NHBC’s commitment to investing in high-quality research, products, and Standards has always been key to supporting the industry in maintaining and improving quality. With around 70,000 plots registered for Buildmark Cover annually on land previously considered Brownfield, NHBC’s commitment to ensuring safe and high-quality developments on these sites cannot be overstated.

In 1999, NHBC became the first UK warranty provider to offer extended warranty provisions for contaminated land. It introduced a new Chapter 4.1 named “Managing Ground Conditions,” which outlines the acceptance requirements for warranty and subsequently published Research & Development  Publication 66 to guide safe housing development affected by contamination.

Although a slow start for those unfamiliar with it, the house-building industry got on board, and with scientific advancements, improved government policy, supporting guidance, and help from specialists like AGS members, the understanding of Contaminated Land Assessment has greatly improved. As a result, contamination claims are now rare.

Recently, the quality and reduced input of geotechnical expertise within submissions has been a concerning trend noticed by the NHBC Land Quality Service team, especially for schemes involving earthworks to prepare sites for residential development. Although NHBC sees exceptional work, we are increasingly seeing evidence, reporting, or designs that are limited or show a lack of understanding of geotechnics.

It is unclear why this is happening; it could be due to an early emphasis on addressing environmental concerns. These are routinely and rigorously enforced through planning but, as a consequence, can often distract applicants or the necessary investment away from the geotechnical needs until the planning consents are secured and certainty of development investment can be realised.

Geotechnics within the house-building industry may be considered less innovative or exciting than other sectors, leading to fewer specialists working in this field. However, as the easier-to-win schemes become less common, this couldn’t be further from the truth. Building more homes on marginal or brownfield development is undeniably becoming more frequent. Indeed, NHBC regularly encounters submissions for warranty on challenging and complex sites. These vary from Greenfields that require flood alleviation measures and significant land raising to existing infilled ground such as former quarries or historic landfills, old collieries, and areas with past industrial land use.

Our registered builders and developers must comply with the technical requirements set out in the NHBC Standards. The NHBC Standards define the technical requirements and performance standards detail how these can be met for the design and construction of homes covered by Buildmark. NHBC regularly reviews and updates the Standards.​

NHBC introduced a new Standards Chapter, 4.6, this year to deal with the increasing number of sites using engineered fills for low-rise residential structures, external works, and infrastructure or below raft foundations on sites with shrinkable clays and trees instead of deep trench fill foundations.

Although Chapter 4.6 may not provide any new information for experienced geotechnical professionals, it does introduce a new performance standard for house building. It gives guidance essential for meeting the technical requirements to achieve NHBC acceptance. It also re-emphasises the significance of geotechnics and the importance of obtaining quality data to enable a suitable design for and to address foundation quality after earthworks.

From January 1st, 2024, any new earthworks tender required to support building foundations where an NHBC warranty will be requested must comply with the performance requirements of Chapter 4.6. From January 2025, all new foundations on engineered fill will be expected to comply fully with the requirements and guidance of the new chapter.

NHBC Standards 2024 became effective on January 1, 2024. Chapter 4.6 includes 13 main performance clauses and spans across 28 pages. At the beginning of the chapter, a flowchart is provided to help practitioners navigate the document.

The clauses cover various requirements relating to competence, geotechnical investigation, ground models, appropriate laboratory testing, compliance testing, earthworks specifications, verification, and reporting expectations. Moreover, this chapter also includes significant requirements and considerations regarding acceptable foundations and design. The main requirements are that:

• Engineered fills do not settle excessively or have the potential to cause excessive differential settlement between properties founded upon the fill and external areas.
• Engineered fill and the underlying ground supporting building foundations shall limit the total building settlements to less than 25mm and minimise angular distortion or tilt to 1:400.
• The design and detailing of foundations, infrastructure, and external works suit the placed fill and underlying ground conditions, considering the overall ground model and any geohazards beneath or nearby.

The introduction of Table 8 is an exciting addition in the new chapter. It provides information on foundations that may be suitable based on three different scenarios with varying standards of fill.

 

But a word of caution: Practitioners should only use this table as guidance. They are reminded not to rely upon it to justify a less robust foundation solution if geotechnical risks remain outside the fill and to remember that all foundations must be designed on a site-by-site basis, considering all relevant geotechnical risks.

