NHBC Standards 2010

4.1 Land quality – managing ground conditions


Hazardous sites

Builders are reminded that where a site* is hazardous, NHBC Rules state that, they must notify NHBC in writing at least 8 weeks before work begins.

Failure to provide NHBC with information about hazardous sites may result in a delay in processing the registration, hold up construction work on site and the issue of the 10 year cover.

* Site is defined in NHBC Rules as an area of land which is covered by a single detailed planning consent.

SCOPE

This Chapter gives guidance on meeting the Technical Requirements and recommendations for assessing the site with regard to managing the ground conditions.

INTRODUCTION

Objectives

This Chapter provides a framework for managing geotechnical and contamination issues with the objective of ensuring that:

  • all sites are properly assessed and investigated
  • foundations and substructure designs are suitable for the ground conditions
  • sites are properly remediated where necessary or appropriate design precautions are taken, and
  • appropriate documentation and validation can be provided to NHBC.

Assessment of geotechnical and contamination issues

Assessment should be carried out by direct investigation and examination of the ground, supplemented where necessary by results of laboratory testing on samples obtained.

Examples of potential hazards and associated risks relating to geotechnical and contamination issues are listed in Appendix 4.1-B.

Additionally, contaminated land should be assessed using the following framework:



Procedural summary

The processes to assess and manage the ground conditions are:

  • illustrated in the Procedural flowchart, and
  • described in detail in the pages that follow.

Useful references are contained in Appendix 4.1-A.

Initial assessment (Clauses D1 to D3)

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 (Clause D4)

Where hazards are not suspected a basic investigation will be required to support the results of the initial assessment.

Detailed investigation (Clause D5)

Where hazards are known or suspected a detailed investigation will be required.

Further investigation

Where the results of the basic investigation or detailed investigation are inconclusive, further site investigation will be required.

Where hazards are found (Clause D6)

Where hazards are identified, design precautions or remediation will be required to minimise their effects.

Documentation and validation (Clause D7)

NHBC will require documentation 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.

Unforeseen hazards (Clause D8)

If any unforeseen hazards are found during the course of construction, further investigation may be required.

Procedural flowchart


DESIGN STANDARDS

INITIAL ASSESSMENT

DESK STUDY

4.1 - D1
A desk study of the site and the surrounding area shall be undertaken by a suitable person
 

A desk study is the collection and examination of existing information obtained from a wide variety of sources.

It should indicate any potential hazards at an early stage and provide a basis for the investigation.

A suitable person, as described in Appendix 4.1-D, should carry out the desk study.

Items to be taken into account include:

(a)
soils, geology, surface water and ground water
 

Investigate the soils, geology, surface water and ground water of the site and surrounding area.

(b)
use of the site and surrounding area
 

Research the current use and history of the site and surrounding area to assess the potential problems including those which may have been left by:

  • industrial, commercial and agricultural uses including storage
  • mining
  • quarrying
  • landfilling and tipping.

Some sites may have been associated with more than one process.

(c)
sources of information
 

Refer to key sources of information including:

  • the Environment Agency or its equivalent, for example coastal erosion, landfill sites, details of water abstraction
  • the Local Authority, for example planning and environmental health
  • county records offices, libraries, museums, and local history sources
  • the utility companies
  • the Coal Authority
    • mining reports - past, present and proposed mining
  • the British Geological Survey
    • maps and information
  • soil survey maps
  • the Ordnance Survey
    • current and previous editions of plans and aerial photographs.

The above list is not exhaustive and local sources may be relevant.

(d)
existing site information
 
Review all available information from:
  • the vendor of the site
  • previous in-house information
  • ongoing monitoring.

INITIAL ASSESSMENT

WALKOVER STUDY

4.1 - D2
A walkover survey of the site and the surrounding area shall be undertaken by a suitable person
 

A walkover survey is a direct inspection of the site and the surrounding area carried out in conjunction with the desk study.

Look for indications of any potential hazards to provide a basis for the investigation.

A photographic record of the site can help in the reporting of the walkover survey.

A suitable person, as described in Appendix 4.1-D, should carry out the walkover survey.

Items to be taken into account include:
(a)topography

 
Abrupt changes in slope
What is the significance of any abrupt changes in slope?
Are there any valley bottoms or depressions which may be soft or filled?
Soft or filled depressions
Overburden on slopes
Is there evidence of overburden on slopes?
Is there excavation at the base of a slope?
Excavation at the base of a slope
Signs of landslip
Are there any signs of landslip, e.g. tilting trees, posts or walls?
Is there evidence of imported soil, tipped material or rubbish? Is it hot? Does it have an odour?
Imported soil, tipped material or rubbish
Local subsidence
Are there signs of local subsidence?



