Article Contaminated Land

NHBC Ground Gas Update – Site Assessment, Characterisation and Design of Gas Protection Measures

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NHBC have published an update to their 2007 technical guidance for low-rise residential developments on sites affected by ground gases. The original document, entitled ‘Guidance on evaluation of development proposals on sites where methane and carbon dioxide are present’, included a simple multi stage classification method for low-rise housing, commonly referred to as the “Traffic Light system”.  The fundamental guidance offered in this document remains applicable but, since its publication, there have been a number of advances in knowledge, including guidance on alternative approaches for characterising gas regimes and updated advice on the design of measures to deal with gas risks.  These include the Characterisation of gas risks without gas monitoring on low-risk sites (CL:AIRE, 2012), Verification of gas protection systems (CIRIA C735, 2014) and the British Standard Code of practice for the design of protective measures for methane and carbon dioxide ground gases for new buildings BS8485:2015.

The full article is available at:

NHBC summarise the new guidance and comment on where they consider it to be applicable.  They also explain where their Traffic Light classification can continue to be used, for a “typical house”, as detailed in the figure below.


Model residential property developed for calculated maximum permitted gas concentration within the subfloor void.


They advise that:

  • There have been a number of recent UK publications offering alternative approaches to ground gas risk assessment and improved advice for the design and verification of measures to deal with gas risks.
  • Practitioners undertaking gas surveys and assessing the risks should be conversant with updated guidance.
  • Robust site characterisation is required to design gas protection measures, but designers must also have an understanding of building-related influences, as these significantly govern design and construction options for gas protection measures.
  • Gas protection design, installation approach and verification requirements should be agreed with NHBC in advance of works, as satisfying requirements after construction is extremely difficult, often more costly, and can be disruptive.
  • Specific requirements relating to gas protection measures may be applied under planning and must also be considered.
  • The NHBC Traffic Light guidance can be used where the development proposals are based on the ‘typical house’ used for modelling in the traffic light classification system.
  • Verification evidence will be requested for gas regimes at Amber 2. For developments where the Characteristic Situation is applicable, the BS8485 scoring system requirements should be adopted, and verification evidence could be required for gas regimes at CS2 or above.



NHBC Technical Extra, Issue 20, April 2016.

Contaminated Land: Applications in Real Environments (CL:AIRE) Research Bulletin RB17 – A pragmatic approach to ground gas risk assessment. 2012.

CIRIA C735. Good Practice and verification of protection systems for buildings against hazardous ground gases. 2014.

BSI. Code of practice for the design of protective measures for methane and carbon dioxide ground gases for new buildings, BS8485:2015.


by Neil Parry

Report Contaminated Land

Contaminated Land Working Group Report

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Neil Parry, Chairman, CLG writes:

Our last meeting was held on 10th January 2015, 17 members attended. Below is a summary of ongoing activities.

Environment Agency (EA)

Further to the meeting with Bob MacIntyre (EA Hazardous Waste) a further (or extended) Working Group meeting is proposed once the Waste classification and assessment Technical Guidance WM3 has been published. Agreement from the Main Committee was required (Ann-Marie to forward additional room costs).


A separate working group has been set up to look into sampling protocols for contaminated soil and waste. We have been in contact with Murray Lark from the BGS and he has agreed to contribute to the sub group.


Karen Thornton reported on the statistics for NHBC and research projects on low energy, SUDS, soil stabilisation. A new Basement Construction chapter 5.4. A full review of standards is due to take place up to April this year.

Land Forum

Chaired by Seamus Lefroy-Brooks. Work being carried out on QMLC Scheme for competence.


SAGTA held a meeting at the beginning of February. It was agreed that Karen Thornton would feed back information from the meeting to the CLWG.


A panel discussion on asbestos will be held on Members’ day.


Further to the AGS Position Statement on the UKWIR guidance (guidance for the selection of water pipes in contaminated land) further monitoring of the general water company requirements will be carried out.


Roger Clarke reported that there has been a drive for more assessors as the numbers in SiLC increase.


Code of practice for the characterization and remediation from ground gas in affected developments. Some progress on the new draft reported by CLWG members involved.

Article Contaminated Land Data Management Laboratories

NHBC’s Role in Developing Hazardous Sites

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NHBC (National House-Building Council) is the standard setting body and the leading warranty and insurance provider for new and newly converted homes in the UK. Our role is to work with the house-building and wider construction industry to provide warranty, risk management and compliance services that raise the standards of new homes, and to provide consumer protection to new homebuyers.

Approximately 80% of new homes built in the UK each year are registered with NHBC and benefit from our 10-year warranty and insurance policy called Buildmark. Around 1.6 million homes are currently covered by Buildmark policies and over the past 40 years, NHBC has protected more than 30% of all existing homes in the UK.

In 1999 Buildmark was extended to provide the homeowner with protection cover against the issue of a statutory notice. This was done in the anticipation of Part 2A, which came into force a year later.

The NHBC Foundation

The NHBC Foundation was launched in 2006 in partnership with the BRE Trust. Its remit is to provide the necessary data and intelligence to develop long-term solutions to industry challenges which lie ahead and lead debate and thinking among industry experts. The NHBC Foundation facilitates research and development, and shares pragmatic and relevant guidance and good practice to the homebuilding industry.

Though much of the NHBC Foundation’s research is focused on the challenges of the Government’s 2016 zero carbon homes target, published works do include ground related issues such as ground source heat pumps, the risks associated with basement construction and the efficient design of piling for housing.

NHBC Standards

The 2011 NHBC Standards, effective from 1 January 2011, introduced for the first time a new chapter for low or zero carbon technologies (Chapter 3.1). It also included an update to Chapter 4.1 – Land quality on managing ground conditions, and a major update to Chapter 4.6 – Vibratory and ground improvement techniques, as well as reference to the introduction of Eurocodes in place of British Standards.