If buildings are on piled foundations, it is essential not to overlook considerations regarding any differential settlement between the houses and external areas in designing any services.

Practitioners must remember what is covered and what is not covered in Ch4.6. For instance, assessing historic fills is not included in Ch4.6 and would require additional guidance from other sources. Long-term monitoring or loading trials may sometimes be necessary to determine performance.

If you are unsure whether your site is suitable for residential development, NHBC provides a Land Quality Service. This service involves close collaboration with stakeholders, enabling early engagement and supporting those seeking approval of solutions. We ensure that the risks associated with land quality and foundation solutions are appropriate for the entire 60-year design life of the properties.

NHBC Standards 2024 can be easily accessed online for free.

Article provided by Karen Thornton – NHBC Land Quality Service Manager FGS, MCIWEM, MCABE

 

Introduction

The recent years have seen sustainability and climate change move to the forefront of the construction industry, with these topics being considered by many engineers, contractors and consultants in their working practices.

With a view to ensuring that climate change requirements are reflected in contracts and with a view to drive construction professionals to focus on sustainability and climate change, the NEC introduced a new secondary Option clause for climate change.  Option X29 (climate change) has been drafted as a Secondary Option that parties using the NEC4 suite of contracts can choose to include in their contracts.  Published in August of 2022, Option X29 represents a very positive step in helping the construction industry help tackle climate change.

Since its publication there has been a steady increase in its adoption, with a further increase in the use of this clause expected as practices focus on setting their climate change goals and missions.  However, its use does not come without some additional risks and liabilities that parties intending to use this option clause should be aware of.

In this article, we discuss the use of Secondary Option X29, its key features and the risks that may arise when Option X29 is selected to apply in an NEC4 contract.

Key Features

Option X29 introduces several key elements that will help the construction industry address climate change in their contracts.  Discussed below are some of the key features that will need to be considered carefully when navigating Option X29 if selected to apply on a project.

Climate Change Requirements

The Scope must specify the Climate Change Requirements that must be complied with. This means that any contractor/consultant will be required to comply with the Climate Change Requirements in order to Provide the Service in accordance with the Scope (as is required under the NEC4 PSC, for example).  Therefore, a failure to comply with the requirements will be a Defect if it is considered to relate to the services/works and consequently will require rectification at the contractor’s/consultant’s own cost.

It is therefore essential that careful consideration is given to the contents of the Climate Change Requirements to ensure that they are both achievable and do not place undue risk on the parties.  For example, the Climate Change Requirements will need to be carefully reviewed for obligations that may cause an insurance concern, such as a fitness for purpose obligation.

Climate Change Plan

The Climate Change Plan is to be devised by the contractor/consultant and is required to set out the contractor/consultant’s strategy for achieving the Climate Change Requirements (e.g., setting out stakeholders, roles, timescales. key milestones, tools and tasks to get there). The intention behind the drafting is for the plan to be updated as and when instructed and required. The form of the Climate Change Plan will be for the parties to decide and may be set out in the Climate Change Requirements.

It will again require very careful consideration to ensure that the Climate Change Plan proposed on a project is both appropriate and achievable.

An open dialogue with clients on the Climate Change Requirements and Climate Change Plan on projects will be important (and should be encouraged) to ensure the documents and requirements contained therein are palatable and workable for all involved.

Performance Targets 

The drafting of Option X29 includes the provision of a table for setting out any performance requirements and the targets that must be met. This provides a regime for measuring compliance with specified performance targets, for which the client may set out financial incentives.

It is for the parties to set the targets in the Performance Table, which can be in any form that they wish, whether this be through implementing key performance indicators (“KPIs”) or through providing specific values or requirements that are to be met.  The targets may also go beyond the Climate Change Requirements.  Given the flexibility of the table provided for in the drafting of Option X29, it is key that any targets entered are objectively measurable, with a clear methodology.

If the targets in the Performance Table are not met, the contractor/consultant will be required to pay the amounts stated in the Performance Table to the client, whilst achieving or exceeding the targets will entitle the contractor/consultant to a payment in accordance with the Performance Table.

Whilst largely intended to be a positive incentive mechanism to encourage parties to meet the targets, it will be important that the Performance Table is carefully reviewed to ensure that ideally no negative incentives/damages/penalties are included.  Any negative incentives/damages/penalties are akin to liquidated damages, which are typically excluded from the scope of cover under a consultant’s typical insurance policy.

