Foundations out of position

[MortarThePoint]

Separate to a discussion on tolerances, I wanted to start a new thread focused on a possible issue with how my foundations have been positioned.

 

I used a tape measure to measure lines the groundworks team had sprayed out lines that represented middle of the trenches and they agreed with the drawings to within what I thought was an acceptable amount typically less 30mm difference. Since the trenches have been dug and poured it is hard to measure dimensions, but I am sure that one of the easiest to measure dimensions is out by 200mm. The wife's face was a picture when I explained to her that the kitchen had just shrunk by 200mm though it's actually the whole of one side of the house. Worse, we are preordering precast flooring that needs to fit. This is a bit of a worry.

 

I measured centre of trench to centre of trench using a tape measure in the three positions (along A, B and C) and all three were under by ~200mm. I sent the drone up and I think this image captures the problem. The orange lines superimposed are the walls construction drawing. I laid some timber in the trenches to have reference dimensions. Now I'm hoping the bottom left hand corner is due to lens distortion so let's not go there yet. I don't have an EDM or anything to do better 2D measurements, but those three 1D measurements have me worried enough.

 

 

With the rear wall (top of frame) centred on its trench the wall going left to right along the centre line is off centre of its trench. It just seems like that whole trench is out of place. That trench is specified to be 850mm wide and have a very structural wall on it which is 215mm wide. If the blockwork was laid out ignoring the trench misplacement it would look like the image below, 200mm off centre (blockwork in blue on top of foundation concrete in grey).

 

 

That level of off centre looks alarming to me. What do people think? Am I rightly worried or does everyone have walls well of foundation centre? Some comfort and cold beer required.

[Declan52]

Won't make any difference. Built walls right out to the edge before. That's why you build on a wide Foundation as it so easy to dig slightly off line.

 

[ProDave]

But how did you get 200mm out if both you and the builder measured the ground marks?

 

As well as marking the centres of each trench, did you not also set up profiles that mark the inner and outer edge of each wall?  These are used as a sanity check for digging the trenches and for setting out when you start building the walls.

[MortarThePoint]

3 minutes ago, Declan52 said:

Won't matter any difference. Built walls right out to the edge before. That's why you build on a wide Foundation as it so easy to dig slightly off line.

 

Thanks, this is good to hear. I worry about it being off centre putting a moment on that part of the foundation that effectively increases the pressure below one side of the foundation section. I expect I'll need to check with the Structural Engineer.

 

[MortarThePoint]

11 minutes ago, ProDave said:

But how did you get 200mm out if both you and the builder measured the ground marks?

 

As well as marking the centres of each trench, did you not also set up profiles that mark the inner and outer edge of each wall?  These are used as a sanity check for digging the trenches and for setting out when you start building the walls.

 

They clearly didn't dig centred on the line. They used a 600mm bucket and I guess they may have widened only on one side. That's the only way I can guess they have done it. No profiles unfortunately.

Edited May 9, 2020 by MortarThePoint

[JFDIY]

A bit of tweaking of the position here and there should get it mostly right. i.e move the whole footprint up and rotate it clockwise slightly.

 

At least it's off towards the stronger side where you have more walls forming buttresses to the foundation.  Not worth worrying about really I'd say, at least all the walls will be on the foundation! - I've heard of people having to build right on the edge due to incorrect set-out.

 

 

 

 

[Miek]

If you think about the actual load on the foundation's it's very very small per unit area. The founds are massively overkill in my view, nowt to worry about, and it will all be hidden anyway.

[MortarThePoint]

I'm hearing good vibes which is encouraging ?

[Oz07]

This is why people use engineers to set out, They're all indemnified against these kind of cockups. Still a digger driver could dig wrong side of the line though.

[MortarThePoint]

I think I have probably been too trusting. They used an EDM to mark out and I checked the mark out as best I could, but more experience would probably have made me do some profiles or outside foundation marks for checking as well as a bit of double checking with tape measure or EDM.

[Dreadnaught]

(Great use of photographs (from a drone, I assume) and diagrams in your post!)

[MortarThePoint]

1 minute ago, Dreadnaught said:

(Great use of photographs (from a drone, I assume) and diagrams in your post!)

 

Thanks, I'm a bit of a nerd. That and buying QCAD which was the best £30 I've ever spent.

[epsilonGreedy]

11 hours ago, Declan52 said:

Won't matter any difference. Built walls right out to the edge before. That's why you build on a wide Foundation as it so easy to dig slightly off line.

 

 

Building up from the edge of a concrete foundation would transgress regs, 75mm from the edge is allowed.

[MortarThePoint]

2 minutes ago, epsilonGreedy said:

Building up from the edge of a concrete foundation would transgress regs, 75mm from the edge is allowed.

 

That's worth knowing, thanks.

[epsilonGreedy]

Just now, MortarThePoint said:

That's worth knowing, thanks.

