Substructure and ground-bearing floors

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5.1.9Materials used for fill

Materials used for fill shall be suitable for the intended use and, unless appropriate precautions are taken, free from hazardous materials. Issues to be taken into account include:

  1. sources of fill materials
  2. hazardous materials.

Fill should be:

  • well graded
  • inert and contain no hazardous materials
  • able to pass a 150mm x 150mm screen in all directions.

Fill containing either expansive materials or chemicals is not acceptable for the support of ground-bearing slabs.

The following types of fill should not be used unless written permission has been obtained from NHBC:

  • material obtained from demolition
  • furnace ashes and other products of combustion
  • colliery shale and any other residue from mineral extraction
  • slags
  • on wet sites, or sites with a high water table, crushed or broken bricks which have S1 designation according to BS EN 771.

Sources of fill material

Where the material is of a stable and uniform type, and from one source, it may only be necessary to check its suitability once. Where material is variable, or from a number of sources, it should all be suitable, and regular inspections and/or testing may be required.

Where industrial waste is permitted as fill material, it is essential that sufficient testing is carried out to ensure suitability.

Where material is obtained from stockpiles, check the material is uniform. Different forms of stockpiling can affect particle size/grading. The outside of a stockpile may be weathered and may not be the same as unweathered material.

Hazardous materials

The following fill materials require testing to ensure their suitability for use with ground-bearing slabs or as backfill to associated trenches:

  • reactive materials
  • organic materials
  • toxic materials
  • materials that include sulfates, e.g. gypsum
  • materials that cause noxious fumes, rot, undue settlement or damage to surrounding materials
  • acid wastes.