Soil Compaction below Slab on Grade

[NFExp]

I received a question from a contractor regarding the compaction / bearing required for soil below a slab on grade. He asked if the soil below the slab on grade was required to meet the 95% proctor / 2000psf bearing capacity.

This is for a small residential project. We specify that the soil below the footings be compacted and that a geotechnical engineer verify the 2000psf bearing capacity.

The contractor has hired geotech and he plans to have all tests required for soil compaction.

Originally I assumed that the entire footprint of the building was to be 95% compacted and verified with a 2000psf bearing capacity but now we are considering to have just the footings verified and to consider the slab on grade as a "floating" slab. Obviously the soil below the slab on grade would still need to be compacted somewhat.

My questions are 1. Are we saving the contractor any money if only require the footings to be compacted per 95%?
2. is there a generic spec for the slab on grade compaction that wouldn't require 95% or compaction test? 3. What is the likelihood of slab settlement for a lightly loaded slab (residential loads).

Any thoughts?

 

[Ron]

First of all, don't confuse compaction with bearing capacity. They are two different animals.

Generally, the 95% compaction should be done over the whole building area and extend about 5 feet beyond the building lines. This provides uniformity and it is easier to properly compact such an area than to just compact the footing area.

Yes, slabs can settle from lack of adequate compaction. When shrinkage cracks in the concrete occur, there can be faulting at these cracks where compaction is insufficient.

Do the compaction.

 

[GeoPaveTraffic]

Bearing pressure is different than percent compaction.

The first goal of compaction is to limit settlement. A side effect of compaction is that you tend to get reasonable bearing pressures on well compacted fills.

I would always require compaction under a slab. Normally we require either 98% standard or 92% modified for slabs. However, this can vary depending on fill thickness and soil type.

Mike Lambert

 

[NFExp]

Thanks for the clarification. I told the contractor to go ahead and compact the whole footing area per the 95% spec.

 

[cvg]

why are you cutting the contractor slack and saving money for the contractor? isn't your job to protect the owner? compact the entire building footprint. especially if this building is founded on fill. yes, your slab will settle and possibly crack. ask the geotech to estimate how much.