Pad foundation on top of existing foundation

[^woody^]

My contractors have excavated the pads ready for the pour, however I just checked them and they are way off the intended positioning

You don't need an engineer's second opinion, you need different builders.

You're not even out the ground yet and they have messed up twice with crap advice. What does that tell you?

If you dont' build to the engineers design, then the work is invalid, won't be covered by his insurance and potentially rejected by building control.

Having said that, there is typically a lot of tolerance in pad design. The probelm you have is that there is no-one there experienced enough to know what can and can't be done. I doubt you will get such reasurance from here either, as it's not something that can be answered definitively on a forum.

 

[iwanttodiy]

Yes, agreed. I’m sure the SE would reject any failures as it wasn’t adhered to his design.

Unfortunately, I’ve already paid pretty much the whole sum to the contractors... (I know, rookie mistake).

Managed to get the pad more centred. Probably got a 1200x1200 x500 pad now. I don’t like the idea that part of the pad is sitting on some breeze block type oversite. But hey... Industrial strength concrete!

 

[wessex101]

I don’t like the idea that part of the pad is sitting on some breeze block type oversite.

That doesn't sound good. Has the building inspector seen the excavation for the pad foundation and approved it?

 

[iwanttodiy]

Yes. He approved it with the new foundations.

Mentioned he only needs pictures of the hole once the steel mesh is in.

 

[^woody^]

Yes. He approved it with the new foundations.

Mentioned he only needs pictures of the hole once the steel mesh is in.

He's been out and actually seen it?

 

[iwanttodiy]

No he hasn’t seen the pad, he inspected the excavation.

They basically wrote on the inspection sheet to call them back for DPC. Which would imply, after the foundations are poured and first set of courses laid.

 

[noseall]

No he hasn’t seen the pad

Oh lordy!

Get him out - now.

 

[iwanttodiy]

Oh lordy!

Get him out - now.

Even if he stated the next visit is DPC?

The pad on the flank wall is not yet complete as it will be poured in 2 batches.

 

[wessex101]

Get the building inspector out to look at the hole where the pad foundation is going to be poured if they haven't seen it already. Inspection of the foundation trenches before pouring the concrete is probably the most important inspection so you want to make sure everything has been inspected and approved.

The trouble is this is all very confusing, there are technical terms being mixed up and I suspect used in the wrong context so I am really struggling to understand what the hell is going on. I cannot envisage a situation where a pad foundation could be cast on top of a block oversite, I'm not even sure what a block oversite is. I would usually expect it to be excavated in to the ground down to good firm undisturbed sub-soil, the same depth and on to the same sub-soil type as the main foundation trench so it shouldn't be sat on top of anything.

 

[^woody^]

Even if the inspector comes out for a look or not, he still needs to be made aware that the foundation design is being built differently to the engineer's specification.

It's alright saying "Oh, the inspector does not want to come out until its time to look at the DPC", but the fact is your builder is not building to the specication and so who knows whether his work is correct or not?

If those foundations move, it's no-ones problem but yours.

 

[noseall]

Even if he stated the next visit is DPC?.

Yes, absolutely.

 

[iwanttodiy]

Ive attached a before pour and after pour. BCO have asked it to be poured 1m deep and the builders dug under the existing pad by 1m. To envelope the existing pad. The bit I am worried about is the stuff between the shuttering and the foundation.

To be honest if I was to do this myself, I would probably have broke out that foundation and had a full pour for the pad.

“Sigh” I’m not very good with this contractor game, hence I would try and micromanage or do it all myself. Probably would have been simpler to say, get a builder in and just dig a hole this size. And done.

Once these guys are finished. Is there i possibility, I go on the other side, dig out the breeze block oversite and just fill in 0.24m cubed of C35 concrete with tied in rebar?

 

[wessex101]

Frankly I'm surprised the building inspector approved that.

It will be impossible to fully excavate under that existing foundation so they have probably only gone back as far as they could reach with a hand trowel. It will be extremely difficult to get the concrete to fill that void and if they had a wet mix you will get shrinkage and a gap between the new concrete and existing foundation. That thin mesh reinforcement is far too weak to compensate for the inadequacies in the pad excavation. Your instinct was right, they should have broken out the old foundation and done it properly.

I would only hope that the engineer has massively over engineered that pad design so that the point load from the column will still be within safe tolerances.

 

[iwanttodiy]

I have written confirmation from the SE about enveloping the existing foundations and he was happy to confirm it is suitable if we tie rebar into it.

However in this situation, as we didn’t envelope the foundations on the inside, I believe the confirmation would be void.