Also see:
4.24.1.1 Compliance
Assessment of the site and the surrounding area shall comply with the Technical Requirements. Items to be taken into account include:
- suitability of persons for the level of investigation
- geotechnical and contamination issues
- investigation procedures
- notification in writing to NHBC of hazardous ground conditions.
Ground investigations and management of risk that complies with the guidance in this chapter will generally be acceptable.
Suitable persons for the level of investigation#
The following skills and knowledge are required from the person responsible for the Initial Assessment, Basic Investigation and documentation and verification. They should:
- understand the hazards that can affect the development and where they originate
- recognise the signs of potential hazards
- conduct a desk study and walkover survey
- collect information relating to such hazards on and adjacent to the site
- report the findings in a clear and concise manner
- determine when specialist advice and detailed testing is required.
The following criteria should be used as guidance for the appointment of a consultant or specialist responsible for Detailed Investigation, management of hazards, documentation and verification:
| Experience | Similar types of site and development |
| Appropriate discipline(s) | Understanding of all relevant skills required on the project and access to other disciplines, including geologists, hydrogeologists, toxicologists and environmental chemists |
| Legislation | Understanding of legislation and liabilities associated with the site |
| Professional indemnity insurance | Appropriate cover for the work being carried out |
| Health and safety | Awareness of occupational hygiene issues and Health and Safety legislation |
| Quality assurance | Use of a quality management system, including appropriately accredited laboratories |
| Project management | Ability to manage a project team consisting of the appropriate disciplines |
| Site investigation | Ability to design site investigation programmes, including soil sampling, testing and laboratory analysis |
| Risk management | Ability to conduct risk assessments as required by the risk management process |
| Reporting and communication | Ability to prepare comprehensive and well presented reports. Effective communication within their organisation and with the client, statutory authorities and the general public |
| Engineering design | Understanding of effective risk reduction techniques, eg engineered foundations and substructure details of suitable remediation |
Geotechnical and contamination issues#
Assessment should be carried out by direct investigation and examination of the ground, supplemented by laboratory testing where necessary, in order to determine the geotechnical and contamination characteristics of the site.
Specifically, where contamination is suspected or found, the site should be assessed using the Source-Pathway-Receptor framework (known as the pollutant linkage).
For land contamination to occur, a source, pathway and receptor must all exist. A written or diagrammatic representation of the land contamination (known as a Conceptual Model), should be produced to show the possible relationships between each.
Procedure#
The process to assess and manage the ground conditions is as follows:
Initial Assessment
NHBC requires all sites to be assessed by a desk study and a walkover survey. The results should be used to determine whether or not hazards are known or suspected.
Basic Investigation
Required to support the results of the Initial Assessment where hazards are not suspected.
Detailed Investigation
Required where hazards are known or suspected.
Further Assessment
Required after the Basic or Detailed Investigation has been conducted, to confirm that all objectives have been met. Where results are inconclusive, further investigation will be required.
Hazards
Where hazards are identified, design precautions or remediation will be required to minimise their effects.
If any unforeseen hazards are found during the course of construction, further investigation is likely to be required.
Documentation and verification
NHBC requires documentation and verification to show that:
- the site has been properly assessed and investigated
- where necessary, suitable precautions are incorporated into the design
- all necessary remediation has been carried out.
Notification of potential hazards and associated risks#
If a site (defined in the Rules as an area of land that is covered by a single detailed planning consent or series of consents relating to continuous development) is classed as ‘hazardous’, NHBC must be notified in writing a minimum of eight weeks before work starts. Failure to provide such information may delay the registration process, the construction work and the issuing of NHBC warranty.
Table 1: Potential hazards and associated risks
| Potential hazard | Associated risk |
|---|---|
| High water table or low-lying land | • flooding
• the effects from toxic or noxious materials which could be concentrated or transported by ground water |
| Mining (past, present and proposed) | • ground movement as a result of the type of mining and materials extracted
• ground gasses, including methane and carbon dioxide |
| Trees and vegetation (eg Japanese knotweed or Bamboo – ‘running’ type) | • shrinkage and heave of clay soils
• physical damage caused by roots |
| Peat | • acid attack
• changes in volume due to variations in moisture content • production of methane and carbon dioxide |
| Infill and made ground, including tipping | • release of gases which may be explosive or asphyxiating
• low bearing capacity causing excessive total and/or differential settlements • consolidation characteristics which may result in subsidence, settlement and/or excessive tilt • localised ground variability (laterally and with depth) which may result in subsidence, settlement and/or excessive tilt • collapse compression or inundation settlement of non-cohesive fills which may result in subsidence, settlement and/or excessive tilt |
| Low bearing capacity ground | • settlement of foundations and substructures |
| Former buildings or structures | • underground obstructions producing variations in bearing capacity and settlement characteristics |
| Adjacent buildings | • effect on stability of both new and existing buildings |
| Drains, including land drains | • contamination, flooding, waterlogging and interruption of land drainage systems |
| Sulfates and sulfides in ground or ground water | • expansive reaction
• chemical attack on concrete, mortar and bricks or blocks made with cement |
| Contamination | • from substances which may be carcinogenic, toxic, asphyxiating, corrosive, phytotoxic, combustive, explosive or radioactive |
| Solution features in chalk, gypsum, and limestone, including swallow holes | • underground cavities |
| Unstable ground subject to landslip | • ground movement |
| Seas, lakes and rivers adjacent to land | • erosion
• exposure to saline |
Last updated: 9th January 2024