foundation advice pls

[diydad888]

Hi,

Apologies for this question list.
I am going to build a couple of small single story extensions to my house here in Edinburgh and need some foundation advice. I used a draughtsman to get my plans and building control through but there is very poor info on what is required, he has not given enough design info regarding the foundations, the council are hopeless for advice, just repeat..see your designer.
All the founds are straightforwards apart from one side which runs beside a neighbours extension, this needs an eccentric loaded foundation(I am building right next to her wall) of 500 mm wide by 400 mm deep..this must connect to the rest which are 700X200 (600X200 seems more than enough,it is boulder clay here)
How to do the foundations if one side is to be 400 deep? should that be cast first and allowed to set ?
if so should I set some reinforcing bars in situ to cast onto next? or shutter that side pour it all and have the space where shuttering was (15 mm)?
I want to get founds in there asap as her ext is on a shallow trenchfill though the soil is very strong.
My draughtsman spec'd C40 concrete for all founds I think this is way way over the top,any views? the soil is type 3 stiff boulder clay, the house founds are 450mm to found base from ground level.
I need to do as much of this myself, is it feasible to rent a large cement mixer and pour my own founds (1.8m3) for the rear? I am worried about it setting too fast as I try keeping the mixer fed, plasticiser ?
what concrete mix to make? do i really need C40?

Thanks for reading,

 

[RedHerring2]

I would have thought that you'll just dig the side foundations 200mm deeper than the rear and pour it all in one to the same top level.

You say the rear alone is 1.8M³ . for 2 fit active guys 2M³ mixed concrete is a long hard day's work with one mixer. And you'll not be able to guarantee the mix strength.

I'd go down the ready mix, without a doubt. And why change the specification? To save a few pounds?

Your BCO will inspect the trench before pouring and may want to afterwards, so why adjust the depth specification from the designer? The BCO may not pass it, although he may agree with you. You need to discuss it with him first.

 

[diydad888]

Thanks for the help, the foundation design I got is unclear and the guy rarely answers my calls to sort it out.
The neighbours boundry extension wall has a trench fill to about 300 to the base from ground level, our (semi detached houses) are on a found base of 450 mm from ground level.
I thought it was best to keep the eccentric found higher (still to 450 base) to offer support to her side but I really dont know. Your suggestion of going deeper and making the top level all the same would be easier.
There wont be much room between her wall and my new one <50mm, I guess if I go with the lower level, I could fill the gap with concrete up to ground level between walls or am I thinking too much?
The council BCO was not worried about her trench fill wall, he said just make the eccentric found as I would not be undercutting her founds at all.
I wish the council had made the draughtsman submit more detailed plans as I feel he got off too easy.

Cheers

 

[RedHerring2]

Go with what your BCO is suggesting.
It does sound as though your BCO is suggesting the top of the side foundation to be 200mm higher than the rear.
I'd place a step of shuttering at the joint of the side to rear, so that the bottom of the step is at the top of the 200mm rear foundations. Then pour the 200mm all round, finally pour the remaining 200mm on top of the side foundations. Explain to the concrete supplier and they should be able to adjust the slump value of the concrete to allow this.
I'm surprised your neighbour's extension foundations were not required to go as deep as the house foundations. I thought that was normal practice.

Another thing to think about is a party wall agreement and the effect or potential effect of your foundations on your neighbour's.

Plenty of threads on this, if you do a search.

 

[diydad888]

Thanks,

yes, I don't understand how they got her ext passed ! I don't think we have party wall agreements here in Scotland.
Cheers