lateral force of 57 gravel on filled porches & unbalanced fill heights?
I’m running into conflicting recommendations / opinions on how high I can safely fill a porch with 57 gravel that will ultimately have a stamped concrete surface. For reference, my tallest porch wall is a little over 6′ high as measured above the footing (which is 22″ below grade), my walls are 12″ hollow CMU on the interior side of the porch and 10″ CMU + brick facing on the exterior.
According to the building code chart R404.1.1(1) for plain masonry foundation walls, I can have between 5′ & 6′ of unbalanced fill height. Once my final grading is done I should end up with 4′ or less of unbalanced height but my concern is the order of operation. If the crew fills the porches before the backfill is done, the unsupported height could exceed the code limit, if they backfill first, the weight of the equipment on the angled backfill could push the foundation wall in and break it. If they fill before the floor framing is done, there isn’t as much support on the interior top of the foundation walls to keep them in place.
I have talked to an engineer who said that 57 gravel has very low lateral forces and that I could fill it 8′ high with no issues but my builder isn’t comfortable going more than 42″ high and has seen walls blow out that were filled with too much gravel.
Am I being overly concerned about this? Is there a commonly accepted solution here so that I can mitigate the blow out / in risk?
Thanks
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I recently had a wall blow out on a project when it was backfilled without being braced. It was designed to meet IRC standards but that wasn't enough; I recommend caution.
#57 gravel isn't universally known; it's crushed limestone that passed through a 1" sieve but is retained by a 1/2" sieve. Like other types of crushed stone, its angular nature means it has a high angle of repose--it can stack pretty high--but your crew should be mechanically compacting/consolidating the crushed stone, which will result in outward pressure.
Can you wait to backfill the porch until the exterior is being backfilled?
Michael, thx for the note. The plan by the crew is now to do it all in one day where they will do an initial backfill on the exterior and crawlspace side of the foundation then fill the porches with gravel. I guess if they are aware enough of the risks it should be ok. I am also nervous about compacting the 57…I thought it was essentially self compacting?
Bruno, whether crushed stone is really "self-compacting" is a matter of some debate. Here's a good overview of the topic: https://icpi.org/blog/the-myth-of-self-compacting-aggregates
Because #57 includes irregularly shaped particles that vary in size from 1/2" to 1", the spaces between aggregate will be irregular and will be reduced by mechanical compaction. The angular surface of the stone might make it able to resist loads without compaction. Truly non-compactible gravel would be perfectly round and all the same size--you can't compact a bucket full of billiard balls. Tumbled pea stone and coffee beans are almost as efficient at filling the air spaces without mechanical assistance. But with angular stone, mechanical compaction forces the particles to arrange themselves to reduce the air spaces between particles.
BrunoF,
You can see the compaction of seemingly un-compactable hard materials that Micheal is describing when you fill jars with food. Things like split peas, which are similar to rock or gravel, can be compacted by tapping the jar on the counter.
Michael, would you care to elaborate on the wall blowing out a little? Type of foundation, height, type of backfill, height of lifts, compacted, etc? Thanks
Bruno-
Just to throw an idea out there: maybe some strategically placed temporary bracing? I'm not sure whether or not it would be needed, but it may give you some peace of mind.
One more idea if it’s not too late -
This would be a good use for a soil stabilization fabric. They work by resisting the shear loads with tensile strength. A layer of that every few feet is enough to stabilize soil/aggregate to the point that the wall is essentially cosmetic.
Alternatively, if you have access to a bunch of old, clean tires, you could accomplish the same thing.
It isn't too late and according to my buddy who is a grading contractor, this is a brilliant idea; thank you! He happens to have a few rolls of biaxial geotextile for this purpose laying around so I should be good to go with some cheap insurance.
Thanks!
Kyle,
Good call. They rebuilt the highway near us with layers of geo-tech mesh every foot, and the almost vertical drop on one side didn't need more than cursory retaining.
This is a good video on the subject (practical engineering).
https://youtu.be/0olpSN6_TCc?si=0-a0itTjB4rBtOv_
Awesome video. Thank you for sharing it.
Patrick,
All his videos are great. He's the structural engineer I wish I had had as a prof.
Bruno, any update on this? How’d it go? I am in a similar position. What was the process you used?