A geotechnical survey has been done?

How deep is the land reclamation? What is it reclaimed on? What is the water regime?

If it is a cellular raft, how deep is it?

It should at least go down to the natural ground below the peat layer. General rule is to not put foundations of important buildings on organic ground e.g. peat.

An advantage of cellular rafts is that if the reclaimed land is still consolidating, then the building will go down at the same rate as the rest of the land (when properly calculated) and so you won't get differential settlement between the surrounding land and the building.

On a 15kN/m2 per floor estimate then the floors literally above ground level will be 4x15=60kN/m2. With a soil that is about 18kN/m3, then the cellular raft needs to be about 3m deep so that the soil at 3m down feels the same weight as if the soil that was there before was still there.

Obviously more work needs to be done, particularly with respect to the water regime, but this is a start.

Thanks all, we finally decided to loose a floor.

Even with a properly engineered geocell/geogrid raft foundation, expect some settlement. This is especially true of peat type soils as the weight of the foundation will compress water out of the soil (and consolidate it.) And of course no matter how well you calculate, the building will refuse to settle uniformly.

In Japan, buildings built on landfill are designed to be jacked and leveled as the foundation settles. I would consider placing the ground floor somewhat above grade, so as to allow for settlement. (The jacking mechanism goes underneath.) Also, reducing the overall weight of the building as much as possible will help.

Of course, you could also consider engineering the building to accept additional floors in the future. That way, as it sinks into the ground, you can keep adding floors on top.

More years ago than I care to remember, I read about the effects of lime slurry injection on soil strength.

Injected into peat or clay, the lime displaces water from the soil matrix, thereby consolidating it and increasing it's strength.

In the case of clay, the lime also has some reactions with the fine alumino-silicate particles, giving a considerable increase in strength.

If you inject lime/fly ash (about 1:1) slurry into the peat at about 2.5m centers and about 3.5m deep, water will be displaced and come to the surface.

The resultant mess looks most discouraging, but if you drain the surplus water, the soil underneath will have settled and over the next month or two, will show a considerable increase in bearing strength.

OMW7 showed how to calculate the bearing load the building will exert and which the underlying soil must resist. To go beyond this, you need to increase the bearing capacity of the soil, or face increasingly expensive foundations.

Another alternative is to use friction piles and support the building on those.

It seems your soil conditions make a raft dubious for the height of building which is proposed.

Take heart, in the 15th century (?), Pizza Constructions stuffed up their foundations and their tower became a world renowned tourist destination!

I used cellular raft foundation on soft clay soil , but no more than 2 storey bldg.because that tipe of foundation is for lightly loaded bldg. on poor soil.