NHBC’s Role in Developing Hazardous Sites

Article Contaminated Land Data Management Laboratories by

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.