Mattg4321 Posted June 7, 2023 Share Posted June 7, 2023 Hi all, I've been lurking here for a while and trying to contribute where possible, but it's now time to start asking some questions of my own. We are about to have some planning drawings done for our rear extension and having worked in construction on domestic properties for 20 years, I'm quite confident in my knowledge of a lot of things construction related, but foundations/groundworks are not one of them! We have done various other projects in the past, including a new build in our garden a few years back, so I'm not new to this, but this time we have a large oak tree roughly 10m from where the closest point of the extension will be. I know from experience that architects/structural engineers etc will generally look out for their own interests/do what's easiest/just not have a clue. I don't want to be spending any unnecessary money going down blind alleys. I don't know what the existing foundations are, but with no evidence whatsoever, I assume strip foundations. Existing house is 13m from the oak tree at closest point and was built in 1970. Anyone care to comment on this assumption? Would piles/a raft have been used in 1970? Either way, I think for an extension a raft is pretty much off the table. From using the LABC online foundation depth calculator, it does appear very likely we will need to pile. The ground is clay (we're just north of the South Downs and the whole area is clay) and with the tree the calculator is showing that it's outside the scope/deeper than 2.5m for strip foundations. To get to the point of my post, should I commission a ground/soil report, and what exact tests do I need to have carried out? Alternatively, should I just accept now it will need piling and go straight to a specialist and have them design it after their own ground/soil survey presumably? Will the piling companies want a report to have already been carried out, or will they want to do their own? Sorry for the rambling post, thanks in advance for any help offered. Link to comment Share on other sites More sharing options...
Jilly Posted June 7, 2023 Share Posted June 7, 2023 (edited) Your structural engineer would decide usually, but you are right, on clay and near trees trees, piling is likely to be required. Get several quotes as they can vary wildly. I used a firm owned by an SE and he provided the calculations for Building Control. I would speak to an SE before the soil survey. Edited June 7, 2023 by Jilly Link to comment Share on other sites More sharing options...
Temp Posted June 7, 2023 Share Posted June 7, 2023 We have clay soil and got a soil condition report. That showed the clay wasn't really a bad type and strip foundations with expansion boards were allowed for our new house. Not even terribly deep. And we have trees nearby. One just 3m away in neighbours garden. Several houses down the road they just assumed they would need piles and they had to go so deep they joke about a man in Australia using the other end as fence posts. I would ask around and see if anyone in the area has built recently. 1 Link to comment Share on other sites More sharing options...
Mattg4321 Posted June 8, 2023 Author Share Posted June 8, 2023 The neighbour’s opposite had an extension 13 years ago, but quite a bit farther away from any trees (and I think a less thirsty tree than an oak). I’ve asked them a few questions already, but I don’t think they got too involved with anything by the sounds of it, or they’ve just forgotten. If I need a soil condition report, are those the exact words I should ask for? Link to comment Share on other sites More sharing options...
Dave Jones Posted June 8, 2023 Share Posted June 8, 2023 yes you will need a report. Ask the SE he will need to spec the piles after obtaining the report. You will need a mini piling rig who will take core samples do a set depth. Expect £2-4K + the SE design fee. Lots of ££££££ lost in the ground. Link to comment Share on other sites More sharing options...
Temp Posted June 8, 2023 Share Posted June 8, 2023 55 minutes ago, Mattg4321 said: If I need a soil condition report, are those the exact words I should ask for? Sometimes called a Ground Condition report. Usually involves digging one or two pits and pushing an instrument into the ground, also sending samples off to a lab. Link to comment Share on other sites More sharing options...
George Posted June 8, 2023 Share Posted June 8, 2023 You'll need GI if you have piles so I'd risk it and get one done up front - it may turn out that you do not need piles and if you do, you've already got the information ready to go. The key thing will be the plasticity of the soil. Link to comment Share on other sites More sharing options...
Mattg4321 Posted June 8, 2023 Author Share Posted June 8, 2023 Thanks, it sounds like it’s worth doing then 1 Link to comment Share on other sites More sharing options...
saveasteading Posted June 8, 2023 Share Posted June 8, 2023 Yes. Ask locals and builders what they did, but as a starting point. Your house presumably isn't on piles so probably the exension shouldn't be either. Don't assume that the SE knows the costs of things. Link to comment Share on other sites More sharing options...
saveasteading Posted June 8, 2023 Share Posted June 8, 2023 , Link to comment Share on other sites More sharing options...
saveasteading Posted June 8, 2023 Share Posted June 8, 2023 My rule of thumb on heavy clay near trees is Strip footings down to 2.4m Piling if below 2.5m But depends on job size and details 1 Link to comment Share on other sites More sharing options...
