Advice sought for Foundation System/Drains/Sewer

[Narthecius]

Hello all, I have posted this elsewhere on the web but thought I'd ask here too in case anyone had a similar experience.

I am hoping to build a small (5.5mx4m) single-storey rear extension on the back of our 1930's end-of-terrace property. Prior to getting planning/building regs drawings approved, I want to confirm the feasibility of the project and approximate cost of the foundation system before I proceed.

Soil surveys I was given when I bought the property indicate the ground conditions are poor (made-up ground) and it's possible a pile or raft system is needed.

There is an adopted sewer (invert level 1m and all pipes <160mm) that converges in the alley next to my property. As far as I understand, building over pipes is relatively simple when using traditional footings, but more complicated with pile/raft systems. Having read through Thames Water's build over guide, piled foundations must be 1.5m from the sewer.

I have attached a (guessed & not to scale) drawing of the sewer system, and the soil surveys. Does anyone have a recommendation on how to proceed from here? CCTV drain survey? Engage a structural engineer? Ask Thames Water? Many thanks!

 

[^woody^]

You need a structural engineer to advise you. And you need to give them clear instructions on exactly what you want - ie is a foundation viable, if so what type and at what cost. They will need to be aware of the build-over criteria, and take that into account.

You'll need to have an idea of what extension size and design you propose to build (planning and b/regs constraints), and a budget in mind too, for the build and the ground work.

 

[Narthecius]

Thanks @^woody^ . I've got a budget, and a very good idea of what I want in mind. As you've probably worked out, whether or not it can come in to existence depends on how expensive getting out of the ground is going to be.

I'm loathed to spend thousands on planning/building regs drawings if it isn't feasible, but I understand I will likely need to chuck something at it if I want to get going.

I guess I'm hoping someone might be able to look at the soil survey (in the context of a block built single storey extension) and say "you're definitely going to need/not need piles". If I do, I can start figuring out how that might work with the drains. I'm probably looking at it too simplistically, though.

 

[^woody^]

Interpretation of ground surveys is really the realm of structural engineers, and if you are basing the costs of an extension on that interpretation, then you don't want to be relying on some randomer on a forum. Made up ground invariably means that a traditional strip foundation is not an option.

It looks like the pipework would prevent a raft in context of a build over agreement, so that would narrow it down.

Initially, you'll need to get an engineers opinion on what foundations are likely. Make sure that your water company's criteria for a build over agreement is considered. Then once that is known, say it is piles, then you can get an estimate for costs before actually proceeding with the design. Then you could proceed with the planning and building regs feasibility/design and cost study.

Remember that the party wall act may apply for deep foundations. You may choose not to follow it, or may need to, but there are costs there too.

 

[Notch7]

Having read through Thames Water's build over guide, piled foundations must be 1.5m from the sewer.

probably due to concerns of vibration damage whilst the driven piles are installed -I wouldve thought piles are perfectly good solution long term as they wouldnt put any pressure on foul drainage at just 1 metre deep

a raft maybe possible but you would need a structural engineer to design it -and liaise with local water authority