Focused on Engineered Fill, chapter 4.6 of the new guidance continues the advancement of build quality and standards in housebuilding and development, bringing the industry in line with a benchmark routinely met by McAuliffe in our brownfield services.
In this guide, McAuliffe’s Managing Director John McAuliffe takes a look at the new quality standards and what they mean for the industry.
Everything You Need To Know About The NHBC Chapter 4.6 – Engineered Fill
The National House Building Council (NHBC) has recently launched its 2024 edition of Technical Standards, which includes a brand new chapter of guidance that provides a quality benchmark that all new developments must meet for earthworks.
Specifically focusing on Engineered Fill, Chapter 4.6 of the new guidance has been launched to continue the advancement of build quality and standards in housebuilding and development.
They say you can't build on weak foundations, and as a leading expert in remediation and enabling works for brownfield development, it’s a philosophy we live by.
John McAuliffe, Managing Director of McAuliffe Group - which has been providing a complete brownfield service and supporting the country’s largest housebuilders, developers and contractors on new housing developments, major infrastructure projects and legacy clean-ups for more than 50 years - takes a look at the new standards and what it means for the industry.
Why have the new standards been introduced?
When new legislation or guidance is unveiled by organisations responsible for monitoring industry standards, there’s a tendency to think the worst, as concerns grow over ever-more hoops to jump through, additional costs, hassle and headaches.
But for those who have already been working to the highest possible standards of brownfield site preparation with Engineered Fill, the guidance set out by NHBC in its brand new Chapter 4.6 is really business as usual.
The chapter lays out the NHBC’s Technical Requirements for Engineered Fill along with recommendations for developments where buildings are founded in Engineered Fill, or where Engineered Fill is used to support external works and roads or pavements.
For some companies, the rigid standards set out in the document will mean the initial investigation, testing, design costs and timescales of site development are higher. But these moves are being made for the wider good of the industry and its reputation, as well as future occupants - while leaving companies far less susceptible to any future comebacks from ill-prepared land that would cost far more in the long run and not allow the NHBC to issue a compliant CML for each relevant property. It is helpful to consider the benefits of the thorough strategic planning outlined in the document alongside the alternate options and their costs to realise why they are so important.
On construction sites, cut and fill or earthworks are often required for a number of reasons, including the formation of development platforms or below-raft foundations on sites with shrinkable clays and trees in lieu of deep trench foundations. But before the NHBC’s new guidance announced in January 2024, there had been little or no official standards on achieving performance standards for Engineered Filling or suitable foundations bearing onto Engineered Fill for housing other than application of the Specification for highway works.
There’s a number of considerations when implementing Engineered Fill on residential projects and without detailed design and preparation, developers can find themselves with land that is unsuitable for the intended use.
Ahead of releasing this new guidance, NHBC revealed that there had been numerous occasions where companies had failed to adequately identify, address or mitigate geotechnical risks while site specifications frequently lacked sufficient detail or neglected residual geopolitical hazards.
This new chapter aims to provide guidance and consistency to the industry. And as a company that has built a formidable reputation founded on attention to detail in all aspects of the pre-acquisition, preparation, sale and development of brownfield sites this can only be a welcome thing.
Striving for a best practice approach, earlier contractor involvement and aligned strategic thinking.
Without any official national standards in place with regards to front-end remedial strategies, it is not surprising that documentation submitted to NHBC for land development ranged from the brilliant to the downright dreadful.
In some cases there has been little or no understanding of geotechnics, with risks either dismissed or not properly mitigated against, while similarly some back-end verification reports have been incomplete or over brief.
It is only right, then, that NHBC has taken the lead to ensure a standard best practice is agreed upon and adhered to. In short, companies must display a clear alignment and understanding of proposed sites, the required improvement measures and the efforts taking place in the intended building foundations. It is joining the dots from inception to concept with a clear strategy running through the work.
The new guidance gives a benchmark of what is expected with the aim of improving standards of earthworks in the housebuilding industry and therefore reducing the chances of future claims or disruption for the homeowner.
The key guidance to ensure industry best practice.
