Strip foundations/possible piling/raft.

[joydivision]

Dug out for a small conservatory yesterday. Dug to 600mm, poor ground and found the extension the conservatory will be connected to is on a raft. Then spoke to the customer who then informed me that the extension has been rebuilt after subsidence in the 1st build. The extension also has been piled under the raft too, and the original house is on a raft, with enough space under the downstairs floor to stand up!

So, spoke to the customer about my obviously concerns, dug a test hole next to extension, and found clay at 1200mm, thought we might be ok, so dug enough out to test the clay, to find out it was only a layer of clay approx 200mm thick with more poor ground below, so back to square one.
Test hole is now around 1400mm, I dont fancy taking more out knowing the depth of the house raft, the customer also thinks that the piles on the extension went in about 15ft.

So my questions are:

Whats the best option piling/raft?
A raft would surley needed to be calculated, i dont fancy putting my name to one that hasnt, any thoughts?
I can get it piled for £1000 excluding concreting them up.

Obviously its not under building control, and im sure im correct in thinking of the above options, but does anyone else think im massivley overkilling it?
You always hear, 'its only a conservatory', and i guess many would have poured at the 600mm mark, maybe less and thought no more, but to me thats a gamble im not prepared to take!

Cheers

Allan

[lutonlagerlout]

i would say a raft will be easier
we have done them in luton where the ol river used to run,but was then diverted so that gravel could be extracted,and then the riverbed was used as landfill
IIRC we couldn't fix any part of the extension to the main house,just mastic joints to allow for movement
LLL

[joydivision]

Cheers for the reply.

Apparently the area was farmland years ago.

Any ideas how much a calc would cost of a structural eng?

Cheers

Al

[joydivision]

IIRC we couldn't fix any part of the extension to the main house,just mastic joints to allow for movement
LLL

Yeah, just built one similar myself, expansion joints against house and used movement starter ties and mastic up!

[lutonlagerlout]

it might cost 300 quid to design the raft
hard to say
yeah those starter ties with a rubber sleeve on
LLL

[GB_Groundworks]

depends on your relationship with structural engineer if you use them regular then it wont be much but one off should be 250/300 +vat like tony said

mini pile and ring beam but then still got to concrete floor. so raft and screed might be easier

[joydivision]

depends on your relationship with structural engineer if you use them regular then it wont be much but one off should be 250/300 +vat like tony said

mini pile and ring beam but then still got to concrete floor. so raft and screed might be easier

Yeah thats the one, mini pile and ringbeam for 1k.
I didnt think that to be too bad.
The concrete floor would have to be cast with mesh in onto a sleeper sat onto the ringbeam also i guess.

Il get a price on a raft calc tomorrow.

Cheers for the advice

Allan