(b)
soils and rocks
 
Ground type
What is the basic type of ground?
Is there any evidence of peat, silt or other highly compressible material at or below the surface?
Peat, silt or other highly compressible material
Cracking or stickiness of the surface
Is there cracking or stickiness of the surface which may indicate a shrinkable sub-soil?
Are there sudden changes in conditions e.g. clay to chalk or soil to rock?
Sudden changes in conditions



(c)
surface water and ground water
 
waterlogged ground
Is a high water table indicated, e.g. by waterlogged ground?
Are there any signs of flooding?
Flooding
Reeds or water loving plants
Are there any reeds or water-loving plants?
Are there any springs, ponds, wells, ditches or streams?
Springs, ponds, wells, ditches or streams
Discoloured water
Is there any discoloured water? What is its source?



(d)
vegetation (which may indicate the nature of the soils)
 
Sparse, dead or dying vegetation
Is the vegetation sparse, dead or dying?
What is the type and condition of vegetation on land adjoining the site?
vegetation on land adjoining the site
Trees
What are the species, height and condition of the trees?
What are the species, height, spread and condition of hedges and scrub on clay?
Hedges and scrub
Former trees
Is there evidence of former trees, hedges or scrub on clay?



(e)
structural information
 
Cracking in buildings
Is there evidence of damage to structures, e.g. cracking in buildings, on or around the site?
Is there other evidence of movement?
Movement
Structures or services below ground
Is there evidence of any structures or services below ground?



(f)
local information
 
Local knowledge
Is there local knowledge of the site e.g. mining, refuse tipping, flooding?
Are there local industrial history records indicating past and present uses of the site?
Industrial records
Street name
Do local place names and street names give clues e.g. Brickfield Cottage, Water Lane?



INITIAL ASSESSMENT

RESULTS

4.1 - D3
The results of the desk study and walkover survey shall be recorded and evaluated by a suitable person
 

A suitable person, as described in Appendix 4.1-D, should record the results of the initial assessment and evaluate whether hazards are suspected.

The record should include the following as appropriate:
  • site plans with dates, showing:
    • previous uses of the site
    • current uses of the site
    • the proposed site layout
  • details of the geology of the site from:
    • geological maps
    • previous site investigations
    • laboratory test results
  • photographs of the site to show particular points of interest or concern, (e.g. areas of ground instability), with dates
  • copies and interpretation of aerial photographs, with dates
  • a list of sources of information consulted (e.g. Environment Agency, Coal Authority, etc.) and copies of the information obtained.

Sites where hazards are not suspected

BASIC INVESTIGATION

4.1 - D4
A basic investigation of the site shall be carried out and recorded by a suitable person to the satisfaction of NHBC
 

Where the results of the initial assessment indicate that hazards are not suspected on the site, this should be substantiated by carrying out a basic investigation.

This approach is to provide assurance for all sites, regardless of how free of hazards they may appear.

Only suitable persons with the skills and knowledge described in Appendix 4.1-D should carry out the basic investigation.

The following provides a specification for the basic investigation for all sites.

Trial pits should be located so as to be representative of the site. (For more detailed information refer to BS 5930.)

The number and depth of trial pits needed depends upon:

  • the proposed development
  • how inconsistent the soil and geology is across the site
  • the nature of the site.

The depth of the trial pits should not usually be less than 3m.

Items to be taken into account include:

(a) geotechnical investigation

(see Appendix 4.1-C)

A basic geotechnical investigation should be carried out. This will include trial pits and, where they do not provide sufficient information, boreholes will be necessary.

Physical tests, such as plasticity index tests, should be carried out as appropriate to support the results of the initial assessment.

Trial pits should be located outside the likely foundation area. The distance from the edge of the foundation should not be less than the trial pit depth.

(b) contamination investigation

(see Appendix 4.1-C)

A basic contamination investigation should be carried out as part of the basic geotechnical investigation. This should consist of sampling and testing of soil taken from trial pits during the geotechnical investigation, as found to be necessary from the outcome of the initial assessment.

During the excavation of the trial pits the use of sight and smell may help to identify certain contaminants.

Where there is any doubt about the condition of the ground a detailed investigation should be carried out (see 4.1 - D5).

FURTHER INVESTIGATION

If the basic investigation reveals the presence of geotechnical and/or contamination hazards further assessment is required and a detailed investigation should be carried out (see Clause D5).

Now refer to Clause D7, Documentation and Validation.

Sites where hazards are suspected

DETAILED INVESTIGATION

4.1 - D5
Where hazards are suspected a detailed investigation of the site shall be carried out, under the supervision of a consultant or specialist acceptable to NHBC, to determine and report on the nature and extent of all hazardous ground conditions
 

A detailed investigation should be carried out where:

  • hazards are suspected from the outset
  • the initial assessment identified hazards, or
  • the basic investigation identified hazards.

The basic (geotechnical and contamination) investigation should form the minimum requirement for any site investigation.

In addition to the basic investigation, the detailed investigation should:

  • adopt a structured and staged approach
  • gather information based on clearly defined stages of investigation
  • consider the immediate site and the adjacent area
  • take into account the possibility of future development in the vicinity of the site
  • consider the nature of the development
  • consider the complexity of the ground conditions
  • consider the presence of soil gas; if there is any possibility of gas being present, then a full gas investigation should be carried out, which should include flow measurements
  • provide a clear understanding of the problems, and an understanding of the liabilities, which have to be managed in order to develop the site
  • consider:
    • the surface water and groundwater conditions
    • the soils and geology, and
    • the previous site history.

