Existing Slab on ground can be considered as compacted granular fill ?

[tngv752]

I try to design the RC slab and footing located on an existing slab on ground. This is 1 storey residential bld.
As the soil is high reactive and the owner doesn't want to remove the existing slab as much as possible to save the cost.
The new slab will be located above the existing slab on ground and can be designed as slab on ground or suspended slab with strip footing system.
If the new slab is designed as suspended slab with strip footing and bored pier system, I can ignore the compacted granular fill. Just loose fill as form work.
If the new slab can be designed as a slab on ground with strip footing system (no piers), can the existing slab on ground can be considered as compacted granular filled ? If not, the existing slab must be removed.

Thanks.

 

[msquared48]

If there are no voids below the existing slab, I see no problem with that since the bearing stress would go vertically directly into the soil. I also assume that the soil can handle the vertical bearing stress imposed as placing a slab-on-grade over 500 psf bearing soil will not give 3000 psf bearing.

Not so if there are voids, as the existing slab will have to span the voids, and may not be reinforced to do so, which also locally increases the bearing stress seen by the soil.

Mike McCann
MMC Engineering

 

[tngv752]

The existing slab is slab on grade and include external and internal slabs.
I see the internal slab have a big crack (10mm wide approx.). I think that there was a ground movement. That's why I have to be careful.

 

[cvg]

the existing slab will spread the load to some degree acting similarly to well compacted granular material. however if the subgrade below the existing slab is poor, (uncompacted, settling, poor soil or voids) than you need to consider that in your design. So you may not want to consider it to be "compacted granular fill" as you really cannot verify if there was any "compaction" done nor can you verify that it is granular.

 

[Ron]

The existing slab was likely not designed for the loads that an overlying footing/column would impose (shear). Given this, I would sawcut the existing slab so that is is the same size as the footing you will install above the sawcut area (you can leave the saw cut section in place, just isolate it from the other part of the slab). That way, if the new footing imposes too much shear on the existing slab, it won't fail suddenly and cause problems. If you cut and isolate, it will elastically settle as load is placed on it and that is tolerable.