Dave Bailey
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| posted on 3/12/16 at 10:05 PM |
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Any builders on here?
Daughter is having a double storey extension built on the back of a 1930's property built on clay... building inspector mandated 2.5m for the
footings due to some trees being about 15m away and stated clay boards to be used on the inside wall... floor is to be beam and block. The footings
were shored up but only with braced 4x2. Trench became a bit unstable and the clay boards were not pinned to the wall because it was just unsafe to go
down. Cut a long story short the clay boards popped up resulting in the concrete setting with no clay boards in. I don't want to get into a war
of words with the builder if the risk of issue is low. Next time the building inspector comes out the beam and block will be in. Any advice? I would
like to have the building inspector out and have a chat but I'm pretty sure the builder isn't going to be over keen on this...
Cheers
Dave B
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JoelP
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| posted on 3/12/16 at 11:40 PM |
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I'd make no mention to the building inspector! Christ that's asking for trouble. 2.5m is shitloads of concrete to resist heave.
Note for next time - get rid of all trace of the trees before the building inspector gets there, and save thousands on the concrete!
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Dave Bailey
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| posted on 3/12/16 at 11:49 PM |
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Good idea cutting the trees down but they are mature oak trees and pretty big. First one has a TPO on it...
The clay boards are heave protection for under the beam and block floor... trench is 2.5m but concrete depth is about 1.8 - 2m
Dave B
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Sam_68
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| posted on 4/12/16 at 08:45 AM |
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At risk of stating the bleedin obvious, the reason you get heave under the floor is that you've cut the tree roots by digging the foundations
trenches... it's because it's without a damned great oak tree sucking the moisture out of it at several hundred litres per day
(yes, really) anymore that the clay starts absorbing water, swells up, and shoves your walls/floor out of the way.
Cutting the trees down wouldn't have helped - if you're on shrinkable clay, it will go through several cycles of seasonal heave and
contraction after the trees have been cut down, before everything settles down, anyway....so if you want to solve the problem by cutting the
trees down, you need to do it several years in advance of construction.
This being an internet forum, we have no idea of how shrinkable your clay is, or the zone of influence of the tree roots relative to the
position of the building, but I'd certainly want heave protection in the top 500-700mm. if the Building Inspector has mandated it. The risk to a
trench fill foundation below that depth is lower (obviously mass concrete, even reinforced, is better able to resist lateral forces than brickwork) ,
but...
If you want a proper second opinion, get a Geotechnical Engineer (or a Structural Engineer with good experience of foundations design) to come
out on site and have a look. Maybe, if your builder is trying to get away with it, tell him that you are prepared accept the situation if he can get
such an engineer to sign it off (and negotiate with the BCO likewise) at his expense?
[Edited on 4/12/16 by Sam_68]
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Smokey mow
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| posted on 4/12/16 at 09:34 AM |
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quote:
Originally posted by Sam_68
At risk of stating the bleedin obvious, the reason you get heave under the floor is that you've cut the tree roots by digging the foundations
trenches... it's because it's without a damned great oak tree sucking the moisture out of it at several hundred litres per day
(yes, really) anymore that the clay starts absorbing water, swells up, and shoves your walls/floor out of the way.
Cutting the trees down wouldn't have helped - if you're on shrinkable clay, it will go through several cycles of seasonal heave and
contraction after the trees have been cut down, before everything settles down, anyway....so if you want to solve the problem by cutting the
trees down, you need to do it several years in advance of construction.
This being an internet forum, we have no idea of how shrinkable your clay is, or the zone of influence of the tree roots relative to the
position of the building, but I'd certainly want heave protection in the top 500-700mm. if the Building Inspector has mandated it. The risk to a
trench fill foundation below that depth is lower (obviously mass concrete, even reinforced, is better able to resist lateral forces than brickwork) ,
but...
If you want a proper second opinion, get a Geotechnical Engineer (or a Structural Engineer with good experience of foundations design) to come
out on site and have a look. Maybe, if your builder is trying to get away with it, tell him that you are prepared accept the situation if he can get
such and engineer to sign it off (and negotiate with the BCO likewise) at his expense?
100% this ^^^^
Its still possible to retrofit the clay board to the foundations at this stage and is something many developers i know do as its easier than trying to
hold it in place during the concrete pour.
If you wait till the concrete has cured you can dig down the inside face of the foundation with a narrow trenching bucket and then drop the clayboard
Down then backfill the void.
[Edited on 4/12/16 by Smokey mow]
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nick205
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| posted on 5/12/16 at 09:02 AM |
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My parents added a single storey extension to the rear of their house 2 years ago. Original parts of the house are early 1800's and the site is
clay and chalk. No trees involved, but they too were instructed to go to 2.5m footings (and did so).
1. Most of the original house doesn't really have footings of any kind. It's been their a long time and there are no problems with it.
2. 2.5m of spoil is a hell of a lot to remove and dispose of.
3. I suspect "planning" are often ticking boxes and wonder how much "real" observation goes on in these things.
Of course nobody wants potentially difficult and expensive long term issues with their house. Best case you may want to sell the property and having
such issues (or not having the correct approvals) could impede the sale and cost you (or the buyer) money.
My parents didn't have another similar local project to compare to. Does your daughter have anything local (a neighbour perhaps) to use as a
reference?
BTW - I'm not a builder! I just take an interest in such projects and the often underestimated scale of cost/issue they bring with them.
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Sam_68
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| posted on 5/12/16 at 09:39 AM |
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quote:
Originally posted by nick205
I suspect "planning" are often ticking boxes and wonder how much "real" observation goes on in these things.
Planning has nothing to do with foundations design or construction. The process is controlled by Building Regulations (which is a quite separate
legislative process, administered by different people).
I've lost count of the number of older properties I've encountered, of the sort you describe, with failed foundations or other serious
structural defects.
The idea that 'they don't build 'em like they used to' is true enough, but not in the way that most people imagine it. 
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SJ
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| posted on 5/12/16 at 01:15 PM |
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quote:
quote:
Originally posted by nick205
I suspect "planning" are often ticking boxes and wonder how much "real" observation goes on in these things.
Planning has nothing to do with foundations design or construction. The process is controlled by Building Regulations (which is a quite separate
legislative process, administered by different people).
I've lost count of the number of older properties I've encountered, of the sort you describe, with failed foundations or other serious
structural defects.
The idea that 'they don't build 'em like they used to' is true enough, but not in the way that most people imagine it.
From my albeit limited experience, I think local factors come into play with how keen building inspectors are on things like foundations. Where I live
the inspector reckoned the ground is very firm, and as a result aren't that worried about footings. Our '70's extension that we
built on top of had 2-3ft of concrete. Much of the original 1930's house has next to none.
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nick205
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| posted on 5/12/16 at 02:51 PM |
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quote:
Originally posted by Sam_68
quote:
Originally posted by nick205
I suspect "planning" are often ticking boxes and wonder how much "real" observation goes on in these things.
Planning has nothing to do with foundations design or construction. The process is controlled by Building Regulations (which is a quite separate
legislative process, administered by different people).
I've lost count of the number of older properties I've encountered, of the sort you describe, with failed foundations or other serious
structural defects.
The idea that 'they don't build 'em like they used to' is true enough, but not in the way that most people imagine it.
Fair point - as I say I'm not a builder so don't know the ins/outs of these things.
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