Failing to prepare is failing to prepare, as they say - and details found in the guidance are nothing new for companies like ours. They do, however, confirm the understanding of what we already know about site preparation and builds on this to provide a performance standard all companies must meet.
The guidance aims to ensure:
-Ground investigation is appropriate to the proposed development, site conditions and nature of the filling
-Geotechnical ground models are developed and appropriate for the proposed development and underlying ground and groundwater conditions
-Engineered Fill is placed to a suitable earthwork specification
-Appropriate geotechnical laboratory acceptability testing and in-situ compliance testing is undertaken
-Evidence is documented and verification provided
When does the new guidance come into force?
The new guidance was introduced in and becomes effective from 1st January 2024. However for complicated tenders or ongoing earthworks signed prior to this date , the NHBC has said that projects will initially be monitored on a site-by-site basis. All new earthworks from January 2025 should meet the new standards set out by NHBC.
If nothing else, the guidance highlights the importance of working with experts who understand the rules, including a suitably qualified engineer responsible for the design and specification of earthworks taking into account the future of the development and type of building foundations.
Earthworks are a critical component of residential development, and the NHBC is rightly requesting that design and specifications shall be produced in a clearly understandable format, including all relevant information, to be distributed to all relevant parties.
Under the new guidance, this detailed strategy documentation, including relevant drawings, is required eight weeks ahead of any filling or associated works so potential issues can be overcome well in advance of any machine touching the ground, while retrospective reviews of already completed works will no longer be considered acceptable. All Engineered Fill material classification should be clearly stated, while the use of recycled aggregate as fill should comply with BRE Digest 433 or other suitable guidance as agreed with NHBC.
An earthworks method statement, from the Earthworks Contractor or equivalent, should be produced and issued to all relevant personnel, which describes how the requirements of the specification are to be delivered, the plant to be used and arrangements for supervision and reporting.
In addition, the following supporting information should also be provided to NHBC:
-Desk study and ground investigation report
-Materials classification (acceptability) assessment
-Geotechnical design report or geotechnical design statement (depending on which is relevant and the complexity of the earthworks project)
With the level of detail now required by the NHBC ahead of works commencing, developers should be aware of what that means for them but more importantly the additional time and preparation required to assess land and make suitable submissions to allow developments to go ahead.
For example, the geotechnical design should be prepared by a suitably qualified and experienced engineer who should assess the ground and be satisfied that it is suitable for Engineered Fill.
Prior to earthworks commencing, the formation level should be inspected and tested in accordance with the earthworks specification, while unsuitable ground conditions need to be assessed, including presence of obstructions, compressible soils, unstable ground and groundwater.
Details are required of the acceptability of materials to be re-used as Engineered Fill. In particular this includes for clays with a plasticity index of less than 40% (less than 65% in SWH 600) and undrained shear strength in excess of 40kPa, materials requiring written NHBC acceptance, such as soils with a silt content greater than 20% and unsuitable materials, such as materials with high sulphate content, where Total Potential Sulphate (TPS) is in excess of 0.25% and also structureless chalk (with grades Dm and Dc).
Where the scope of the earthworks is defined as Geotechnical Category 1 (Geo Cat 1 - simple), a Geotechnical Design Statement (GDS) must be prepared and submitted to NHBC.
Most earthworks are classified as Geotechnical Category 2 (Geo Cat 2 - conventional) in accordance with BS EN 1997- 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 (complex), a Geotechnical Design Report (GDR) needs to be prepared and submitted to NHBC.
The Earthworks Specification should clearly describe the design requirements to be satisfied, such as detail of how the Engineered Fill is to be placed and what criteria will apply to prove its compliance.
Such a 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.
The three traditional types of standard Specifications for Earthworks are generally acceptable:
Method:- not acceptable to support buildings
End Product:- linked to Geotech properties, so suitable to support buildings
Performance:- must be used with End Product.
An expert knowledge is required to judge the type of specification required and should be based on industry guidance such as the Specification for Highway Works (SHW), BS EN 16907 ‘Earthworks’, ICE Publishing – Earthworks: A Guide ‘2nd Edition’, or on trials/previous experience, that is acceptable to NHBC.
It is essential that the quality of the earthworks and building foundations are aligned.