The latest update to the NHBC Standards continues our corporate mission to work with the house-building and wider construction industry to provide guidance, inspection and technical services to raise the standard of new build UK homes to protection homeowners. The identification of geotechnical risk assessment and the implementation of robust site investigations and geotechnical and remediation designs are therefore essential to NHBC, our developer customers and ultimately the homeowner.

Chapter 4.1 Land quality – managing ground conditions

Chapter 4.1 was first published in 1998 and, since that time, few changes have been made. The Chapter has now been updated to reflect recent technical changes and developments, made to reflect the changes to British Standards and the development of European Standards. It now includes technical guidance produced since the Chapter was last revised and better aligns the process for assessing contaminated land with the Government’s guidance document CLR 11 (Contaminated Land Report 11): Model Procedures for the Management of Land Contamination (2004).

Chapter – CH4.6 Vibratory ground improvement techniques

The update to Chapter 4.6 reflects changes and innovations in ground improvement techniques. It outlines current industry practice, provides additional guidance on the suitability of ground to be treated, clarifies the objective of vibro treatment, and updates the range of suitable stone fill for vibro column materials by permitting the use of suitable recycled aggregates. It now also references Eurocode EC7 (BS EN 1997 – Geotechnical Design).

The new Standards reflect the EU wide transition to Eurocodes for the design of structural elements following the withdrawal of the existing British Standards in March 2010. It is proposed that the Building Regulations in England and Wales will be revised in 2013, with the structural Eurocodes becoming the standard reference document for demonstrating compliance. In the interim, the Public Contract Regulations 2006 require Eurocodes to be used for the design and construction of publically funded building projects.

For geotechnically challenging sites, such as those where vibro improvement, piling or engineered fill is required, the management of geotechnical risk is likely to be enhanced by adherence to EC7. Additionally, in the UK, the British Standard for Earthworks (BS6031:2009) has also been extensively revised and is now compatible with the Eurocodes. These documents set out the requirements for assessing the geotechnical suitability of the ground for development and the execution of stabilisation works and foundations.

Some of the changes include:

  • References to the 15 kPa absolute limit on soft clay strength has been dropped
  • The 30 kPa limit on soft clays is maintained as not being generally acceptable unless the suitability of the treatment can be demonstrated, taking due account of the impact of group effects, ground heave and settlement
  • Requirement to consider inundation settlement risk issues of poorly compacted fill
  • Requirement to consider surcharging settlement effects
  • References to chalk or clay fills have been omitted and replaced with the generic ‘loose or un-engineered fills’
  • Requirement to consider effects on ground gas and contamination
  • Recycled aggregates can be used subject to compliance with BRE Digest 433 or other suitable guidance, such as WRAP
  • Validation testing is required of treated ground to confirm that the proposed load-settlement performance has been achieved
  • Requirement to produce validation reports confirming that the proposed load settlement performance of treated ground has been achieved
  • Clarification that plate load tests on stone columns alone are not acceptable to NHBC for treatment validation


Land Quality Endorsement (LQE)

For housing developments on major Brownfield sites requiring significant geotechnical and contamination remediation, NHBC has increasingly noted that many of the sites developed for housing in the UK are remediated by specialist remediation companies, landowners, private developers, regeneration specialists, development agencies and similar companies.

These organisations are responsible for or own contaminated land and are remediating them for residential development. However, they are not themselves NHBC registered builders or developers, and are therefore outside NHBC risk management processes and may not be aware of NHBC’s requirements.

NHBC introduced Land Quality Endorsement (LQE) in 2005 as a consultancy service providing technical risk management for sites being remediated befoere residential development. LQE allows the assessment of contaminated and brownfield sites against the requirements of the NHBC Standards.

This determines the suitability of these sites for Buildmark cover in advance of the formal registration of residential properties. Sites are assessed against the requirements of NHBC Standards Chapter 4.1, including a review of geotechnical and foundation proposals alongside contamination assessments.

The pre-registration assessment of sites affected by contamination and the remediation adopted will potentially enhance the marketability of a site by reducing the potential risks to the builder or developer, whilst saving time and effort.

Article Contaminated Land Laboratories Safety

What Is Finalling?

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In future the NHBC will adopt a consistent approach to determining what will and what will not prevent the necessary confirmation that a warranty is in place. In order to facilitate this, any outstanding information, defective design or non-compliance with standards on site must be classified as either RED (prevents warranty) or GREEN (will not prevent warranty) by applying the following sequential logic test:

Will the outstanding issue result in:-

1. A risk to health and safety?
2. A claim against the warranty?
3. Significant disruption to the occupier (in order to rectify the issue)?

If the answer to any of the above questions is yes then the item will be classified as RED and confirmation that a full warranty will be in place will be withheld until the relevant issue is resolved. If the answer to the three standard questions is no then the item will be classified GREEN and confirmation would be provided. It remains the builder’s responsibility to address any outstanding Green issues.

In relation to design issues, inadequate or unsatisfactory information is generally the reason preventing a warranty being in place. Hazards where information will normally be requested by NHBC Engineering include:-

High water table, Made Ground, Mining or other Cavities, Multiple Hazard, Peat, Soft Ground, Steep Slopes (more than 1 in 10), Sulphates, Landfill gas and Peat, Contaminated Land other than Landfill Gas.

Geotechnical site investigation report, gas test results and proposals for gas membranes received from a builder for a site with known made ground, past shallow mining and within 250m of a landfill site.

Mining report and foundation proposals not received, therefore fails the logic test and hence would be classified as RED until such information is received and approved.