Vibratory ground improvement techniques

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4.5.8Materials for use as fill

Stone fill for forming columns shall be compatible with the ground conditions, and be suitable for the vibratory ground improvement process.

Column fill should be a clean, hard, inert material. Limestone fill may not be acceptable in acidic ground conditions.

Suitable sources for fill material

All material used for fill should be suitable.

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

Where material is obtained from stockpiles, the uniformity should be checked. Different forms of stockpiling can affect particle size and grading. The outside of a stockpile may be weathered and may not be the same as unweathered material. The use of recycled aggregate as fill should comply with BRE Digest 433 or other suitable guidance as agreed with NHBC.

Hazardous materials

The following materials require testing to ensure their suitability for use as fill to support structural foundations and slabs, or as backfill to associated trenches:

  • Acid wastes.
  • Reactive materials.
  • Materials that include sulfates, e.g. gypsum.
  • Organic materials.
  • Toxic materials.
  • Materials that cause noxious fumes, rot, undue settlement or damage to surrounding materials.

Test requirements for fill material

Tests should be carried out by a suitably qualified person with a detailed knowledge of the:

  • material to be tested
  • proposed conditions of use.

The samples which are tested must be representative of the true nature of the material. It may be necessary to take a number of samples to find out the material characteristics of the fill.

Sulfate content should be expressed as a percentage SO4 by weight on the basis of acid soluble testing, taking full account of the recommendations of BRE Special Digest 1 Part 1.

Fill material requiring NHBC acceptance

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

  • Colliery shale and any other residue from mineral extraction.
  • Slags.
  • Furnace ashes and other products of combustion.
  • Material obtained from demolition.
  • On wet sites, or sites with a high water table, crushed or broken bricks which have no limit on their soluble salt content (as defined in BS EN 771).

Expansive fill materials

Fill containing expansive material is not acceptable for use as support to structural foundations and slabs, or as backfill to associated trenches.