Raft foundation .. depth of hardcore

[RonBowater]

We're digging the footings for my daughter's single storey, timber framed extension (3.5m x 7m) and the architect/structural engineer has specified a concrete raft foundation with steel reinforcement and a ring beam around the edge (presumably because strip foundations would need to be over 2 metres deep to get past clay to solid ground). Depth of concrete is specced at 175mm with 225mm hardcore beneath with a greater (unspecified) depth and width at the edges.

This 225mm seems to be a hell of a lot of hardcore and means a load of digging which we could do without (we can't get a mini-digger onto the site so it's all manual hard labour).

Questions:

1) Is the 225mm reasonable and/or necessary ? Can I safely reduce this maybe depending upon ground conditions after consultation with the BCO ?

2) Typically, what is the additional depth/width needed for the edge footings to accommodate the ring beam ?

Thanks
Ron

 

[Blagard]

Is the house on a Raft as well?

How much lower is the ground around your house from the inside ground floor level ?

It is possible they think that after stripping off the topsoil / vegitable matter you may need that amount of hardcore to get back up to the underside of the slab.

I'm not into Raft designs myself so no comment on the edge thickening details. I know that local BC officers here don't like them unless really necessary so have only seen them on poor ground that would otherwise need a 13 foot deep foundation and a local one that went wrong!

By all means ask the BC officer if after the site strip it suggests you don't need that much hardcore. If he says no, it's no more work than you anticipated!