Pit Fill

[cfuller]

I've got to fill in a 15' W x 15' L x 33' Deep concrete pit then pour an 8" slab for a machine. I'm a mechanical engineer this subject really isn't my wheel house. It's been suggested to fill in gravel and compact every 4'. My question is that typical way filling a pit? If so how do safely do this operation in somewhat confined space?

Thanks

 

[EireChch]

What depth is the slab at?

You can do it with gravel and 4 inch lifts but would take a while

Flowable fill is likely more economical.

 

[jayrod12]

Get a geotechnical engineer involved locally. For something that large (deep) it may be beneficial to fill with a wide gradient of stone. 4 ft between compaction efforts is not often enough in my experience. Usually around here it's maximum 6" lifts, but that's for 3/4" granular limestone.

 

[EireChch]

I missed that and thought it was a 4 inch. I was going to say that you could likely do a larger lift of 2ft to within 4 ft of the slab and then do 4-6” lifts. You will have a lot of confinement which is good for a large lift. the only thing is that you would need a heavier compactor for large lifts. Which you won’t be able to fit in the pit. You could look into a compactor fitted to excavator arm.

It's a bit non standard and you would need to prove that the larger lift is meeting your compaction criteria.

A plate load test could test the entire 2ft lift, but it would be difficult to get a reaction load for the test.

Just use a low density flowable fill.

 

[GeoEnvGuy]

33 feet deep you would need a long stick excavator to reach bottom from surface. You can attach a compaction plate to the excavator. Or you would need to get a push diesel plate compactor into the hole but you would also need to be blowing in ventilation as standing in the hole running a diesel engine can be hazardous.

For a machine foundation you would want a crushed stone 1 foot at depth with 6 inch lifts up higher or flowable fill as your backfill subgrade.

Also consider if you need to drill holes above the water table to let out any water which may accumulate.

 

[BridgeSmith]

It would seem to me that it would be more economical to just backfill the pit, only compact the top few feet to temporarily support a slab that is structurally adequate to span the 15', and not worry about future settlement.

Rod Smith, P.E., The artist formerly known as HotRod10