Advice on extending a raft foundation

[evo111]

Hi There

I am in the process of extending above my double garage. My existing house and garage have been built on a raft foundation. The structural engineer involved in the design of the extension has advised me to extend the perimeter of the raft foundation to compensate for the additional load above the garage.

Does anybody know what this will involve ??

Any advice or help gratefully received !!

Many thanks
Simon

 

[noseall]

The structural engineer involved in the design of the extension has advised me to extend the perimeter of the raft foundation to compensate for the additional load above the garage.

Does anybody know what this will involve ??

maybe you should be asking him. has he not detailed the additions?

i can see his point if the extended raft was cast as one with the original, but adding a bit or joining to the existing is only going to be as strong as the join surely?

 

[Shytalkz]

Pure rafts (as opposed to piled ones) work by spreading the loading over its entire footprint. Whilst the ideal is to get the bearing pressures equal under all areas, if the load and raft centres of gravity are not coincident, the pressures will vary. Sometimes a raft is extended beyond the building, so as to move its centre of gravity to that of the load and thereby even up the pressure underneath.

If you're just going up into the roofspace over the garage, then there will not be that much increase in dead (building) loading, but the live (occupancy) loading will increase by quite a bit. This will move the load CofG slightly, the impact of which is dependent on the layout as a whole and the current bearing pressures.

If you're going outside the footprint of the existing raft and building with your extension, then this will alter the raft and load CofGs - again, the extent of this is dependent on the layout and bearing pressures and this will have an impact on the advice that the SE gives you.

It is possible to add to existing rafts, though there is a fair wodge of calculating/detailing to do. Jointing is not an issue: it is possible to accurately predict the forces that will occur at the joints and design the connection accordingly.

 

[evo111]

Thanks for your reply.
The existing raft extends about 2 feet from the house on all three sides except on the garage side wherei want to extend.

Structural engineer basically wants the raft extended by two feet along the length of the garage. He has given details of steel to useand drawings etc... He wants it dug down about 2 feet deep as well.

My question is does anybody know whether this is expensive work??

I appreciate that without seeing drawings etc it is impossible to answer a question like this accurately - but I have zero knowledge of this type of building work.

Help and advice gratefully received.

 

[Shytalkz]

Shouldn't be an horrendous cost, but without seeing exactly what he's come up with - and why - it's not possible to say. The 2' deep bit is intriguing: is the rest of the raft that thick? Do you live in a mining area?

 

[evo111]

Thanks for your reply.

No I dont live in a mining area - I live in the same county as yourself !! Just outisde Kings Lynn - Norfolk.

Looking at the engineers drawings a little more carefully - it looks as if the raft wil have to be extended 600mm laterally and 300mm in depth.

 

[Shytalkz]

Ah, you're on the shítty stuff! Can probably guess who the SE company is as well...

Quite often over there, the ground is surcharged, to squash the crud underneath down and reduce the raft settlement . If that was what happened with yours, adding to the raft can be problematic.

Have they done an SI to determine the depth of the muck underneath?

 

[evo111]

The Structural engineers have no concerns about settlement or the ground condition.
They want it widened so that the it extends outwards two feet the same as it does the rest of the house.
I can only assume that it is so that the weight distribution is spread evenly over the whole raft perimeter.
Still a real pain in the ass - especially when ones budget for extending is tight anyway - this is just additional expense and hassle !!!!

 

[Shytalkz]

Hmmm.

The two (no concerns about the ground and requiring it widened) don't equate to my mind. Still, it's not my PI insurance .

 

[joe-90]

Shouldn't be an horrendous cost,

Apart from the SE at £720 per day +travel.

 

[Shytalkz]

Lol - that drum just fall out of the cupboard again, did it?