Raft foundations

[fattony]

Hi

I'm looking at building a 2nd story on top of my single story garage, I've been told 90% of the houses on my estate have raft foundations as the area used to be a quarry.

I assume there would be a minimum depth for the raft foundations originally, I doubt anyone would be able to tell me what this is as it would vary depending on the locations but would the local bco keep a record of this?

Is the process dig a trench to establish the depth of the existing raft foundations, employ a structual engineer to assess and calculate if it could support a 2nd story?

Thanks

 

[RonnyRaygun]

Is the process dig a trench to establish the depth of the existing raft foundations, employ a structual engineer to assess and calculate if it could support a 2nd story?

Thanks

Digging a trial hole next to the edge of the slab will only expose the probable edge thickening, which may be around 450mm deep, while the rest of the slab considerably less thick than this.

It's certainly possible to engage an SE to determine whether or not a second storey would be possible, but they would need to carry out a site investigation to determine the ground bearing capacity.

If the raft for the garage is similar to the raft for the house then it is likely that a second storey will be possible, as the loads will be no greater.