4.1 Land quality – managing ground conditions
SCOPE
INTRODUCTION
Objectives |
Procedural summary |
Procedural flowchart |
DESIGN STANDARDS
DESIGN STANDARDS
INITIAL ASSESSMENT
DESK STUDY
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:
Investigate the soils, geology, surface water and ground water 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.
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.
- the vendor of the site
- previous in-house information
- ongoing monitoring.
INITIAL ASSESSMENT
WALKOVER STUDY
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:
What is the significance of any abrupt changes in slope? | ||
Are there any valley bottoms or depressions which may be soft or filled? | ||
Is there evidence of overburden on slopes? | ||
Is there excavation at the base of a slope? | ||
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? | ||
Are there signs of local subsidence? | ||
What is the basic type of ground? | ||
Is there any evidence of peat, silt or other highly compressible material at or below 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? | ||
Is a high water table indicated, e.g. by waterlogged ground? | ||
Are there any signs of flooding? | ||
Are there any reeds or water-loving plants? | ||
Are there any springs, ponds, wells, ditches or streams? | ||
Is there any discoloured water? What is its source? | ||
Is the vegetation sparse, dead or dying? | ||
What is the type and condition of vegetation on land adjoining the site? | ||
What are the species, height and condidion of the trees? | ||
What are the species, height, spread and condition of hedges and scrub on clay? | ||
Is there evidence of former trees, hedges or scrub on clay? | ||
Is there evidence of damage to structures, e.g. cracking in buildings, on or around the site? | ||
Is there other evidence of movement? | ||
Is there evidence of any structures or services below ground? | ||
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? | ||
Do local place names and street names give clues e.g. Brickfield Cottage, Water Lane? | ||
INITIAL ASSESSMENT
RESULTS
A suitable person, as described in Appendix 4.1-D, should record the results of the initial assessment and evaluate whether hazards are suspected.
- 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
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
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
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
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 present | Geotechnical hazards present (but no contamination hazards) | Contamination hazards present (but no geotechnical hazards) | Geotechnical and contamination hazards present | |
Initial assessment and basic investigation | ||||
Detailed investigation results | ||||
Proposals to manage geotechnical hazards | ||||
Proposals to manage contamination |
UNFORESEEN HAZARDS
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.