Multiple cracks in raised slab!

[Inspector807]

In the area i'm in a great deal of slabs are being used instead of crawlspaces, some as high as 7' which i consider a problem waiting to happen,but the average height is around 3', i don't believe you should be allowed to build up a foundation full of fill for a slab that high, especially the way they are prepared, they bring the sand clay in with a bobcat and push it around run over it a few times and bring in the remaining loads, no tamp is used to be compacted in levels they are relying on the bobcat to compact it which is unable to do so around the edges of the slab, i've called for a test recently and it came back with a 75% compaction, does any have any idea how long before the fill starts to settle and effect the structural capability of the slab not to speak of the possible problems with the plumbing that may occur once the soil under them start to settle, any help would be appreciated

 

[JAE]

I would think that at 75% compaction, there would definitely be some consolidation over time but this depends upon the pressure placed upon the soil. Also, in filling up to 7 feet, the underlying soils will now experience much higher compressive stresses (7 x 100 pcf = 700 psf) that will also create some settlements of the underlying materials.

You might try posting this question in the Foundation engineering forum.

 

[Ron]

If you have a test result with 75% compaction, I would suspect the test result. Almost impossible to get compaction that low. Would be higher just from dumping in place and running over with light traffic.

Having said that, the consolidation from undercompaction can be significant if the fill volume/depth is great. You can compute this just from weight-volume relationships with proper compaction information.

As JAE has said, the fill likely represents a greater stress on the near-surface soils than the slab.