use of existing concrete ground slab as foundation

huangyhg

use of existing concrete ground slab as foundation
i am working in project in which the project is a fast track. the project involves refurbishment works in which some 230mm thick masonry walls 6m high need to be constructed on the existing concrete ground slab. there are no as built drawings for the existing building. i am thinking of designing aa raft foundation to be constructed on this existing slab. and the wall to be a cellular masonry wall free standing to 6m above the proposed slab. any advise?
you need to know what the subsurface conditions are under the existing slab, in other words get geotech advice.  drilling through the slab for your borings will give you the ground slab thickness.
what provides out of plane strength for these 6 metre high walls?  as you say they are 230 thick, i assume that means double brick.  reinforced vertically and braced at top?
when you say a "raft foundation", do you mean a spread footing cast on top of the slab?
think we just had this conversation:
in some older buildings that we have dealt with there has been some localized (and sometimes substantial) settlement of the original grade below the slab.  the older the building, the more suspect, imho.  they haven't always had vibratory plate compactors.  though not intended to, the slab on grade then ends up with voids and pockets below as it spans over these hollow spots.  usually fine with foot or light wheel traffic, but put a wall or new column on it and it can give you trouble.
beyond the other issues, i'd carry some exploratory holes and injection grouting if necessary before i'd set a new wall on an existing slab.
.02

chedim,
i assume you are in australia due to the references to 230mm wall and raft slab.
i have actually seen slabs on ground where they never even bothered to remove the grass. your job as a structural engineer is not to make assumptions (e.g. that the existing slab on ground is okay) your job is to make sure that things are okay in a scientific and thorough manner. get a boring done, they are very cheap compared to the consequences.
if you do end up using the existing slab then put 50mm of sand blinding between it and the new footing to minimise shrinkage restraint from friction.