Raft, pile, pier and beam foundations

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4.4.6Safe transmission of loads

Raft, pile, pier and beam foundations shall be designed to transmit loads from the structure to the ground safely, without excessive settlement.

Raft, pile, pier and beam foundations should safely transmit loads. The following issues should be taken into account:

  • adequate stiffness to ensure that differential movement does not adversely affect the supported structure
  • nature and bearing capacity of the fill material to be placed under the foundation
  • specification of concrete and cover to reinforcement.

Raft and semi-raft foundations:

Raft and semi-raft foundations should be designed:

  • to prevent the erosion of ground beneath the raft
  • (where required) to accommodate warm air ducts, service ducts or services without any adverse effect upon the performance of the foundation
  • to limit the risk of ducts becoming flooded
  • to support the building envelop without the risk of differential movement between leaves of cavity walls.

Fill for raft foundations should be in accordance with Chapter 5.1 ‘Substructure and ground-bearing floors’.

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.

  • 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
  • sufficient internal beams are to be provided to stiffen the slab adequately
  • the area between downstand beams should not be greater than 35m2
  • the ratio of adjacent sides on plan should not exceed 2:1
  • the minimum depth of perimeter and party wall beams is to be 450mm. On larger homes, some internal beams should be of the same depth as the perimeter beams
  • perimeter and internal beams should be sufficiently wide at their base to carry their total loading at the allowable bearing pressure for the site
  • beams are to be designed to span 3m simply supported and cantilever 1.5m
  • beams are to use properly formed reinforcement in accordance with BS EN 1992-1-1
  • where mesh is used in beams, it should be delivered to the site pre-bent
  • all beams should be cast on a minimum of 50mm concrete blinding
  • minimum cover to reinforcement should be 40mm
  • floor slabs should be a minimum 150mm thick, with nominal top face reinforcement as a minimum and anti-crack reinforcement in the bottom face, where appropriate
  • stools or similar should be used to support floor slab mesh during casting
  • corners and junctions to beams should be adequately tied using similar reinforcement to the beams
  • a minimum cavity drain of 225mm below the DPC is to be maintained.

Piled foundations:

The design of all piled foundations should specify precautions for cohesive soils where volume changes can occur.

The bearing capacity and integrity of piles should be confirmed by testing, when required.