Raft, pile, pier and beam foundations

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4.4.3Site conditions

Raft, pile, pier and beam foundations shall be designed to take account of site conditions. Issues to be taken into account include:

  1. the results of the site and ground appraisal
  2. dwelling design, layout and site levels
  3. trees and hedges
  4. frost susceptible soils
  5. potential for differential settlement.

Site and ground appraisal

All information relating to the site and its ground conditions which is necessary for full and proper foundation design should be obtained.

Building over changes in ground characteristics should be avoided.

Dwelling design, layout and site levels

Foundation design should take account of site layout, shape, size and construction of the dwelling.

Stepped foundations and suspended floors may be required for sloping sites.

Trees and hedges

Where the soil is shrinkable and nearby trees and hedges are existing, proposed or have been recently removed, foundations should be designed as shown in Chapter 4.2 ‘Building near trees’.

Frost susceptible soils

To avoid damage from frost action, the depth to the underside of the foundation in frost susceptible ground should be at least 450mm below finished ground level.

Differential settlement

Foundations should be designed to avoid any local stress points or any differential settlement.

Foundations for terraced homes, or those adjoining an existing building, may require special precautions to prevent damage from differential settlement. Foundations for attached bays, porches, garages, conservatories and other structures should be a continuation of those for the main home, unless the design indicates an alternative which takes account of differential movement.