Raft foundation design

[RME1]

I am looking to design a single story extention (approx 3m x 4.5m) on the back of my house and wanted a bit of guidence from those of you who have done this sort of thing before (preferably in the UK).
My background is mainly in bridge design working for large consultants, so typically will have an array of software and full desktop/site investigation at my disposal. This is at the other end of the scale in that I have a pencil, calculator and the view out of my window to go off.
From what I can gather the house is on the site of an old quarry, and is likely to be piled through made ground. I've spoken to building control, who have advised me that other extensions in the vicinity have been on raft foundations. To further complicate matters I'm looking to bridge over 2 drains (foul and surface) with invert levels at approx 750 and 850mm depth.
I'm thinking a fairly simple bridging structure containing some compressible fill will be adequate for the drains with the raft then sitting on top (with some compacted type 1 and blinding etc. in between).
My main question relates to the design of the raft and what sort of analysis methods would be deemed appropriate. The NHBC standards give guidence for the edge beams, but models for the slab itself could range from the very simple to incedibly complicated.
I know the easy answer is to keep it simple and over design it, but just wondered what was typically done.
Any thoughts/advice welcome

Thanks.

 

[BAretired]

I would start by obtaining a soil report. If it is "made ground" you will get some idea of its consistency. You could end up using drilled piles instead of a raft.

BA

 

[RME1]

Thanks for replying BA. I'm trying to avoid a piled solution, mainly because of access problems (everything has to cme through a gate at the side of the house), and frankly I'm trying to keep the cost down as well. I thought the bearing pressures would be so low under the raft that I may not need a detailed ground investigation.

 

[BAretired]

In my neck of the woods, a pile foundation would be more economical than a raft foundation unless the piles were required to be ridiculously deep because of pockets of uncompacted soil or organic matter in the backfill. But if it was an engineered fill, that should not be the case. I had a similar situation locally and the piles were all 12" dia. by 15' deep, easily excavated by a drill mounted on a bobcat (which might fit through your gate). The piles were friction piles with no reliance on end bearing. The fill was compacted clay.

BA

 

[valleyboy]

The issue here I guess is the potential for differential settlement. Rme you are right that for such a small extension that should you adopt a raft the contact pressures will be very low. However unless you know the nature and consistency of the fill material you are taking a bit of a risk. Can you make intact with the LA building control section to see if they have any historic SI data you could have a look over? This could give you a heads up.

If you do design as a raft, I've seen a range of options ranging from flat slabs stiffened with ground beams, rafts placed on a depth of engineered fill etc. you could do an FE analysis of a flat slab with a range of spring stiffnesses and deign for the resulting moments.

The access you describe could be suitable for installation of either screw piles or mini piles installed with a grundemat internal hammer. If the main house is piled this would probably be my preference.

DW