Not All Earthworks Are Created Equal

The National House Building Council (NHBC) Standards define the technical and performance requirements for designing and constructing new homes, and are the ‘go-to guide’ for many developers and construction professionals.

The standards are revised yearly and 2024 saw a critical update in the form of the new Chapter 4.6 which references engineered fill. While engineered fill is not unusual, it is the first time the NHBC has brought their requirements together and written them down.

Chapter 4.6 outlines critical requirements for sites involving filling, which include:

  1. Earthworks Specification.
  2. Geotechnical Design Report or Geotechnical Design Statement.
  3. Geotechnical testing to confirm earthworks parameters for any fill material.
  4. Third-party supervision during earthworks.
  5. Geotechnical verification testing during the process.
  6. A comprehensive verification report upon completion of earthworks.

The details of the earthworks specification, testing frequency, and supervision intensity will vary based on factors such as whether the fill serves as a bearing stratum, the nature of the fill material, and the extent of filling required.

The NHBC’s categorisation of suitable and unsuitable materials for earthworks is of particular significance. Unsuitable materials include clays with a Plasticity Index (PI) exceeding 40%, soils with an undrained shear strength less than 40kPa, material with a Total Potential Sulphate exceeding 0.25% and Class Dm and Dc chalks.

Some aspects will also need to be confirmed, such as if there is a minimum thickness of earthworks this applies to and if there is any wiggle room on the 2% upper limit on lime or cement when modifying the moisture content of fill materials.

The requirements will be all too familiar for individuals well-versed in earthworks practices. Yet, for those less familiar, GRM’s team of geotechnical engineers are ready to provide help and support in implementing the new guidelines. GRM’s inspection team is also available to conduct thorough inspections and verify adherence to these requirements.

If you have any development or construction projects, then please get in touch to find out how we can help save both time and costs. Please use your main point of contact at GRM or for new enquiries email or call 01283 551249.