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Compaction requirements for subgrade or fill below a pile supported slab?

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abusementpark

abusementpark

Structural
Dec 23, 2007
1,086
Are there any compaction requirements for the subgrade or fill below a pile supported slab (i.e. no reliance on the soil)?

We usually still call for 95% percent compaction on the standard Proctor, but I often wonder if this requirement could be relaxed since we aren't relying on the soil to support any of the building weight. But maybe there are other reasons why such a high level of compaction would be desirable.
 
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In my view, so long as the soil can support the concrete until the slab has cured, no special requirements are necessary

Kieran
 
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Are you compacting it before or after the piles are driven? If you're doing it after the piles are driven, it will be hard to access with large equipment.
I'm no expert, but I would compact to 95% before the piles are driven. You might have to add a little bit of soil after they're driven as the soil will tend to self compact due to the pile driving vibration.
 
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Are you compacting it before or after the piles are driven? If you're doing it after the piles are driven, it will be hard to access with large equipment.
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Usually before the piles are driven.

I'm no expert, but I would compact to 95% before the piles are driven.
Click to expand...

Why is 95% compaction needed? Why not something less stringent like 90% or even less?
 
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Depending on the soil... I've used non-compacted fill... on several projects. If the slab is structural, no need to pack it down... even the slight loading from the plastic concrete will not likely cause consolidation in a short time.

Dik
 
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I would use 95% because if the soil settles below the slab, a family of critters is likely to set up housekeeping. With 90% it's pretty much just dumped out of the dump truck.
 
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Have you considered the compaction requirements for the piling rig access/stability?
 
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That was my thought surely the piling mat will give you all need anyhow
 
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Just a thought in another direction: we know that very often in piles, in presence of horizontal stress components, soil-foundation interaction is very sensible to conditions in the uppermost soil layer, so a greater rigidity of the fill might improve significantly pile behaviour under lateral loads. This of course if the thickness is not negligible at the scale of interest and if such horizontal components are not negligible themselves.
 
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I would use 95% because if the soil settles below the slab, a family of critters is likely to set up housekeeping.
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What kind of critters are we talking about here?

With 90% it's pretty much just dumped out of the dump truck.
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Oh, it's that easy to achieve 90%?
 
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Just a thought in another direction: we know that very often in piles, in presence of horizontal stress components, soil-foundation interaction is very sensible to conditions in the uppermost soil layer, so a greater rigidity of the fill might improve significantly pile behaviour under lateral loads. This of course if the thickness is not negligible at the scale of interest and if such horizontal components are not negligible themselves.
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Interesting point.
 
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The surface needs to support construction loading, including the piling rig. However, compaction may be only part of the problem. Beware of conditions where the material under the slab is prone to volume change with moisture.
 
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jedclampett said:
With 90% it's pretty much just dumped out of the dump truck.
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Really? Perhaps my location is different, but a typical load of granular material used for structural backfill weights approximately 100 lbs./c.f. in loose condition in a truck or stockpile and has a modified proctor of approximately 145 lbs/c.f., or 69% of modified proctor.
 
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