Raft, pile, pier and beam foundations

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Raft, pile, pier and beam foundations shall be constructed in accordance with the design. Issues to be taken into account include:

  1. setting out and excavations
  2. localised effects and trench bottoms
  3. installation of piles, piers and ground beams
  4. load capacity verification of piles.

Setting out and excavations

The accuracy of setting out should be checked by control measurements of trenches, including their location relative to site boundaries and adjacent buildings. Levels should be checked against benchmarks, where appropriate. For excavations, check:

  • trench lengths
  • trench widths
  • length of diagonals between external corners.

In addition, for piled, pier and beam foundations, check:

  • spacing
  • alignment
  • positions in relation to the proposed superstructure

Walls should be located centrally on the foundation, unless specifically designed otherwise. Inaccuracy may prevent walls and piers being located centrally, resulting in eccentric loading and possible foundation failure.

Discrepancies to the design of the foundations or variations in the ground conditions should be reported formally to the engineer.

Variations in design or ground conditions should be recorded and distributed to NHBC and others concerned with sitework.

Foundation excavations should:

  • be kept free from water
  • not be excessive.

Localised effects and trench bottoms

Trench bottoms affected by rain water, ground water or drying should be rebottomed to form a sound surface.

Table 1: Localised effects

Differences in bearing capacity (e.g. from localised changes in strata)Consult the engineer.
Soft spotsExcavations should be deepened locally to a sound bottom, or the concrete should be reinforced.
Hard spotsShould be removed.
Visible roots, especially in clay soilsConsult the engineer and modify the design depth.

Installation of piles, piers and ground beams

Piles are to be installed by an appropriate specialist and under the supervision of an engineer.

Piles are to be vertical, unless designed otherwise.

Where piles are more than 75mm out of position, or out of alignment by more than 1:75, the engineer should reconsider the adequacy of the foundation design.

Where piles are misaligned by more than 150mm in any direction, or more than 5° from their specified rake, they should be replaced, unless otherwise recommended by the engineer. Alternatively, additional piles should be provided in accordance with the design modifications provided by the engineer.

Care should be taken to ensure that the bond of beams to piers and piles is in accordance with the design and is adequate.

Load capacity verification of piles

Test loading of piles should be undertaken when required. The builder is to obtain written confirmation that the piles are suitable for their design load.