To pile or to pad and beam, that is the question...

[Richrara]

Cheers for the thorough insights.

We meet all the criteria of our water company not to need a build-over agreement - unless when we get underground we find that the pipe would unexpectedly sit underneath the wall (it's 500mm away we believe - and on that basis we def meet all the eight criteria not to need a build-over). The issue with the survey was that the overground piece of testing kit wasn't able to detect the thing underground as the sewer is 2.6m deep. Our architect says it would need to travel at an angle for 4m before straightening up - which we're pretty sure it doesn't (I could see when they were removing the camera that the bend was close to the manhole) - but I'm baffled as to why they couldn't measure where the bend was just from the CCTV footage. I highlighted this with the company owner and he's not getting back to me, and hasn't mentioned charging - so I think he realises his lads on the job perhaps didn't do what they should have (it was a £220 fee).

We did dig down 2-metres and hit grey clay (when I roll it it's like blue tack). I know there's a chance it could change when we get lower than that - but the cost of a soil test I'm led to believe can be around the £1500 mark. And there are borehole records from nearby that show it as being clay down there too. So I think it's fairly safe to say it's clay.

Notch - I noticed you mentioned beams straight away (rather than rafts) - presumably that's as - as you say - rafts could be more complicated with the tree and drain issues? To be honest, the SE who recommended the pile and beam solution comes very highly recommended by several people in the industry, so I should probably just trust his judgement. Just need to make sure we're making the right choices up-front as budget is tight.

 

[SpecialK]

What water company do you fall under? Often useful information on their websites.
Did the CCTV drain company provide you with a copy of the video? Might be helpful if the water company asks for it?
I have just built over a sewer and found the best way was to instruct a private building control company (for the whole build) who then wrote to the owner for permission. This then lists the specific details you need to adhere to when building over. In the end I didn't need to pay £800 for a build over agreement!


Piling works for Foundations (southern water)

• Where piling works are proposed the position and condition of the sewer should be established by means of electronic survey. Should the survey show that the sewer is likely to be within three metres of the proposed piles, trial holes must be taken to establish the exact location of the sewer. The piling method e.g. percussive/driven or auger bored and the acceptable distance of the piles from the sewer must be agreed with us prior to the commencement of any work on site. Upon completion of the works a further CCTV survey may be required to ensure that the sewer has not been damaged during the piling operation.
• It should be noted that where piling works are to be carried out care must be taken to avoid any damage to the public sewer. Should the survey suggest that the sewer is likely to be within 2 metres of the piling, as in this case, trial holes must be excavated to establish the exact location of the sewer? No piling should be permitted within 1.0m of the sewer. No driven or percussive methods of piling works will be accepted within 3.0 metres of the existing public sewers. Detailed plans and the type piling works to be utilised will be have to be submitted to this office for approval. A repeat CCTV survey will be required on completion of the works to confirm that no damage has been caused as a direct result of any piling works.

 

[Notch7]

Notch - I noticed you mentioned beams straight away (rather than rafts) - presumably that's as - as you say - rafts could be more complicated with the tree and drain issues

Rafts are a great solution, but for a 2 story building the raft will be pretty substantial, lots of spoil removal, loads of reinforcing bar, formwork needed to create the step.

And with a raft you end up with an extension that floats on the soil, moving independently of the house.