
RE: Can replace 57 with 4 and not compact?
i learned the lesson of compacting #57 stone the hard way back before i was an engineer and was a contractor. i did a project where we had one area that had to be backfilled about 6' on top of the footing to get up to sugrade level and a second area where a footing was undercut about 5' and backfilled similarly. the material was dumped in and leveled off. the slabs/footings around these areas were poured and building construction went on. about 3 months later (in the middle of winter), i started noticing little swirling holes showing up adjacent to the exterior footing area (the area where the footing was undercut) that was backfilled as described. i took a 7' long piece of rebar and sunk it about 1 foot off of the footing. that's when i realized the material was washing in to the backfill. at the end of the project, we had problems with cracks showing up in the masonry at these two locations. i'm fairly confident it was because there was some subsidence in the slab/thickened slab or footing that was holding the walls. the cracks were worst just outside the actual backfill area and lessened with distance away from that. the cracks did extend to probably 30-40' away from the area before they disappeared.
i have to argue with contractors all the time, so i have actually bet (bragging rights) a contactor before. the contractor was going to backfill 7' in a 10'x20' area and didn't want to compact it because he thought i was full of bs. i told him that he had to densify the backfill or they'd have problems. since he didn't believe me (and apparently had the time), he decided to first backfill as he had planned. we painted a line around the top of the backfill and then he dug it back out to within a couple of feet of the bottom. then he backfilled it again while beating in each 1-2 foot layer with the trackhoe bucket. when he got back to the top, the backfill was almost a foot lower (call it 6-8"+ since he left a little material laying on the ground and the paint line was 2 inches thick,etc). his only comment was "YIKES!"...and i never had to argue with him over densifying #57 stone backfill after that.
RE: Can replace 57 with 4 and not compact?
RE: Can replace 57 with 4 and not compact?
The fine migration is from the trench walls and the soil you place on top of the stone deposit. since clean has a large empty void ratio, soils from around it can migrate into the voids and then the question is what replaces those soils. there is a judgment call for the fabric that has to take into account the site, soils, depth of stone, and [fill in the blank]....
If you have groundwater entering the trenchs, it makes it a lot harder to place & compact backfill. Some clean stone could be warrented to get to a point where fill soils can be put (while running a pump in the stone on the downslope side of the trench), but the deeper the stone trench the more potential problems that i posted before. With water entering the trench, i'm a little more sympathetic to the contractor. But, beware of the ease with which contractor's will want to haul in stone over structural fill once they get going and get a write-off on having to compact.
RE: Can replace 57 with 4 and not compact?
Clean stone is not compacted to a proctor because 1. Proctors are not designed to measure compaction of stone sized particles 2.difficult to accurately measure in place density 3. Since the density is rarely measured, there is no tables date etc. to correlate it to.
Fillig on top of stone can be accomplished with filter fabric or choking with 1/4 in and peastone.
RE: Can replace 57 with 4 and not compact?
The fairy tale of non compacting gravel you mention with regards to concrete foundations on soft soils I have used for many large commerical buildings with no problems. I have placed #57 gravel as much as 8 feet thick down to virgin soil BUT the base was "over excavated" with a 2 on 1 distribution load path. I never compacted #57 stone and never had any settlement issues on many brick buildings. Pipe trenches is a different story and may need a filter fabric. Hopefully no other future work will ever get done next to it. I have seen some paper work from some stone suppliers lab here in NJ that by just dumping the stone out of the truck that will give you a relative density of around 90%. Not bad.
RE: Can replace 57 with 4 and not compact?
RE: Can replace 57 with 4 and not compact?
RE: Can replace 57 with 4 and not compact?
RE: Can replace 57 with 4 and not compact?
I will admit that there some areas where the crushed stone densifies easily. You are lucky to be in an area of anomalous materials. In the last 35 years, I have run into such materials in Colorado and surrounding states, BUT very rare. I encountered a similar crushed material in Viet Nam, south of Hue. I read about a few instances on this Forum. I consider them to be anomalies.
The vast majority of recommendations using coarse rock stabilization on this forum involved working the material into place, whether above or below the water table. True, this material usually (and I stress usually)readily achieves maximum density fairly quickly. If the material is angular and properly graded, it is usually fairly stable. If the gradation is not right, though still meeting the published gradation requirements, or if the stone and sands are rounded to subrounded, densification may or may not be achieved and stability can range from moderate to extremely low.
RE: Can replace 57 with 4 and not compact?
FYI, Not only is it angular, the gradation is perfect here in northern New Jersey. Its a diabase rock called "black granite". Diabase is actually much stronger than granite.
RE: Can replace 57 with 4 and not compact?
I suspect the issues of migration of fines, and uneven pipe support etc. surrounding such local voids (if some effort is not made, by mechanical compaction and/or I will note many other means such as shovel slicing or narrow roller compaction wheels alongside the pipe to somehow move the rock more evenly up under the haunches), could well be even more important per se than how much can one compact a particular layer of crushed rock. I would add that I would suspect the overall volume of voids that might occur under the pipe with dumped rock might generally well be much larger with very large pipes, whereas it could in general be much smaller e.g. with minimum size 8” sewer pipes. Also, some types of pipes and joining systems might well have adequate strength, long-term ring stiffness, and ductility etc. to tolerate such imperfect support, and some might not. Perhaps the same could be said of any deleterious effects of voids etc. on the overlying parking lot. If however the pipe/joint system is compromised by non-uniform bedding, resulting in broken pipes or infiltration/sinkholes etc., I suspect most would agree this is not a desirable situation for either the pipeline or the parking lot.
RE: Can replace 57 with 4 and not compact?
One thing about 'self compacting' gravel is that you also don't know how the soil around it is going to react. When you place that #4 (which should be angular to even consider this in the first place) you want those stones to lock in with the excavated wall- this requires tamping of some sort. The surrounding soil also may be softer than your fill and might benefit from the pressure of that compacted fill. If you don't compact the backfill against the wall, the wall fill might just settle back against the fill. If you don't compact the fill at all, you don't get the benefit of densification of the wall material.
I agree with everyone else, compaction's good all around. What does it cost for a jumping jack, $20 a day? Easily accomplished on a job like that. Clean washed rock is a great solution for a wet trench, but don't skimp on the compaction. You may just be perpetuating the problem you were trying to avoid.
RE: Can replace 57 with 4 and not compact?
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".
RE: Can replace 57 with 4 and not compact?
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)
RE: Can replace 57 with 4 and not compact?
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?
RE: Can replace 57 with 4 and not compact?
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.
RE: Can replace 57 with 4 and not compact?
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
RE: Can replace 57 with 4 and not compact?
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.
RE: Can replace 57 with 4 and not compact?
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.
RE: Can replace 57 with 4 and not compact?
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.