Extension help! Existing foundations not as expected

[warmington_ash]

Hi,

 

We've just started our small single storey extension, and we've dug out the new footings, and to my surprise, the existing foundations are not as drawn. The house in only 7 years old and the existing details show the top of foundation 600mm below FFL, however its actually 1400mm below. There extras courses of below ground blockwork that are not shown on the plans.

 

We have dug out our new footing to the depth proposed by our engineer (to allow for 1500mm of concrete), however as it stands, the new footing will interface with only 775mm of the existing concrete footing and 725mm of existing blocks. I've contacted my engineer, but the digger driver is here again tomorrow so I was keen to hear what the likely outcome will be if anyone has any views?

 

Will he likely tell me that I need to dig deeper to ensure a greater concrete to concrete interface? If so, I'm a bit snookered. Access is tight and this is the deepest that a mini digger can go.

 

Any help or advice would be greatly appreciated.

 

 

[Marvin]

I would be surprised if the concrete to concrete interface was a problem.

 

The depth is usually to do with the stability of the ground. I presume the deeper block work was a design choice, however I'm sure someone will correct me.

 

Hopefully the blockwork below ground is either solid or the cavity is filled - possibly with concrete.

 

M

[Radian]

I would be asking building control if they were happy with it as is. They might be able to respond to an email with that photo attached.

[ETC]

I suspect that your BCO and engineer will want you to go down to the bottom of the existing house foundation for the bottom of your new foundation - that is where the good ground was located when the house was built and presumably still is. 
 

If the house is relatively new your local BC should have a record of the depth the original trenches were excavated to. Ask them.

 

Looking at the drawing and the photograph it looks like you have a suspended ground floor structure - this could suggest that the ground wasn’t good enough for a ground bearing slab or for a shallow strip foundation.

 

My advice would be to get your engineer and your local BCO out and for them to advise you on your next step. Your BCO will want to inspect the ground before you pour concrete and he/she will more than likely take advice from your SE.

 

Don’t panic it can be sorted out.

[warmington_ash]

20 minutes ago, ETC said:

I suspect that your BCO and engineer will want you to go down to the bottom of the existing house foundation for the bottom of your new foundation - that is where the good ground was located when the house was built and presumably still is. 
 

If the house is relatively new your local BC should have a record of the depth the original trenches were excavated to. Ask them.

 

Looking at the drawing and the photograph it looks like you have a suspended ground floor structure - this could suggest that the ground wasn’t good enough for a ground bearing slab or for a shallow strip foundation.

 

My advice would be to get your engineer and your local BCO out and for them to advise you on your next step. Your BCO will want to inspect the ground before you pour concrete and he/she will more than likely take advice from your SE.

 

Don’t panic it can be sorted out.

 

The existing house founds are a combination of 2.4m and 2.9m deep. It was an old orchard so heave was an issue which dictated the depths. The ground is good solid clay, so it it wasnt for the trees, my understanding is that the foundations (new and old) could be much shallower.

 

My engineer has advised a new depth of 1.5m with dowel rebar joints. The depth of 1.5m is based on the NHBC guidance regarding trees (wild cherry tress at the back of the garden).

 

Maybe the unexpected blocks I've found that sit on the existing foundations are effectively acting as the foundation? But I'm not sure why they would rather have done that compared to just concreting higher? and why wouldn't the drawing have been followed? All a bit odd.

 

I'm just praying that we don't need to go deeper, because that will be a real problem.

 

[tonyshouse]

I would place a sheet of 25mm eps berween the new and existing, on the top of old and up to under the drain against the wall 

 

I would cut the bottom of the trench clean and squared up.

 

get concrete in tomorrow at all costs , try to finish exactly on a block course or 5mm below. 

Edited April 28, 2022 by tonyshouse

[nod]

Building control will be less concerned with the interface of foundations Only that the new foundations have hit solid ground 

You have good depth As long as the bottom of the new trench is solid BC will tell you to pour 

[PeterW]

I’d be more concerned about that pipe running on the face of the founds. You can’t just bury it in concrete without movement joints so that needs addressing. 
 

In terms of dowels then it looks a simple case of 16mm dowels resin bonded into the concrete and the blockwork and pour away. Has the engineer specified dowels or hoops or is it just JFDI..?