NHBC Issues New Chapter – 4.6 Engineered Fill

In January of this year the NHBC rolled out their newest chapter – 4.6 Engineered Fill.

Amongst a number of additional updates, this new chapter covers guidance on meeting their Technical Requirements for engineered fill along with recommendations for developments where buildings are founded in engineered fill, or where fill is used to support external works and roads or pavements etc.

Mark Russell, Standards & Policy Manager at NHBC commented, “Our standards team is committed to continuous improvement and revising the Standards annually to ensure they remain current and relevant. I am proud to be part of such a robust process that continues to support the ongoing advancement of build quality and helps builders and developers to keep raising standards in house building.”

The launch of this chapter and the dates for roll out are interesting as, whilst on the surface there appears to be quite an addition, and potentially a welcome one, in many cases it will be business as usual, particularly with regard to founding within engineered fill.

In our experience, our developer clients have long avoided this particular route which, as geotechnical and environmental consultants, can be inwardly frustrating when we see the value that can be gained through good earthworks design and construction. Too often we see trenchblock/reduce level digs/over designed rafts or piles being adopted in order to get through newly placed fill. The environmental and financial cost of this is significantly higher when compared to a traditional shallow foundation within the engineered fill.

reusing soil on site
materials management plan

Why is it avoided? Historically, we have found this to be a knock on effect of a lack of confidence from the warranty providers themselves, which is likely down to bad experience and increased risk of excessive settlement or heave resulting from poorly designed and controlled earthworks, possibly resulting through the absence of a set of technical standards to follow!

The thing is, a robust and well considered earthworks design, based on a full and detailed high quality ground investigation, constructed under the supervision of an experienced engineer can usually mitigate this type of concern. This approach is often demonstrated in the commercial non-warranted world as well as in the public sector with earth structures and highways.

You’ve probably driven on a road or motorway constructed on engineered fill in the past week, walked along a footpath on a constructed embankment with a retaining wall set in or travelled on a train that runs along an embankment constructed with engineered fill.

How useful will this chapter really be?

Whilst this new set of conditions may give rise to concerns over ‘another load of hoops to jump through’ or similar, it could, and arguably should, be welcomed and used to add value back into certain schemes where practicable. Although the initial investigation, testing and design costs, and timescales, may seem higher than some clients are used to, they should be considered alongside the alternate options and their costs to realise the benefit.

Founding in engineered fill can not only reduce the project costs with regards to foundation options, but can also reduce the requirement to dispose of material off-site. Through a well thought out cut and fill model, and use of the CL:AIRE DoWCOP Materials Management Plan (where suitable and applicable), the majority of soils can be kept on-site. This not only leads to cost savings, but importantly is a more sustainable option, ensuring valuable soil resources are re-used rather than disposed of, and limits that amount of truck movements to and from the site.

Omnia have been pushing this approach with engineered fill, where practical and of value, for a few years now and have demonstrable experience of successful schemes. Tying together the Ground Investigation, Testing, Geotechnical Design, Material Management Plan, Supervision, QA and sign off can be a challenge but those of us that have been practicing this for a number of years should have no issue in fitting neatly into this NHBC Condition 4.6 for Engineered Fill and assist our clients in delivering more sustainable value engineered projects.

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