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

Discussion in 'Building' started by Richrara, 22 Dec 2020.

  1. Richrara

    Richrara

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    Hi, we're about to do an 8m x 4m 2-storey extension and have an oak tree 4metres from the proposed extension flank wall. We're also on heavily clay-based soil. One structural engineer has suggested pad and beams as being the easiest and most cost-effective solution, whilst another has said we should definitely pile it. Anyone have any experience/knowledge/opinion on what's best given our constraints?
     
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  3. Richrara

    Richrara

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    And having just had a look around the forum, I'm now concerned about the cost of piling: I've been given a quote by a piling contractor of £3k for 6 piles - which is doable, but I'm seeing threads describing piled foundations for smaller extensions costing £10k+. In which case, would my best bet be - whichever route we decide on - to get the structural engineer's drawings and consult both the piling contractor and my builder before proceeding with any works - just to make sure that the builder doesn't have to add costs to his quote (which was based on 1m foundations) if we pile it. I know that the piles will cost £3k, but is there something else with this type of founation that could add to the builders costs? Sorry - realise that's two questions now, but advise on both would be much appreciated!
     
  4. SpecialK

    SpecialK

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    Does your piling quote spec how deep the piles will go without extra charges? Does it mention how much extra per metre they charge or is the quote for a completed job? There is an extension build in projects section that's just been piled and I'm sure he added the costs in there...
     
  5. Mag2.0

    Mag2.0

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    If your concerned with ground movement i'm not sure how a pad resolved your situation given that a pad sits close to the formation level.
    Are we talking screw piles / concrete piles / stone piles?
    Maybe some more information would help give some context to the situation.
     
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  6. Richrara

    Richrara

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    Thanks for the replies. Mag, I was told by a structural engineer that a pad and beam solution would be signed off by building control without any issues as being a suitable solution to having an oak tree so nearby? Re the type of piles, all I know is they're 'auger' piles.
    Special K - the quote was over the phone, based on the size of the extension and there being a total of six piles. So in essence are you saying that it can vary quite widely depending on how deep they need to go? I'm going to go back to the structural engineer who is suggesting piles and find out a) why (over pad and beam) and b) what the variables are. I'll update here once I know.
     
  7. ^woody^

    ^woody^

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    Were those the only two criteria?

    Whilst pad and beams allow for roots to go in between, and are useful for bridging large roots, there is certainty more risk of movement long term if things change significantly. That's not to say it will occur, just the risk is there, and that risk may be small or larger depending on several factors.

    With piles you are paying more to reduce/remove that risk.
     
  8. Richrara

    Richrara

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    Cheers woody.

    Update: I've now spoken to the structural engineer who said if you require foundations that go below 2.5m's, it's cheaper just to pile. The piling company hadn't mentioned the cost of ground beams would also need to be taken into consideration for a piled solution (boy am I glad I looked on this forum!), so what I though was going to cost 3kish, they're now saying 8-9k to get it piled and everything else done and ready for brickwork. My builder said he'd knock 4k off his costs as he wouldn't have to do the foundations, so we're not that much out anyway. In a further twist, I spoke to another piling company who said that - as we're on level ground and the level of the existing building is only a few inches higher than the ground we're extending on to - it's going to be cheaper to do rafts! So I'm waiting on a second quote for a complete foundation solution on rafts. I'll update here when I have that in case it's useful to anyone else.
     
  9. ^woody^

    ^woody^

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    Whether it is or not is not the issue. What matters is whether the piling company are qualified to determine if a raft is actually suitable. An independent structural engineer is the person to decide. You will get a floating extension tied to a fixed house. You can probably visually what could happen.

    You seem to be concentrating on price, which is a bit dangerous for foundations. Paramount, is "what foundations are suitable", and then comes cost (vs. risk).
     
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  11. kingandy2nd

    kingandy2nd

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    6 piles doesn't seem like all that many. My 11 x 5.5m extension had 14 or 15 (I can't remember now). As has been mentioned, normally the price is only to a certain depth, with every additional metre costing you more.

    Once you have your plans, email round to various companies - I found quite a variation in the prices and what was included in the 'normal' prices.

    Check whom will do the groundworks so there's no misunderstanding between the builder and the pilers.

    Good luck :)
     
  12. Richrara

    Richrara

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    Cheers both - all sound advice. The piling company suggesting rafts have been established locally since the 1970's so it should be safe to assume they know what they're talking about. They also provide a guarantee with the work. That said, if the price difference between rafts and beams isn't significant, then I'll probably go with what the SE suggested I.e. beams (however I've spoken to 3 now and they've all said different things, ha!).

    One final thing if anyone has any experience of this: we have a mains sewer that we approximate - based on manhole positions - to be 500mm from the flank wall of the new building. Providing that's the case, and it doesn't take a turn under the building, we fulfil all the criteria not to have a buildover agreement. What we need to avoid however is getting down there with a piling machine on site, and finding we're going to need a buildover agreement. Anyone have any suggestions of how we might best establish the sewer position more precisely. I had a cctv survey done, which was beyond useless; is trial pits the only way to be certain?
     
  13. sxturbo

    sxturbo

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    A drain survey by a company that knows what they are doing will be able to advise the location of drains and pipework.

    The water board usually have a specific contractor you must use for build over, so speak to the water company, they will either carry out a drain survey against your plans at your cost or advise you of the contractor (think they give a choice of 2) you must use.

    My parents had to have a buildover agreement last year and the above advice is from this. Your local water company might be different to my parents.
     
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  14. Richrara

    Richrara

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    The issue we have is that the drain bends and then straightens up. We just don't know how quickly it bends, and - bizzarely(?) - the guys doing the CCTV drain survey said that CCTV wouldn't be able to establish this. Does anyone know if that's correct or not? As long as the drain bends before 4m were fine. But I would much rather know in advance for certain where it bends and then runs straight. Would CCTV normally be able to tell this?
     
  15. sxturbo

    sxturbo

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    They should know, they can zero the counter at the start of the drain and then from there the software calculates the distance cover.

    For the bend they should be able to measure the length of the bend they can then calculate the radius of the bend.

    How much was your drain survey?

    Normally for a decent one looking at a few hundred £
     
  16. Notch7

    Notch7

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    Have you spoken to your water Company?

    Build over agreements are really build near agreements.....you need one if you are building within 3m of a public sewer.

    To position it you need a thing that goes down the sewer, then you track it with another thing on the ground.


    Have you dug down? Are you definitely on shrinkable clay?

    I did a job with an oak tree 4 metres away, the client got the soil tested, it was found to be load bearing mudstone lower down and just a 1 metre footing.


    For piling, you need: driven piles, they are filled up with concrete and some rebar is cast in so it pokes out the top.

    Then a load of rebar with special bends to a schedule is wired in place

    Then the whole lot is cast in a concrete skirt

    Then you need beam and block suspended flooring.

    And you will need a structural engineer to design the piling and ring beam


    Alternative is a raft, which again needs an engineer to design, could be more awkward with tree and drain issues. Rafts float around, piles stand on stilts. Neither will move around with the house unless that has same foundation spec.
     
  17. Mag2.0

    Mag2.0

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    I'm pretty sure the water company will not allow you to pile within 15m of their assets unless its screw piles only. I can't remember the exact requirements so may be wrong but worth checking out.
     
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