If the Performance Table includes any KPIs, these should be reflected in the contractor’s/consultant’s commercial model (e.g., pricing etc) for the project. It will be important that any KPIs are understood and reviewed to ensure they are acceptable (and are achievable – especially where any negative incentive/damages/penalties are proposed).

Contractor’s/Consultant’s Proposals 

One positive feature of Option X29 is that the contractor/consultant can propose changes to the Scope if they believe that they can reduce the environmental impact of the lifecycle of the asset.

The process requires the proposal to be ratified by the client before any change can occur. Whilst the client does not have to accept the proposal, it is hoped that this feature will be positively accepted by clients to reduce environmental impacts on projects.

Limit of liability

The standard NEC4 liability provisions introduce a concept of ‘excluded matters’, for which liability is unlimited.  Any amounts payable by the contractor/consultant in accordance with the Performance Table will be an excluded matter and therefore excluded from the cap on liability (if secondary Option X18 is used). This means that liability for payments of any amounts in the Performance Table is unlimited.

The potential of exposure to significant and/or unlimited liability under this clause emphasises the need for careful consideration to be given to the Performance Table, particularly if any negative incentives/damages/penalties are included.

Conclusion

The intention of this clause is a positive step in helping the industry tackle net zero and other related climate change and biodiversity goals, and its inception by NEC is one that should be lauded.

However, it is not a “complete solution”, and parties will need to consider carefully how it should be used and whether any amendments are necessary to balance commercial needs and reflect project-specific standards.  The use of this clause and the implications therefore require a great deal of thought.

Option X29 has been drafted in a deliberately flexible manner that enables the parties to decide upon the approach that they want to take on a project-by-project basis. This means that project specific amendments may be made to the standard positions discussed within this article. Therefore, the drafting (and implications) of Option X29 will need to carefully considered on a project-by-project basis.

However, there are a few key points to highlight in relation to Option X29 before it is used:

• As mentioned, the clause is not a complete solution that can just be inserted into a contract. It requires a great deal of thought and discussion between the parties as to how it will be used and the targets that will be set. These targets need to be both attainable and measurable for the clause to be used effectively.

 

• The definitions of Climate Change Requirements and the Performance Table do not include a reference to any of the emerging standards or definitions that we are seeing developed, such as the UK or World Green Building Council’s definition of ‘net zero’.

Whilst the drafting promotes flexibility, it does enable the client to specify the determination of each definition. Contractors and consultants should assess the definitions and metrics for each and every contract that it enters.

• Consultants and contractors will need to carefully consider the requirements of the clause in line with their professional indemnity insurance policies. As mentioned in this article, there is a risk that some of the documents discussed may contain provisions that are inconsistent with insurance cover.

It goes without saying that the potential benefits that Option X29 (and “green drafting” more generally) can bring may make the effort worthwhile, but it is important that the risks and liabilities associated with and being imposed on the Contractor/Consultant are understood at the outset. Option X29 and “green drafting” more generally, therefore need to be considered carefully as it could lead to additional risks and liabilities being imposed on the contractor/consultant in the guise of tackling climate change.

If you would like any further information or would like to discuss this note or the use of Option X29, please contact the authors.

Article provided by Kathryn Eva (Associate at Beale & Co) and Harry Coates (Solicitor at Beale & Co)

Since 2007 when the first SuDS manual was published, AGS members have increasingly been asked to provide services to support design of sustainable urban drainage systems. The author of this article has been working with the BRE365 test since the 1990s and is still supervising, designing and reporting infiltration rate results to designers from trial pit and borehole methods.

AGS has recently been investigating how our industry manages undertaking and reporting infiltration rates for design of devices for discharge of surface water into land including; soakaways, basins, swales and permeable paving. Amongst members, the almost exclusive method requested by clients is to follow the BRE DG365 procedure as discussed in the AGS article (ref AGS magazine Oct 2021).