Big Jimbo Posted June 8, 2023 Share Posted June 8, 2023 @saveasteading Just out of curiosity, 2.4m down for strip footing. (or any distance) from where ? Existing ground level, or ground level once stripped ready for footings to be dug ? The reason i ask is that i always strip the ground. Usually down to the level of the bottom of the slab, or the distance that is required under the block and beam floor. That distance could be 300mm if clay. I always dig my footing from that point when in fact i could have already removed 4 or 500mm. So my 2.4m footing is effectively 2.9m deep. Am i digging to deep ? Link to comment Share on other sites More sharing options...
saveasteading Posted June 8, 2023 Share Posted June 8, 2023 (edited) Trench depth whether original or reduced ground level. Reach of bucket and stability of ground The require depth isforbelow thel finished ground level....where dampness will be steady. Edited June 8, 2023 by saveasteading 1 Link to comment Share on other sites More sharing options...
Lofty718 Posted June 8, 2023 Share Posted June 8, 2023 (edited) I had this issue on a recent build. SE was pushing for piles, wanted 1k + vat to dessign them, then quotes from piling companies were insane. I also spent about 500 quid messing about with a soil test and investigation. I paid to have the neighbours tree removed, 1100 all in including grinding the stump. It was an ugly unkept oak tree that blocked a lot of light and fllled all the neighbours gardens with leaves Also the tree was actually 12m away but the SE put in the drawings that it was 7m away. Not sure if done on purpose, but removing the tree saved a lot of money and I recommend you do the same way before getting started. check what the nhbc calculator says regarding depth if you remove the tree, it will be significantly less than keeping it, because the calculator assumes the height of the tree and imo is overkill. Edited June 8, 2023 by Lofty718 Link to comment Share on other sites More sharing options...
Mattg4321 Posted June 9, 2023 Author Share Posted June 9, 2023 I have been considering taking the tree out, but it’s likely to upset the neighbours. I’ve been there before and it’s not something I want to repeat. Maybe I need to start thinking of a way to sell a story to them and hopefully keep them onside. Link to comment Share on other sites More sharing options...
Dave Jones Posted June 9, 2023 Share Posted June 9, 2023 you may only have to dig deep the element that is within the trees effect radius (they are all different, all the rest of the footing can step back up. Oak are high water demand Link to comment Share on other sites More sharing options...
saveasteading Posted June 9, 2023 Share Posted June 9, 2023 7 hours ago, Lofty718 said: calculator assumes the height of the tree and imo is overkill. Not really, because trees continue to grow to the heights shown, and the growing requires water, and roots spreading too. Killing a tree stops the annual cycle of absorbing the water in summer, so the ground will still expand in winter..perhaps for 2 years. Most people think that the roots cause physical damage. This happens, but shri nkage of clay is the main risk. Link to comment Share on other sites More sharing options...
Marvin Posted June 9, 2023 Share Posted June 9, 2023 The roots from the oak tree, to the side of our back garden at our last house, streched further than the distance of the branches by about 50% The extension foundations was trench filled concrete 2.5 meters deep with a beam and block floor. The whole place was high shrinkability clay. The oak tree roots ended up in our front garden! I understand there are different species of oak tree so they may not all be the same.. Link to comment Share on other sites More sharing options...
Mattg4321 Posted June 9, 2023 Author Share Posted June 9, 2023 I’ve seen that table before. So if the tree is 10m away, then if it’s over 20m tall (which it probably is), it’s likely to need foundations deeper than 2.5m. Sounds like a soil investigation report is the next step. Meeting architect again on Tuesday, so will discuss a way forward with him. Anything I should be asking him/pointing him in the right direction with anything? Thanks for all the help so far. Link to comment Share on other sites More sharing options...
saveasteading Posted June 9, 2023 Share Posted June 9, 2023 See what he already knows based on the above. He should know what your ground is likely to be, from experience. Until an oak, or similar, is about 23m from the house, it has an effect. Link to comment Share on other sites More sharing options...
Lofty718 Posted June 9, 2023 Share Posted June 9, 2023 Oaks can definitely be destructive, but a lot of it is guesswork from SE's covering their own backs (understandably) The roots can apparently travel up to 3x in distance of the the height of the tree. Removing can also cause heave where the ground is used to the water demand from the tree. There is usually always a less costly solution than piling though, which in these current times can make your build prohibitively expensive. Link to comment Share on other sites More sharing options...
saveasteading Posted June 9, 2023 Share Posted June 9, 2023 The thing with piling is the cost of mobilising. The piles themselves can be less. Then you break the tops off and do trench fill concrete. Link to comment Share on other sites More sharing options...
Lofty718 Posted June 9, 2023 Share Posted June 9, 2023 Mine would of been very expensive. Design, piles, ring beam, block and beam floor. Then there's the unknown of how deep the piles need to go and the extra costs that this can incur Link to comment Share on other sites More sharing options...
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