Also see:
4.6.7 Design and specification of earthworks
A suitably qualified engineer shall be responsible for the design and specification of earthworks, taking into account future development and type of building foundations. Engineered fill shall be placed in accordance with a suitable earthworks specification. Issues to be taken into account include:
- geotechnical design
- earthworks specification
- types of specification.
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 Geotechnical design, 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 Table 4, 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 outline from Managing Geotechnical Risks DMRB CD 622, as a guide. Both the GDS and GDR should be prepared by a suitably qualified engineer.
Table 4 should be used to attribute a geotechnical category to projects, and the subsequent requirements for GDS or GDR.
Table 4: Geotechnical design categories
Assessment of the site and proposals |
---|
Geohazards – legacy of mining/quarrying, un-engineered fill, low strength/bearing capacity, slope stability, etc |
Preparatory works – removal of remnant foundations, obstructions, settlement monitoring, mining treatment, etc |
Requirement for ground improvement – preloading, surcharging, dynamic compaction, etc |
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.
A typical earthworks specification should include the following:
- Description of scope and aims of the earthworks including reference and brief summaries of supporting documents:
a. Desk study and ground investigation report.
b. Geotechnical Design Report (GDR) or Geotechnical Design Statement (GDS)
c. Materials classification (acceptability) assessment - The intended end-use of the engineered fill should be defined
- Details of any site preparation works required prior to earthworks filling and the treatment of exposed surfaces to prevent deterioration
- Types of materials permitted for use together with material properties, including arrangements to prevent the deterioration of cohesive materials
- Classification testing may include particle size distribution, plasticity (Atterberg Limits), compaction tests or moisture content tests should be undertaken to confirm that the fill materials remain within the earthworks specification
- Types and frequency of classification tests should be site and material specific and confirmed in the specification.
Acceptable test methods and frequencies of classification tests given in Tables 9 and 10 of BS 6031: 2009 - Requirements for the placement, spreading and compaction of earthwork materials, including compliance criteria
- Requirements for the disposal of unsuitable material
- Testing proposals for the verification of compliance criteria and performance requirements to be met for end product and performance specifications, including load-settlement testing
- Arrangements for supervision
- Proposed as-built records to reflect the completed works
- Construction drawings showing the proposed extent of the filling works (including thickness)
- All other site-specific requirements and relevant construction drawings showing retained features, constraints, etc
- The form and proposed content of an earthworks validation report
4.6.7.3 Types of specification
Three types of standard Specifications for Earthworks are acceptable to NHBC:
- method
- end product
- performance.
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 Tables 5 and 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
End use | Acceptance criteria |
---|---|
Building foundations | >95% MDD* and <5% air voids (4.5kg or vibrating rammer) >98% MDD* and <5% air voids (2.5kg rammer) |
External/infrastructure | >95% MDD and <5% air voids where fill >5m thick (2.5kg/4.5kg rammer) |
*MDD = Maximum Dry Density
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 specification shall include relevant conditions to be satisfied during construction to ensure the following are met:
- materials are chemically suitable for the project and the surrounding environment
- materials should be durable, and not prone to deterioration or non-biodegradable
- earthworks should provide a stable finished surface that will limit post-construction settlement or movement within the engineered fill
- the earthworks should provide a surface of sufficient stiffness and/or shear strength for the intended use.
Once agreed with NHBC, the earthworks specification should not be amended on site without prior agreement with 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 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
Note(s)
1. Acceptable minimum depth of engineered fill beneath building foundations = 600mm.
Figure 2 illustrates the guidelines for the use of differing earthwork specification for different end-use, located in close proximity.
Note(s)
1. Specifications A and B to be to Table 3
2. Specifications C may be method or end product or no defined compaction regime
Last updated: 2nd January 2025