Structural slab on Dry Fine Sand Backfill

[1437n6]

I have a concerete slab that has fork truck traffic on it, which has settled in one section. The settled section shows some brick and debris (fill material), however the remainder of the borings show Dry Brown Fine Sand, Little Silt down 15'. Should I be concerned with the Dry Brown Fine Sand as a support material for the concrete slab?

 

[msucog]

if it's also fill, i'd say it depends on the consistency/relative compaction of that material. the question is pretty open ended depending on if you're worried about slab performance due to traffic, slab performance relative to cracks and subsidence due to the old fill, slab performance relative to levelness and flatness, all the above, etc. i suspect you're worried about having some nasty cracks and floor issues bad enough to cause trouble for fork lifts...

 

[1437n6]

That is correct, I am mainly concerned with floor issues bad enough to cause trouble for fork lifts, specifically additional slab settlement. The fine sand material that the slab bears on averages 5 hammer blows on a 140lb weight falling 30' on 2"O.D. sampler, which is medium dense. My original concern was a solution to repair the section that had poor fill by some means of piers, however my new question was concrening the structural ability for the fine sand to support any loading above.

 

[msucog]

5bpf for sand fill is pretty poor. it's not nearly as bad for say a sandy silt (where the silt is the primary constituent) fill but it's still not good. without many other details, it appears to me that you definitely may see additional problems. the thickness of the slab may (or may not) help alleviate some of the underlying conditions.

the fork lifts loads (wheel loads in particular), number of forklift passes, adjacent rack loads, slab reinforcement and thickness, subgrade conditions, foundation conditions, expected performance, etc will all play a role in the overall assessment of your situation. hope my rather general explanation helps to shed some light on your situation. good luck.

 

[1437n6]

Thanks for your help, it does provide insight.