Residential Slab on Grade with Grade Beam on piles

[hairman]

I have to design a residential foundation (slab on grade) on a site where peat is located from 5' to 15' below grade.
1. Do the gradebeams have to be below the frost line?
2. Can the slab itself bear on granular material compacted on the existing inorganic material with the peat below?
Thanks.

 

[dik]

Depends on the soil and local practice. In this area, we typically use 24 or 30" deep gradebeams with 6" of foam void underneath (a product called FrostCushion). You may have problems with your slab on grade sitting on the peat. and you might consider structuring the slab on voidform.

Dik

 

[eric1037]

Exterior grade beam should be below frost.

If you have inorganic fill already existing on top of the peat, that should be fine to cast a floor slab against. No need to compact it.

This is not technically, a "slab-on-grade" since it will be supported by grade beams. It should be suitably reinforced anticipating loss of support.

A word of caution with these: Make sure any utilities below the slab are tied up to it from below. I have seen several unfortunate cases where utilities broke under structural slabs.