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4.6.6 Materials for use as engineered fill
Engineered fill materials shall be suitable for the site conditions, proposed end-use and techniques of placement and compaction. Issues to be taken into account include:
- suitable sources of engineered fill materials
- treatment of fill materials.
The earthworks specification should indicate the materials to be used for filling including their geotechnical classification and range of properties that should be subject to monitoring and testing. Material classification should be clearly stated eg Class 1C Coarse Granular General fill or a Class 6N Well Graded Selected Granular fill in accordance with Specification for Highway Works.
Materials that do not fully comply with the earthworks specification must not be used as engineered fill.
4.6.6.1 Suitable sources for engineered fill materials#
Engineered fill material may be site-won or imported from appropriate suppliers or donor sites. Where differing engineered fill types are placed in subsequent layers or distinct zones, it is important to ensure compatibility – for example, to ensure that fines are not washed out of one material into a more coarsely graded material, geotextile separators may be required.
Where material is obtained from stockpiles, the homogeneity should be confirmed. 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 the unweathered material within. The use of recycled aggregate as fill should comply with BRE Digest 433 or other suitable guidance as agreed with NHBC.
Details of considerations regarding the acceptability of materials for re-use as engineered fill are provided in Table 3. Tests undertaken to confirm acceptability should be carried out by a UKAS and/or MCERT Accredited laboratory (or other specialist laboratories) in accordance with the earthworks specification, appropriate standards and industry guidance, with a detailed knowledge of the materials to be tested and proposed end use. The samples which are tested must be representative of the true nature of the material.
Table 3: Acceptability of materials for re-use as engineered fill
4.6.6.2 Treatment of fill materials#
The following techniques may be used to modify the moisture content of materials to render them suitable for use as engineered fill:
- placement in windrows or layers with mechanical turning as required for drying purposes
- the addition of lime and/or cement, typically up to 2% by weight to reduce the moisture content to nearer the optimum moisture content (OMC) for maximum dry density
- the addition of water by spraying or similar to increase the moisture content to nearer the OMC for maximum dry density.
The following techniques are not acceptable to NHBC for the treatment of materials for use as engineered fill:
- stabilisation to improve strength by the addition of over 2% lime or the addition of any cement or other hydraulic materials
- the addition of chemical binders or grouts.
Last updated: 2nd January 2024