Trench fill foundations,

[willismp]

Hi,

My builder was constructing some steps into a trench fill foundation.

The building inspector (private company) said the overlap had to be 300mm, so the builder made the overlaps 300mm.

Reading the Approved building regaulations they say minimum 1m overlap.

When questioning the buidling inspector he said "the regulations are a guide only".

Is this correct? It sounds like a lack of knowledge to me, should I insist on strucutal calcs to prove the overlap is ok?

Thanks

 

[RonnyRaygun]

Your BCO is getting confused between trenchfill and strip foundations.

The minimum step for trench fill is 1.0m, the minimum step for strip footings is 300mm.

The regulations (assuming you mean Approved Documents) should be adhered to unless it can be proved that a different design is suitable and/or sufficient.

Is your BCO going to do the calcs to prove 300mm is OK?

 

[willismp]

I have asked the BC company, they have refused saying "in their professional opinion its fine" and they are providing the certificate.

Upon checking both the 'approved documents' and 'NHBC guidance' all state >1m.

It's a concern as not sure if to build on top, as it will be expensive to sort.

I have made a formal complaint to try and push for some calcs, but frankly its a lot of hassle, when it should be quite simple!

Lesson learned to always use the council BC officers!

 

[RonnyRaygun]

I have asked the BC company, they have refused saying "in their professional opinion its fine" and they are providing the certificate.

Upon checking both the 'approved documents' and 'NHBC guidance' all state >1m.

It's a concern as not sure if to build on top, as it will be expensive to sort.

I have made a formal complaint to try and push for some calcs, but frankly its a lot of hassle, when it should be quite simple!

Lesson learned to always use the council BC officers!

You won't get calcs from a BCO. You need a structural engineer for that.

It's not just in the NHBC guidelines, it's also in Approved Document Part A.

Don't be under any illusions that Council BCO are any different though.

I'm not saying you will have any problems but you will have little luck making a claim if you do.

 

[noseall]

I think you could be fretting over nowt.

Consider what some older buildings are built off then look at your own concrete and compare.

How thin will the concrete be at the thinnest point?

 

[RonnyRaygun]

In all honesty I think Noseall is right. Depth and width are the most important things in trench fill foundation design.
If the foundation is wide enough to sufficiently spread the load and deep enough that it is below the influence of trees, below any made ground, below frost action, and below any drains, then it shouldn't move anyway.

 

[willismp]

Thanks to all for your responses,

I still feel the BCO should justify with calc's that it ok, as it was his decision alone to move away from the regulations.

I am only concerned as looking at the regulations, a strip foundation should overlap 300mm or twice the step. Trench fill overlap >1m. My initial thought would be the opposite.

To remove & replace now would cost me £3/4k, but the extension shell is £65k, so it needs to be right now.

 

[tony1851]

Strictly speaking, the foundation overlaps specified in Approved Document A are not 'the regulations'. They are advisory and are only 'deemed to satisfy' provisions. This means that if a builder follows those dimensions, then an inspector has to accept that the foundation is sound.

If a builder departs from the guidance, it does not mean that the design is unsound, but he may , at the discretion of the inspector, be required to provide calculations to justify the departure.

But there will be a considerable factor of safety built into the guidance details, and as N and RR have both stated, in practice you will almost certainly not have a problem. The vertical rigidity of the walls of the extension will help to spread the load evenly along the foundation.

 

[stuart45]

I can understand you wanting to get the foundations right, but I don't think I would be ripping them out because the overlap on the step is 700mm too short. Even at the worst there are ways of adding concrete to concrete.

 

[maltaron]

It always amuses me when you build a single storey extension to a pre war 2 storey house. The house has stood with no problems for a hundred years on a foot (300mm in new money) wide foundation a foot deep. Yet the little extension has to have 600mm wide at least 1000mm deep.

 

[RonnyRaygun]

It always amuses me when you build a single storey extension to a pre war 2 storey house. The house has stood with no problems for a hundred years on a foot (300mm in new money) wide foundation a foot deep. Yet the little extension has to have 600mm wide at least 1000mm deep.

450mm is wide enough for most low rise buildings, but as a rule of thumb to avoid calculation, 600mm is often used. It also gives a bit more leeway in the setting out.

1000mm is a minimum when there is high volume change potential. Depth can be 750mm in low volume change clay and as low as 450mm in some gravels.

 

[tony1851]

It always amuses me when you build a single storey extension to a pre war 2 storey house. The house has stood with no problems for a hundred years on a foot (300mm in new money) wide foundation a foot deep. Yet the little extension has to have 600mm wide at least 1000mm deep.

That's becaue the old building was built using lime mortar. It will have settled soon after completion, but lime mortar remains plastic for many years and will accomodate movement (within reason) without cracking.

Modern brickwork built using cement mortar is rigid and brittle and although strong (in compression) compared to lime mortar, it cannot accomodate the tensile stresses caused by even slight differential settlement without cracking. That is why modern foundations have to be rigid.