4.6.7 Design and specification of earthworks

Engineered fill should be placed to a suitable earthworks specification – the scope, details and requirements of the specification should be appropriate to the site, the nature of the fill, the techniques of placement and the end-use of the site.

4.6.7.1 Geotechnical design#

On engineered fill sites, an element of geotechnical design will be required by NHBC. The complexity of the design and its supporting earthworks specification should reflect the scope of works and the associated risk. Geotechnical design is covered in BS EN 1997, with earthworks specifically covered by the requirements set out in BS 6031 (Code of practice for earthworks) and BS EN 16907 (Earthworks).

It is recognised that on certain residential developments, the level of earthworks required can be minimal, with a low to negligible risk associated with the project. Where the scope of the earthworks is defined as Geotechnical Category 1 (Geo Cat 1), based on the Table below, a Geotechnical Design Statement (GDS) shall be prepared and submitted to NHBC.

Most earthworks are classified as Geotechnical Category 2 (Geo Cat 2) in accordance with BS EN 1997-1 and are required to be designed by an appropriately qualified and experienced person. Where the scope of the earthworks is defined as Geo Cat 2 or 3, a Geotechnical Design Report (GDR) shall be prepared and submitted to NHBC.

The content lists of a GDS or GDR may be prepared using guidance from DMRB CD 622 as a guide. Both the GDS and GDR should be prepared by a suitably qualified Engineer.

The following Table 4 – ‘Geotechnical design categories’ should be used to attribute a geotechnical category to projects, and the subsequent requirements for Geotechnical Design Statement (GDS) or Report (GDR).

Table 4: Geotechnical design categories

4.6.7.2 Earthworks Specification#

The earthworks specification should be prepared once the earthworks design has been undertaken and the properties of the proposed fill material have been established. It should clearly describe the design requirements to be satisfied, ie, detail how the engineered fill is to be placed and what criteria will apply to prove its compliance and include the information detailed below. An earthworks specification should be practicable and capable of both measurement and enforcement and should be capable of being monitored by an effective form of quality assurance procedure.

Typical Earthworks Specification should include the following:

4.6.7.3 Types of specification#

Three types of standard Specifications for Earthworks are acceptable to NHBC, and these are:

Method specification defines how compaction should be conducted in terms of the types of compaction plant, method of operation, number of passes of the plant and the final thickness of the compacted layer.

The End Product specification defines the degree of compaction necessary, and if appropriate stiffness requirement, for the given material by reference to criteria linked to either serviceability or ultimate limit states. The level of compaction required is normally expressed in terms of selected geotechnical properties eg percentage of maximum dry density or prescribed minimum stiffness and is supported by on-site earthworks testing. Where an end product specification is used, the requirements normally define overall targets to be achieved without detailing the methods to be used to achieve the targets.

Performance specifications require the works to be defined relative to long-term project requirements, which are commonly set at a relatively high level. For example, the criteria may be defined based on the long-term fitness for use of a building that is to be formed upon the completed earthwork. NHBC requires a performance specification to be used in combination with an end product specification.

NHBC specification requirements for use on residential development are summarised in the following Tables 5 & 6.

Table 5: Types of Specification for earthworks acceptable to NHBC

Table 6 provides the minimum level of compaction required throughout the placement of all engineered fill for both the support of building foundations and external and infrastructure works.

Table 6: End product acceptance criteria

It may be appropriate for a project technical specification to incorporate different forms of earthworks specification for different types of fill material, eg method compaction for general fills and end product compaction for coarse granular fills.

The earthworks technical specification shall include relevant conditions to be satisfied during construction to ensure the following are met:

Once agreed with NHBC, the earthworks specification should not be amended on site without prior agreement with the NHBC.

The earthworks specification should be based on industry guidance such as the Specification for Highway Works, BS EN 16907 ‘Earthworks’, ICE Publishing – Earthworks: A Guide ‘2nd Edition’, or on trials/previous experience, that is acceptable to NHBC.

The earthworks specification should outline if placing, testing and verification of earthworks fill is to be completed on the whole site, or be undertaken and presented in a phased approach to facilitate the build programme.

Table 7 shows the types of earthworks specification that are acceptable to NHBC for different types of engineered fill and subsequent end-use, considering the starting ground (formation) level has been assessed as suitably stable and appropriate for the proposed earthworks and end-use (as per clause 4.6.5).

Table 7: Matrix for determining appropriate types of earthworks specification

Figure 2 illustrates the guidelines for the use of differing earthwork specification for different end-use, located in close proximity.

Last updated: 2nd January 2024

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