Structural Stone Fill

[bootlegend]

I designed a large mat foundation for a bin in a quarry that was supposed to be founded on solid bedrock. After blasting out the area the bedrock is about 4 feet too low. The customer is asking what fill I recommend to bring their site back to correct elevation. I only need 5000 psf. Pad is approximately 60' x 30 x 3' thick. Bedrock below is sound. I'm thinking something along the lines of 10" minus crushed limestone for the first two feet, compacted in 1' lifts, and then a #357 stone mixture for the top two feet, compacted in 8" lifts should be good enough. Does this sound reasonable?

 

[BridgeSmith]

With the pad being 3' thick, carrying only 5ksf, I would question whether compaction is necessary. (Actually, the first thing I questioned was why the 3' thickness for such a light load). I get using rock to backfill - it's a quarry, the rock is right there, presumably, but unless the small amount of settlement you'd get with uncompacted rock is not tolerable, why go to the effort of compacting it, when the load pressure will do that for you?

 

[SlideRuleEra]

bootlegend - When considering a solution to an unusual problem, to me, even the crude approximation of a picture is worth a thousand words. Here is my grossly simplified view of the proposed solution:

Does that concept look like the kind of fill I would want under a structural slab (even though the fill and the slab are confined on the bottom and all sides)?
IMHO, no, to many possible voids. I would go with smaller stone backfill.

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[bootlegend]

HotRod10,

The thickness is based on uniform bearing which is conservative and could probably be thinner but there is a piece of machinery on the slab as well and I stick with the 3.5 x the weight of the machine out of ACI guidelines. I don't want to get settlement under some parts of the slab and not others so I think compaction would help prevent that.

SlideRuleEra,

Your sketch is a pretty good approximation of my proposal. Wouldn't most of the upper voids be filled while compacting the upper layer? Wouldn't this prevent the migration of the upper material into the lower voids? I'm not sure so I'm asking the question

I talked to another engineer familiar with quarry procedures and they suggested a 4" to 6" max size but use crusher run for the whole depth so you get the small and large stuff together. Seems to satisfy your concerns with the original suggestion I believe and sounds reasonable to me.

 

[SlideRuleEra]

bootlegend - Voids need to be minimized for full depth... not just the upper layer. Even the #357 stone will have too many voids, IMHO. If there are voids, over time the compacted fill can "ravel" to voids below. Somewhat like a sinkhole.

I like the suggestion the other engineer suggested to you:

...4" to 6" max size but use crusher run for the whole depth so you get the small and large stuff together.

Well mixed "large" and "small" aggregate... just like concrete without the cement.

A little more about my sketch. I used approximate 10" spheres, the best case. Spheres can be "arranged" for for minimum voids by using "cubic closest packing". For this arrangement, there are (theoretically) 26% voids... unacceptably high, and no amount of compaction will make it better (unless the rock is broken). In real practice, voids will be much higher. As a former bridge contractor, we had projects that included hand placement of "one-man rip-rap" (equivalent to the 10" size proposed,supplied directly by a quarry). From the quarry, rip-rap is not spheres... not even close. Even with careful hand positioning, I'd be willing to bet voids were about 50%.

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[BridgeSmith]

I didn't realize the pad was for an equipment load, which presumably are applied point loads to the pad, as well as some dynamic/vibratory loading, so now the thick pad makes sense.

The crusher run seems like a good solution all around - it compacts well and it's the easiest to produce.

 

[bootlegend]

Thank you both for the feedback.

 

[oldestguy]

No mention is made of excavation dimensions. Unless the width of the excavation is wider than the slab, the edge support will be lower. In this case go beyond the slab edge by the thickness of the proposed fill dimension and measure that width from the bottom of the excavation, not the top.

 

[Rich2000]

The solution is very simple, just fill this layer low grade concrete it will sort out the problem!

 

[Jwilks93]

I oversee structural trench backfill quite regularly that sees heavy traffic loads. Per ODOT specifications, fill the area with item 304 (aggregate base material consisting of 1.5" crushed stone down to fines) in lifts. Lift thickness depends on the type of equipment being used to compact. Typically for plate compactors use 4" lifts, but if you have the access for a smooth drum vibratory roller or hoepac you can go to about 12". Make sure you water the material before compaction to ensure you are able to achieve maximum dry density. It doesn't hurt to have a technician familiar with nuclear density testing and a calibrated nuke gauge to ensure you're getting adequate compaction. I would suggest compacting to 98% of lab maximum dry proctor density. If you have the probably of frost heave, you can even top off the aggregate base material with something a little more porous like 57 stone (max nominal size 3/4") to prevent ice lenses from forming. Just make sure you compact it.