A consultant or specialist acceptable to NHBC should be appointed to:

  • design and supervise the detailed investigation
  • present all the factual data obtained from the detailed investigation.

Guidance for the appointment of a consultant or specialist is given in Appendix 4.1-D.

MANAGING THE HAZARDS

4.1 - D6
Any hazardous ground conditions shall be satisfactorily managed under the supervision of a consultant or specialist acceptable to NHBC
 

As appropriate, the consultant or specialist acceptable to NHBC should:

  • identify any results which show that design precautions and/or remediation may be necessary
  • carry out a risk assessment to determine appropriate design precautions and/or remedial treatment
  • specify the options for remediating any contamination that may be present and provide a remediation statement
  • make recommendations as to appropriate design precautions including any ground improvement techniques as necessary
  • make recommendations on appropriate precautions for all underground services serving the site
  • ensure the works are appropriately supervised
  • produce a remediation report.

Items to be taken into account include:

DESIGN CONSIDERATIONS

(a) design precautions

Solutions for dealing with geotechnical hazards include the following:

  • specialist foundations:
    • piling and ground beams
    • rafts
  • ground improvement techniques:
    • vibro
    • dynamic compaction
    • surcharging.

(b) remediation techniques

Solutions for dealing with contamination hazards include the following:

  • risk avoidance - treatment to reduce the risk to the target by changing pathway or isolating the target by:
    • changing layout
    • building protective measures into construction
  • engineering based - treatment to remove or isolate the contaminants or modify the pathway by:
    • excavation
    • providing ground barriers
    • covering and capping
  • process based - treatment to remove, modify, stabilise or destroy the contaminants by:
    • physical means
    • biological means
    • chemical means
    • thermal means.

(c) site location

The identification of any constraints associated with the site and surrounding area which could restrict design precautions or remediation techniques should be identified and specified.

(d) timescale

Time constraints may influence the solution chosen since some techniques are very time consuming. This should not alter the requirement for effective remediation.

(e) consultation

In order to avoid abortive works it is important that the requirements of all statutory authorities are met by the proposed solution for the site.

REMEDIATION

(f) method statement

The method statement should detail the proposed remediation strategy for the site.

The statement should include the following details:

  • original risk assessment, identification of the remediation objectives and outline information of the method chosen
  • remediation objectives for ground, groundwater and soil gas
  • working method for implementation of the remediation
  • waste classification and methods for controlling and disposing of waste
  • proposed supervision and monitoring of remediation
  • all validation sampling and testing to be implemented.

(g) reports

The report should include the following information:

  • photographic records, especially for work which will be buried (e.g. membranes)
  • site diaries or drawings, environmental supervisor's site diary, and independent witness statements where appropriate
  • accurate surveys of the levels and position of all remediated areas
  • a description of any remedial materials used
  • details of soil movements and waste transfer notes
  • results of post-remediation sampling; laboratory certificates should be provided in appendices
  • validation test results
  • monitoring results
  • details of all consultations and meetings with statutory authorities.

Now refer to Clause D7, Documentation and Validation.

All sites

DOCUMENTATION AND VALIDATION

4.1 - D7
Documentation and validation shall be provided to the satisfaction of NHBC that the site is suitable for the proposed development
 

Items to be taken into account include:

(a) geotechnical assessment

WHERE GEOTECHNICAL HAZARDS ARE PRESENT

NHBC should be provided with design proposals to overcome the hazards.

(b) contamination assessment

WHERE CONTAMINATION HAZARDS ARE NOT PRESENT

Evidence to substantiate that the site is not suspected to be hazardous may be asked for.

WHERE CONTAMINATION HAZARDS ARE PRESENT

NHBC should be provided with design proposals to overcome the hazards.

Radon gas

Where the site is within an area susceptible to radon it will be necessary to follow appropriate guidance in Building Regulations and associated documents.

The following table indicates the documentation required by NHBC.

Documentation required by NHBC
 No geotechnical or contamination hazards presentGeotechnical hazards present (but no contamination hazards)Contamination hazards present (but no geotechnical hazards)Geotechnical and contamination hazards present
Initial assessment and basic investigationtick tick tick tick
Detailed investigation results tick ticktick
Proposals to manage geotechnical hazards tick  tick
Proposals to manage contamination  ticktick


UNFORESEEN HAZARDS

4.1 - D8
Where any additional or unforeseen ground conditions are found during construction, the builder shall ensure that they are investigated and managed to the satisfaction of NHBC
 

As construction proceeds, additional or unforeseen hazards may be found. For example, it is possible to have undetected hazards which are missed by the site investigation.

Where additional or unforeseen hazards are found additional specialist advice is required so that the hazard is properly investigated, managed and validated.