BRE DG365 is an empirical methodology for the design of soakaways, which includes the test procedure to provide observational data on infiltration rates. It was not intended for either basins or permeable paving. Because of the sparse nature of the test instructions, how it is carried out and reported can vary between practitioners. Concerns were first raised in the AGS safety working group at the same time as general trial pitting methods came under renewed scrutiny. The geotechnical working group was also requested to look at data quality in context, which resulted in the previous article. It is recognised that AGS members’ risk assessments and method statements on trial pitting may have been extended to cover BRE365 by written procedures to keep inexperienced staff safe in conducting these in-situ tests. Innovations and modifications may also have been made to the BRE365 method, to standardise in-house practice and to avoid taking up excessive resources during what can be a logistical challenge of labour, plant and materials.

This article is intended to build on the previous article, briefly cover the test origins and discuss what alternatives might be possible.

The development of technology of infiltration testing can be traced through publications. The following list shows an incomplete chronology.

1973	BRE151 Soakaways – (also NHBC 5.3.11)	Soakaways BRE Digest 151, Watford. Building Research Establishment (Digest 151 is presented currently as a method in NHBC Standards https://nhbc-standards.co.uk/5-substructure-ground-floors-drainage-and-basements/5-3-drainage-below-ground/5-3-11-surface-water-soakaways/ )
1991	SR 271 The Hydraulic Design of Soakaways Report	The Hydraulic Design of Soakaways Report SR271 1991 by DC Watkins published by HR Wallingford is available as a free resource here https://eprints.hrwallingford.com/311/ and describes the modelled water flow processes in a soakaway.
1991 (latest revision 2016)	BRE365 Soakaway Design	Empirical method of soakaway design including test for infiltration rate.
1996	R156 Infiltration drainage – Manual of good practice	CIRIA report Report 156 1996 Infiltration drainage – Manual of good practice Roger Bettess BSc PhD MCIWEM. Provides an infiltration test procedure similar to BRE365 but with some variations and extra information.
2007 (Version 6 including 2016, 2018, 2019)	C753 The SuDS Manual	The current reference for Suds Approval Boards/Lead Local Flood Authorities (SAB/LLFA) in the UK is the The SuDS Manual (C753) https://www.ciria.org/ItemDetail?iProductCode=C753&Category=BOOK&WebsiteKey=3f18c87a-d62b-4eca-8ef4-9b09309c1c91.

BRE 151 & NHBC 5.3.11 involves recording the time for a fixed height of water to drain in a small trial hole. The time t is graphically translated to a soakaway size using correlations which have uncertain/unknown justification.

SR271 indicates that flow in unsaturated ground is different to saturated ground in four main respects (head in the fluid is due to suction, storage coefficient and conductivity are variables, and gravity induces vertical flow), and compares in-situ test results with those obtained from numerical modelling using Richards modified equation. By relying on testing at the proposed location of the device and use of bulk factors of safety the design philosophy is confirmed to be experimental. Uncertainties do not include the possibility of significant variations in soil type at the location of the test (and by implication the soakaway).  This may be a point of contention with some practitioners used to UK shallow geology.

The work in R156 was carried out by HR Wallingford under contract to CIRIA in the period October 1991 to March 1995. R156 cites many other sources of information (BRE were part of the steering group represented by Dr John Powell). The extra information in R156 does help to confirm the experimental nature of the test specific to the location of the proposed infiltration device and the intrinsic adoption of bulk safety factors. This requires that the location of the device (test site) must be chosen by a designer.

C753 probably deserves an AGS article on its own. In Chapter 01 – The philosophy of SuDs, the 3-D conceptual model in Figure 1.2 models the underlying geology as a grey monoblock rather than layers of strata for example which is a shame. In nearly 1000 pages of C753 the word geology occurs 16 times, frequently associated with land stability; ecology is listed 42 times and archaeology once. Possibly geology is not that important to the delivery of SuDs, but is regarded as a constraint. AGS members are probably most interested in SuDs Section 25 Infiltration design methods, and specifically 25.3 Infiltration Testing Methods and Appendix B.4 Infiltration Assessment. Appendix B.4 includes a checklist (Table B6) intended for use by the approving bodies (and the designers) which assumes competence in ground assessments, which may not be available in all design teams. There are plenty of apparently excellent SuDS case studies, but they don’t generally include details of the geology or infiltration tests and how infiltration challenges were overcome. Competent ground assessors would surely benefit designers.

Discussion

The SUDS manual is probably the most important document for members and designers to understand. Unfortunately, members may only be requested to perform the BRE365 test and nothing else.

In my experience, over-winter (wosrt-case) water level monitoring has been more frequently requested by designers indicating a more favourable approach to ground modelling.

