Barn conversion ground floor slab

[G38]

Good morning All,

Here is one for you...

We are working on a barn conversion which is traditional in the sense it has solid external walls (9" I believe). The footings consist of 'stepped' masonry (standard for UK barns) which are actually exposed currently, probably due to the ground level being reduced during gardening duties of some sort. The roof is to be a lightweight cementious panel system sat on top of a timber raised collar. Due to it being in a flood risk zone the floor also has to be raised 1.0m from current ground level.

We are proposing to build a new internal raft foundation (possible edge thickening dowelled into external walls). From this we are proposing to provide a 100mm (4") cavity and then a new block work skin to support the structure, with the principle the external walls then do not need to be underpinned. Although theoretically we wouldn't have been adding any additional load to the external walls, we are trying to avoid the walls supporting any structure at all, whilst providing additional lateral restraint by tying into the new block wall. I've attached a quick sketch and would appreciate your thoughts on the below questions.

1. Do you see this detail as potentially undermining the already shallow footings?
2. Do you think differential settlement could occur between the external wall and new block wall causing some cracking in the future?
3. Do you think that if we provided an edge thickening it could cause damage to the external wall even if we provide some sort of compressible filler board?
4. Is there a lack of stone at the edge of the raft? Usually I would expect to see a continuation of the stone for edge load distribution.

Thanks a lot for your comments in advance.

 

[dik]

Should work... Can you put an 8" wall down to the level of the footing? Also with climate change, the weather in Europe and UK appears to be getting colder; are there any issues with frost heave?

Dik

 

[G38]

Hi,

We have considered this but it would effectively mean building 2 ground floors. So ground bearing slab at foundation level and then suspended to reach the required finished floor level. Frost heave is a problem in the UK and foundations generally need to be 450mm below ground level for new jobs (of course not taking into consideration the minimum founding depths for strip footings which can be anywhere from 750mm to 1000mm, could be deeper for trees / hedges). Externally we are going to build the level up 450mm and provide a french drain of sorts to minimise the impact on the existing footing (although its been exposed for a while I imagine).

Thanks for your reply.