New footing adjacent to existing (in London) - house extension

[lamh]

Hi!

I'm building a single-storey ground extension as shown in the attached mark-up.

Does anyone know if the proposed section is OK in terms of Building Regs? i.e; the trench foundation to 1m depth up against the existing foundation, presumed to be higher.

The excavation to the existing footing is only local and then the existing foundation will be loading the new trench footing, both of which I think are OK.

I would think this is a standard detail but can't find any guidance in Building Regs, for when your foundation is perpendicular like this, and not parallel.

Any insight appreciated.

Thanks

 

[oldestguy]

This detail likely does not have a standard procedure for an unbraced excavating next to the old for ANY purpose due to the question of support loss to the old. If there was such a standard, it is very likely that this job would not qualify.

 

[lamh]

Hi,

Thanks for the reply.

If you were to underpin an existing foundation you would excavate below it in small widths before pouring the underpin. So I would have thought that the temporary case here, excavating a 500mm width to the face of the existing foundation, would be similar to a hit and miss strategy of underpinning.

Thanks

 

[PEinc]

Your sketch does not show enough information. At what level is the lowest, new, floor slab subgrade compared to the existing building foundation? It looks to me like you need underpinning where the two new foundation walls abut the existing foundation wall. Also, underpinning may need to be installed between the two new foundation walls if the new addition's slab subgrade is below the existing bottom of foundation wall. Also, when underpinning for the two abutting walls, the underpinning will need to extend far enough beyond the edges of the new walls to prevent undermining of the existing wall when sloped excavations are made for the two new abutting walls.

Edit:
What you can or should do greatly depends on the elevation difference between the existing and proposed and on the dimensions of the proposed foundation. As I said, more info is needed.

http://www.PeirceEngineering.com

 

[LittleInch]

I suspect you will really need a T shaped foundation to underpin the existing at the connection point

My experience of foundations is that the decision exists in the head of the particular inspector you get.... So allow for more concrete than you show here.

Is the base soil under the new foundation clay or gravel?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.