I am trying to determine the at rest active pressure when backfilled with clean crushed stone. I have used a frictin angle of 41 degrees but I need some imput as to this being proper. I know from experiece that this type of backfill produces less pressure on the wall than grandular sand-rock fill but I don't know how much. I posted this on the structural site some time back but got no response.
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Friction angle of crushed rock
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The angle of internal friction of a crushed stone will be directly influenced by type of rock and grain size. Forty-one (41) seems to me possible, but dangerous. I would use a safe 36 degrees unless a direct shear test is performed on a sample.
There's at-rest earth pressure and active earth pressure. Which one are you trying to solve? A friction angle of 40 degrees or so is likely appropriate. For Rankine active earth pressure, you'd get a coefficient of 0.22 for at-rest about 0.36.
Hope this helps.
f-d
¡papá gordo ain’t no madre flaca!
Hope this helps.
f-d
¡papá gordo ain’t no madre flaca!
I take it that the crushed rock is too big to run a triaxial test on? If so, a quick way to guesstimate a friction angle is to pile the crushed stone on top of eachother and then measure the angle of repose. A true crushed stone slopes would fail by sliding parallel to the slope. When the slope angle exceeds the angle of internal friction, the stone grains should slide down the slope. The steepest slope that the crushed stone slope can attain, therefore, can be said to equal the internal friction angle of the stone. Crude, but could give you some comfort level.
The answer is: It depends on density.
If you just dump it in, then 41 is far too high.
If it is compacted, then it maybe is OK.
Since you want to limit the pressure on the wall do this, if you can:
Dump it in in lifts and compact only that material more than 2 feet out. This leaves loose stuff against the wall and provides good resistance to movement for the bulk of the backfill. The loose zone acts as a cushion so that you can develope the active out in the compacted zone.
Don't worry about settlement of the loose zone, since it hangs up on the wall and the compacted zone, the silo effect. It does not develop pressure like a liquid as a result.
Never compact next to the wall unless it is designed for far more than active pressure. Many a wall has tilted in because of this effort.
If you just dump it in, then 41 is far too high.
If it is compacted, then it maybe is OK.
Since you want to limit the pressure on the wall do this, if you can:
Dump it in in lifts and compact only that material more than 2 feet out. This leaves loose stuff against the wall and provides good resistance to movement for the bulk of the backfill. The loose zone acts as a cushion so that you can develope the active out in the compacted zone.
Don't worry about settlement of the loose zone, since it hangs up on the wall and the compacted zone, the silo effect. It does not develop pressure like a liquid as a result.
Never compact next to the wall unless it is designed for far more than active pressure. Many a wall has tilted in because of this effort.
i think high 30's to mid 40's (likely higher will high densities but i would do some confirmatory testing on the material before i went out on that ledge) is reasonable for densified crushed stone. (none of that self compacting stuff--should be in high 20's or maybe low 30's even for this). i've got references somewhere but can't find them at the moment. google sites like ready mix, mse wall, aashto and dot sites.
toones: check out george sowers "introductory soil mechanics and foundations: geotechnical engineering 4th edition. pages 583 and 584. please keep in mind that the measured angle of repose will ROUGHLY correspond to the internal friction angle of the crushed stone in a LOOSE state. density of the stone, as stated in previous threads, will have an impact on the friction angle.
"Compaction is not needed"...i disagree with that statement. i'm guessing that phi could possibly be 45 if not well compacted, but there are other considerations that should be given. i wouldn't use 45 degrees and then just disregard the void spaces, settlement potential, migration of fines, etc. since the material was not densified. i've seen so many problems result from not densifying crushed stone. i might use 45 but i would require that it be densified in thin layers just like structural fill.
on one building that the owner asked us to look in to why everything was cracking, we cored several holes in the floor slab. one core was located about 10 feet behind the cast in place wall. the core drop about 6 inches. since i couldn't really see down in there, i stuck my camera down in to the hole and took pictures all the way around. the subgrade under the slab right near the wall had settled at least a foot. the wall was backfilled with #57 stone that was dumped in with a trackhoe bucket and not densified. the entire area was in a cut so the underlying soils were not related to the problem as far as we can tell based on the boring data we had at the site. the wall itself had zero cracks on the face. it all looked to be due to the crushed stone backfill behind the wall settling out over about 3-5 years.
on one building that the owner asked us to look in to why everything was cracking, we cored several holes in the floor slab. one core was located about 10 feet behind the cast in place wall. the core drop about 6 inches. since i couldn't really see down in there, i stuck my camera down in to the hole and took pictures all the way around. the subgrade under the slab right near the wall had settled at least a foot. the wall was backfilled with #57 stone that was dumped in with a trackhoe bucket and not densified. the entire area was in a cut so the underlying soils were not related to the problem as far as we can tell based on the boring data we had at the site. the wall itself had zero cracks on the face. it all looked to be due to the crushed stone backfill behind the wall settling out over about 3-5 years.
oldestguy said:
True enough, last year I saw an RC wall visibly cracked in one place, intact everywhere else. When I questioned the builder, he answered it was due to backfill compaction.
Not too good to crack the wall even before it starts doing its job!!
for that last post, i think that particular statement should be qualified a little for those on the board that may not see the consequences of misreading/misinterpreting the statement. "never compact next to the wall with overly large compaction equipment unless it is designed for....".
and that does not mean that you can't still compact with small equipment in thinner lifts. i've seen contractors argue that since they couldn't use a large sheepsfoot roller right behind the wall, they couldn't compact behind the wall. that's foolish. get a small walk behind compactor and compact in 4-6" max lifts or use a mechanical hand tamp compacting in 4" lifts. if that is still too much stress on the wall, then go to home depot and buy the personal hand tamp and put it in in 2" layers. i doubt anyone would actually argue that a mechnical hand tamp will overstress a wall with 3000psi concrete that has cured out to 75% of the design strength. just keep the larger equipment at least 5-10 feet or more off the wall.
and that does not mean that you can't still compact with small equipment in thinner lifts. i've seen contractors argue that since they couldn't use a large sheepsfoot roller right behind the wall, they couldn't compact behind the wall. that's foolish. get a small walk behind compactor and compact in 4-6" max lifts or use a mechanical hand tamp compacting in 4" lifts. if that is still too much stress on the wall, then go to home depot and buy the personal hand tamp and put it in in 2" layers. i doubt anyone would actually argue that a mechnical hand tamp will overstress a wall with 3000psi concrete that has cured out to 75% of the design strength. just keep the larger equipment at least 5-10 feet or more off the wall.
I'm wtih msucog on this one. Specify what is required for the project. It's up to the contractor to used the means and methods to complet the project without damaging the final product. If that means a walk-behind compactor or "jumpin' jack" then sobeit. Either that or there should be a formal RFI or some other dialog on the subject.
f-d
¡papá gordo ain’t no madre flaca!
f-d
¡papá gordo ain’t no madre flaca!
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If by clean stone you mean AASHTO 57 limestone or similar, you are generally safe using 41 degrees, assuming of course, that it is properly compacted (densified actually). If you want to test it you can use a large (i.e. 300mm+ shear box.)
I'll try to dig out the references and post later.
I'll try to dig out the references and post later.
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