It seems the current SuDS manual was written with Eurocodes in mind, but apparently not integrated possibly due to the embedded modelling from SR271 between test, device design and bulk safety factors. Mention of 14688 & 14689 in the B6 checklist are assumed to enable initial permeability assessments to be made by designers from descriptions correlated to published permeability values. In such cases, ground practitioners might note subtle fines content descriptors in coarse soils which could lead to significantly different engineering properties in tests. Designers and regulators might just see either SAND or CLAY and get a wrong outcome.

It is tempting to assume that permeability and infiltration approximate to the same value but that may not be accurate. Scale & geology variations mean that measurements of permeability in small laboratory samples cannot be representative unless part of a fully developed ground characterisation. In any event permeability cannot be used in exactly the same way as infiltration rate due to the modelling/empirical link between test and the device design. It is the device (soakaway/basin) that has a characteristic design value of infiltration (usually the tested soil worst-case), and potentially too conservative if water will find preferential pathways and bulk safety factors are deployed. Whilst the SuDS manual includes falling head tests in ISO 22282-2:2012 as acceptable in principle, there are conditions and limitations. There may be parallels here with the recent AGS presentation on sample disturbance where taking a few high quality and well-selected data sets might be considered “better” than many more data of variable/unknown control. We might assume that falling head test data should be presented as a part of a full Eurocode characterisation of the ground to be compliant. Simply replacing isolated infiltration data from a trial pit with more falling head tests in a borehole is likely inadequate, although potentially safer for operatives and might use less resources.

Use of boreholes as a device for obtaining infiltration data is a natural ambition for AGS members seeking compliance with standards and health and safety. However, if the depth of infiltration is limited, for example, by the Environment Agency through planning to less than 2m depth (as been the experience of the author) then the preference for using trial pits is understandable. A daisy-chain or in series hierarchy approach from initial water harvesting to bulk basin attenuation with limited shallow infiltration finally to deep soakaways for overflow situations might be good design but is not necessarily supported by case study.

There is also Geotechnical investigation and testing – Geohydraulic testing – Part 5: Infiltrometer tests (ISO 22282-5:2012), which exclusively describes the various types of ring infiltrometer test; single or double ring, open and closed. In these tests, flow through the side is not included, therefore not compatible with the SR271 modelling. However, the ring infiltrometer would logically be more appropriate for plane devices such as permeable pavements if the modelling is different. Again in my experience, It seems shallow depth BRE365s is preferred by designers for permeable paving.

Conclusion

From this discussion, there are no obvious “off the shelf” replacement alternatives to the BRE365 test to recommend to members.  Noting the fundamental link in the digest between the design output and the in-situ test. However, practitioners might note that although BRE365 was most recently updated in 2016, Bettes 1996 (R156) is the primary reference in the SUDs manual.

The alternatives to BRE365 should be ISO 22282-2 or ISO 22282-5 neither is wholly recommendable (or likely to be accepted nationwide) as a direct replacement.

To support a sustainable agenda wider use of understanding ground models at the initial stage is recommended and not to rely on limited study & a small data set of BRE365 tests. A site where there is a 3m thick layer of clay over 10m of unsaturated permeable sands; three 2m deep tests would indicate infiltration is not possible resulting in the design of an attenuation basin occupying potentially unnecessary space and wasting other resources. A conceptual model, initial investigation and targeted deeper BRE365 tests may result in a much more sustainable scheme.

BRE365 and the SuDS manual could be updated to include the modern context such as health and safety including the gravel filling of test pits, responsible use of resources/logistics and adoption of dataloggers. Guidance could be provided on the benefits of a good geological characterisation and what benefits could be gained from having representative/characteristic high quality data rather than the adoption of worst-case values because of limited data. More case studies might be expanded to include problem solving where poor draining geology has been overcome by engineering for example using linear features which can intercept preferential pathways in variable soil/rock. Or combined systems featuring infiltration devices of limited/known capacity linked to bulk attenuation and final overspill into deep borehole soakaways.

The AGS is keen to hear examples of good practice for safely undertaking the BRE365 methodology and examples where SuDs design has been informed by alternatives to the conventional BRE365 method.  Please contact the AGS at ags@ags.org.uk with examples or comments.

Article provided by James Harrison of 4D Geo Limited