Stemwall Foundation

[Whyask]

I have a two story house with 8" cmu exterior walls and all the cells are filled solid. These walls are on top of a 6' cmu stemwall. The interior slab on grade also has load bearing walls on monolithic footings on the imported compacted fill.

The loading from the exterior walls has a range of 3-5 kips/ft. The soils report has total settlement of 3/4" and differential settlement of 1/480 (1 inch over 40 feet). I am varying the widths of the footings to keep a uniform psf less than 2000 psf.

Historically with smaller loadings per foot, I have designed the stemwall to have the top course poured solid with the slab and bend additional stemwall rebar 4' into the 4" thick slab. Then the block walls on the exterior on top of the stemwall.

I have been warned by the Geotech that given the loading and the settlement that this procedure will not work as the wall will settle due to the weight/soil settlement conditions and the lightly loaded slab will not. Thus buckling the slab. I am considering a free floating slab that would be poured after the stemwall and exterior walls have been constructed instead. Is this the correct design procedure or is there a better method?
Thanks

 

[ishvaaag]

Well, it seems the Geotech has a specific idea of the foundation and doesn't want the design to deviate from it. The use of a structural slab on grade may deal with all the issues but of course constitutes a depart from the views of the Geotech and certainly -if no further exam is to be made- may introduce some indeterminacy (because not having been examined) in the ulterior behaviour. So within these parameters leaving the slab separate from walls (and other structure) may be reasonable.