Abutting a new and old foundation - depth

[Eli123]

Hi this is my first post so be gentle

I am planning an extension to my property, a 1950’s brick / brick cavity wall house.

My question relates to the best way of abutting the new and old foundation.
The old foundation is a concrete slab approx 150mm thick at a depth of around 500mm with brick up to DPC blues.

The new foundation is to be a trench fill and will need to be 1000mm (we are on clay).
I have been given conflicting advice from engineers and BCOs in the past re the way they should meet
One take on this is: Step down slowly to the new depth (maintaining a sort of 45 degree step of sorts under the old foundation until the new depth is reached then fill the trench (with an optional bit of rebar at the top where the new foundation will be more ‘shallow’ (Pic B)
The other take is to effectively mass fill undermine / cut straight down and underpin the existing foundation at the junction (Pic C x 2).
My concern RE the second option is that if any settlement occurs the cracks will spread into the existing house and not just be taken up by a wall starter kit etc.
I was hoping some of you may be able to give your experience and take on this
Thanks in advance

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• Eli123
• 22 Sep 2014
• 1

 

[jeds]

It's not usually advisable to join to or undercut existing rafts. Why not just go down to the appropriate depth next to it?

 

[Eli123]

Thanks

That's pretty much what I thought.
I found this diagram in the Scottish Building regs for foundations. This is what I had planned to do but my BCO seems to need persuading otherwise.

He's pretty experienced but I cant fathom the logic!

Untitled

• Eli123
• 22 Sep 2014
• 1

 

[jeds]

BCOs are not supposed to offer design advice. They are not usually qualified to do so and certainly don't carry the necessary PI insurance to back it up. Their job is to check the design of others or else check that it complies with approved methods - i.e. Approved Documents. You do get the odd one that gets a bit up themselves and try to insist on all sorts of pet ideas but they usually back off when you ask them for a copy of their PI insurance.