 

 

I had to look into this subject because I forgot to move a wall profile string when working at 1am the night before the dig started. A 5m trench had a 70mm taper that I dealt with by shuffling the whole house 50mm to get a best overall mid concrete position within my 600mm trenches.

 

I did my own setting out with a steel tape measure plus trigonometry. My digger bloke was excellent and the end result is good.

[MortarThePoint]

Well I had lots of time to think of some ideas overnight:

 

1) It can be off foundation centre and that's fine to a limit which I'll find out from the Structural Engineer.

 

2) The exterior wall at the rear (top of frame) could be pushed back a bit. Currently the wall to foundation is centred on the cavity. Below DPC, the inner leaf is 190mm blockwork and the outer is 100mm blockwork. If that wall had its overall footprint centred then that is a 45mm shift. Due to the walls higher up being of thinner blockwork, the cavity side of the 190mm blockwork will be more highly loaded, but I'd hope that over the two courses below DPC that effect would disappear(?)

 

3) The inner leaf of a cavity wall carries most of the load. I know that the wider you go with the cavity the less structural the outer leaf becomes, but I don't have any load sharing proportions to back that up. My cavities are to be 100mm. *IF* the outer leaf is only supporting its own weight and the inner leaf is supporting its own weight plus all the floors and roof then you could arguably centre the load on that basis and it yields 95mm. If the outer leaf takes 1/3 and the inner leaf 2/3 then its about 45mm. Arguably double those shifts would yield the same moment on the foundations as was originally designed in, just in the outward rather than inward direction.

 

 

4) Use thinner stronger blocks. The spine wall running the mid-length of the house is designed as 215mm/10.4N blockwork below DPC and 190mm/10.4N above DPC (a). If that 190mm/10.4N was thinned to 100mm/21N, but still centred on the lower blockwork, it would yield half the change in block width, so 45mm (b). They would be special order blocks and the wall would be more vulnerable to lateral loads, but there may be something there. Even 140mm/15N blocks offcentre from the lower blockwork could give 40mm or more. Thinner blockwork doesn't help the first floor as that's already 100mm, but could help on the ground floor. Could go as far as 100mm/21N shifted over 100mm (c) but that feels iffy.

 

 

5) Get the groundworks guys to dig and fill next to the misplaced section of foundation, or at least some piers at right angles. I can only imagine their reaction but it is their mistake. I'll see how they navigate it when they measure up for the below DPC blockwork as they are doing that too. I haven't discussed the misplaced foundation section with them yet and want them to kind of work it out for themselves rather than have me point it out.

 

Hopefully some mixture of the first 4 will get things back on track.

[Declan52]

48 minutes ago, epsilonGreedy said:

 

Building up from the edge of a concrete foundation would transgress regs, 75mm from the edge is allowed.

In all my years I have never seen a single building control officer come out and check where the blocks where laid on the concrete.

Checked the ground before the concrete pour then another check when your backfilling with stone and putting any services in.

[JFDIY]

And I thought I was one for over-thinking a job...... you better get that holiday booked. ?

[tonyshouse]

My engineer once allowed us to corbel out ove an obstruction in the ground at the corner of the building, still standing 45 years later 

[MortarThePoint]

9 minutes ago, JFDIY said:

And I thought I was one for over-thinking a job...... you better get that holiday booked. ?

 

No holidays for a while at this rate. (more thinking Covid-19)

Edited May 10, 2020 by MortarThePoint

[PeterW]

Seriously you’re overthinking this ..!!! It’s absolutely fine, just let them get the blocks down. There is nothing wrong with that lot and it’s not going to go anywhere. 

[MortarThePoint]

11 minutes ago, PeterW said:

Seriously you’re overthinking this ..!!! It’s absolutely fine, just let them get the blocks down. There is nothing wrong with that lot and it’s not going to go anywhere. 

 

Thanks, I'm sure you're right. One of those things you churn on in the middle of the night having been woken up by a 4yo.

[Pete]

22 hours ago, MortarThePoint said:

 

Thanks, I'm sure you're right. One of those things you churn on in the middle of the night having been woken up by a 4yo.

Do not worry, it is normal for self builders, WE worry about everything especially if you doing most of the work ourselves!!

[MortarThePoint]

Well a bit of an update here. The Structural Engineer has said he isn't happy about the central wall misalignment but has kept me waiting for about a week now to do calculations as to how much offset is OK.

 

The groundworks guys did day 2 of blockwork yesterday and that looked to be fitting nicely by eye. I went out this morning and they have taken it back down. They checked yesterday's work with the total station and realised they had made a mistake with a mark that meant a wall went 40mm off target (OK one end, straight, but 90 degrees 13 minutes to the other wall). They have now identified that the front area of foundation is out of shape and they are talking about digging next to it and pouring more foundation. 450mm width if just butting up or 600mm if doweled. I prefer the sound of doweled. To their credit they have worked all this out themselves and corrected their work, rather than coming to me and asking if it is OK, which gives me some confidence.