3.1 Site preparation – harmful and dangerous substances
Land contamination is an unwanted legacy of Britain’s long industrial history. Part IIA of the Environmental Protection Act 1990 (inserted by Section 57 of the Environment Act 1995) was introduced to enable the identification and remediation of contaminated land from which contamination currently represents an unacceptable risk. Risks associated with the lands future use will continue to be dealt with under the planning and building standards system. Some functions of Part IIA, planning and building standards regimes may, at times, overlap.
Public registers - Part IIA adopts a ‘suitable for use approach’ that requires the current risks to be assessed and remediated as required, for a sites existing use. The primary regulatory role for this rests with the local authorities. Local authorities and SEPA must establish public registers to record all prescribed regulatory action taken under Part IIA. The register will contain particulars relating to the remediation, as well as notifications of the identification of contaminated land. The registers will expand as new information is identified.
Section 78A(2) of the Act (as amended) provides a specific definition of ‘contaminated land’ for the purpose of the Act. Land that is not ‘contaminated land’ as defined under the Act may still contain harmful or dangerous substances and the following guidance should be useful to the local authority in carrying out its other functions.
Pan 33 - Land confirmed, or suspected of being contaminated is a material consideration when local authorities determine planning applications. The key role of the planning system is to ensure that all the ground included within the planning application is suitable for the proposed future use. Conditions may be added to any permission given to ensure the required remediation takes place. Planning Advice Note (PAN) 33 ‘Development of Contaminated Land’ explains further the role of planning and includes useful cross-references to other relevant publications and regimes.
Harmful or dangerous substances include deposits of faecal or animal matter and any substance, or mixture of substances, which is, or could become, corrosive, explosive, flammable, radioactive or toxic or which produces, or could produce, any gas likely to have any such characteristic.
Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirements of this standard in so far as is reasonably practicable, and in no case be worse than before the conversion (regulation 12, schedule 6).
Surface soil and vegetable matter can be detrimental to a buildings structure if left undisturbed within the building footprint. Therefore, before any work can commence, unsuitable material including turf, vegetable matter, wood, roots and topsoil should be removed from the ground to be covered by the building, and the ground immediately adjoining the building, to a depth of at least that which will prevent later growth that could damage the building. The term ‘ground immediately adjoining’ is intended to cover ground that is disturbed as a direct result of the works.
The solum (prepared area within the containing walls of a building) should be treated to prevent vegetable growth and reduce the evaporation of moisture from the ground to the inner surface of any part of a building that it could damage.
The solum should be brought to an even surface and any upfilling should be of hard, inert material. See guidance to Standard 3.4 relating to building elements adjacent to the ground.
To prevent water collecting under the building, the solum should be not lower than the highest level of the adjoining ground. However where this may not be possible, such as on sloping ground, the solum should be laid to fall to an outlet in the underbuilding above the lowest level of the adjoining ground to prevent any water build up below the building. Any part of the underbuilding that is in contact with the ground, such as on sloping ground, should be tanked (see clause 3.4.7).
Where the site contains fill or made ground, consideration should be given to its compressibility and its collapse potential. Thought should be given to foundation design to prevent the damaging effect of differential settlement.
For the purposes of this Technical Handbook, clause 3.1.0 provides guidance on what harmful or dangerous substances may consist of. Because of their hazardous qualities, any ground below and immediately adjoining (see clause 3.1.1) a building should have them removed or made safe. Guidance on remedial action to deal with such substances is given in clause 3.1.5.
A preliminary desk-top study should be carried out to provide information on the past and present uses of the proposed building site and surrounding area that may give rise to contamination. Examples of land likely to contain contaminants can include, but are not limited to:
gas works, coal carbonisation plants and ancillary by-products works
industries making or using wood preservatives
landfill and other waste disposal sites
metal mines, smelters, foundries, steel works and metal finishing works
munitions production and testing sites
oil storage and distribution sites
paper and printing works
railway land, especially the larger sidings and depots
sewage works, sewage farms and sludge disposal sites
petrol filling stations.
During a walk-over of the area there may be signs of possible contaminants. The preliminary investigation can assist in the design of the exploratory and detailed ground investigation. A detailed ground investigation may be necessary and should provide sufficient information for the risk assessment and the design and specification of any remedial work.
Risk assessment should be specific to each building site and take into account the presence of source, pathways and receptors at a particular building site. Generic assessment criteria may provide an indication of where further consideration of risk to receptors is required. The selection of assessment criteria should take into account the specific circumstances of the building site and the receptors that may be exposed. Assessment criteria should be authoritative and scientifically based. Should a risk be indicated then further consideration would be warranted. This may involve collection and assessment of further information. Useful tools for undertaking detailed assessment of risk are available e.g. CONSIM and CLEA. Reference should be made to SEPA http://www.sepa.org.uk/and DEFRA http://www.defra.gov.uk/web sites which contain details of published and forthcoming guidance.
