NHBC Standards 2006

4.5 Raft, pile, pier and beam foundations

4.5 - Appendix 4.5A
   Guidance for the design of semi-raft foundations on made ground

Appendix 4.5-A

Guidance for the design of semi-raft foundations on made ground

The following notes are to be used as a guide for Engineers designing raft foundations, but are by no means exhaustive. Special consideration will be required for certain sites.

  1. Raft foundations are to be designed by a Chartered Civil or Structural Engineer taking account of ground conditions and the results of the site appraisal and ground assessment.
  2. Sufficient internal beams are to be provided to adequately stiffen the slab.
  3. The area between downstand beams should not be greater than 35m2.
  4. The ratio of adjacent sides on plan should not exceed 2 : 1.
  5. The minimum depth of perimeter and party wall beams is to be 450mm. On larger dwellings some internal beams should be of the same depth as the perimeter beams.
  6. Perimeter and internal beams should be sufficiently wide at their base to carry their total loading at the allowable bearing pressure for the site.
  7. Beams are to be designed to span 3m simply supported and cantilever 1.5m.
  8. Beams are to use properly formed reinforcement in accordance with BS 8110.
  9. Where mesh is used in beams, it should be delivered to the site pre-bent.
  10. All beams should be cast on a minimum of 50mm concrete blinding.
  11. Minimum cover to reinforcement should be 40mm.
  12. Floor slabs should be a minimum 150mm thick, with nominal top face reinforcement as a minimum and anti-crack reinforcement in the bottom face, if appropriate.
  13. Stools or similar should be used to support floor slab mesh during casting.
  14. Corners and junctions to beams should be adequately tied using similar reinforcement to the beams.
  15. A minimum cavity drain of 225mm below dpc is to be maintained.