The Environment Agency manages an extensive research programme related to land contamination http://www.environment-agency.gov.uk/. This programme is ongoing and a check should be made to ensure that the most up-to-date guidance is used. The following are just some of the publications that may be of interest:
assessment of risks to human health from land contamination, an overview of the development of soil guideline values: CLR 7
priority contaminants report: CLR 8
contaminants in soils, collation of toxicological data and intake values for humans: CLR 9
contaminated land exposure assessment (CLEA) model, technical basis and algorithms: CLR 10
land contamination risk assessment tools: an evaluation of some of the commonly used methods: Technical Report P260
secondary model procedure for the development of appropriate soil sampling strategies for land contamination - R&D Technical Report P5
technical aspects of site investigation - R&D Technical report P5.
Where the desk study, records or local knowledge of previous use identifies, land that may contain, or give rise to, harmful or dangerous substances, planning permission will normally be subject to conditions. These conditions may be imposed to ensure that the development proposed for the land will not expose future users or occupiers, or any building or services, to hazards associated with the contaminants.
There may be occasions when land containing harmful or dangerous substances has not been identified at the planning stage, and the presence of contaminants is only suspected later. Some signs of the possible presence of contaminants are given in the table below together with the possible contaminant and the probable remedial action recommended.
Table 3.1. Possible contaminants and actions
|Signs of possible contaminants||Possible contaminant||Possible remedial action recommended|
|Vegetation (absence, poor or unnatural growth)||metals, metal components||none|
|organic compounds, gases||removal or treatment|
|Surface materials (unusual colours and contours may indicate)||metals, metal compounds||none|
|oil and tarry wastes||removal, filling, sealing or treatment|
|asbestos (loose)||removal, filling, sealing or treatment|
|organic compounds including pnenols||removal, filling or treatment|
|potentially combustible material including coal and coke dust||removal, inert filling or treatment|
|refuse and waste||removal or treatment|
|Fumes and odour (may indicate organic chemicals at very low concentrations)||flamable, explosive, toxic and asphyxiating gases including methane and carbon dioxide||removal or treatment; the construction is to be free from unventilated voids|
|corrosive liquids||removal, filling, sealing or treatment|
|faecal, animal and vegetable matter (biologically active)||removal, filling or treatment|
|Drums and containers (whether full or empty)||various||removal with all contaminated ground|
The verifier may require the removal or treatment of any of the contaminants in the table to clause 3.1.5, to be carried out by specialists.
If any signs of possible contaminants are present, the local authority should be told at once. If the presence of any of the contaminants listed in the table to clause 3.1.5 is confirmed, it is likely that some form of remedial action will be required. For guidance, the normal course of remedial action is listed against each contaminant. In all cases these courses of action assume that the ground to be covered by the building will have at least 100mm of in-situ concrete cover. Expert advice may be required to provide an economical and safe solution to the hazards encountered especially where contaminants are present in large amounts or where there is imminent danger to health or safety.
The Construction Industry Research and Information Association (CIRIA) produces many useful guidance documents on the application of different risk management techniques.
There are a range of options for managing the risk of contamination. This can include removal or treatment of the contaminant source or breaking the pathway by which contaminants can present a risk to receptors:
Removal means that the contaminant itself and any contaminated ground to be covered by the building should be taken out to a depth of 1m (or less if the verifier agrees) below the level of the lowest floor. The contaminant should then be taken away to a place to be named by the local authority
Filling means that the ground to be covered by the building should be covered to a depth of 1m (or less if the verifier agrees) with a material which will not react adversely with any contaminant remaining and may be used for making up levels. The type of filling and the design of the ground floor should be considered together
Inert filling means that the filling is wholly non-combustible and not easily changed by chemical reactions
Sealing means that a imperforate barrier is laid between the contaminant and the building and sealed at the joints, around the edges and at the service entries. Note that polyethylene may not be suitable if the contaminant is a liquid such as a tarry waste or organic solvent
Ground treatment may provide a more cost effective and environmentally sustainable solution. Treatment may be the only option where the presence of structures or services prevents excavation. Treatment processes can be biological, chemical or physical and be undertaken either in-situ (contaminants are treated in the ground) or ex-situ (contaminated material is excavated and then treated before being returned). The processes convert the contaminant into a neutral form or render it harmless. There are also solidification and stabilisation processes that can ‘fix’ contaminants in the soil so as to reduce the harm, and thermal processes that alter the contaminant by incineration or by volatilisation. The exact process to use will depend on the contaminant present and the soil type. Expert advice should be sought.
CIRIA and the EA websites http://www.ciria.org/ also contain useful data sheets on remedial treatment options.
The National House Building Council (NHBC) http://www.nhbc.co.uk/ together with the Environment Agency, has produced a guidance document ‘Guidance for the Safe Development of Housing on Land Affected by Contamination’. Although relating primarily to dwellings, the document provides useful guidance for all building types. It aims to promote the adoption of good practice in the identification, investigation, assessment and remedial treatment of land affected by contamination, so that the development of housing on such land can be undertaken safely and with confidence that no unacceptable risks remain.
With the increasing re-development of former industrial land, attention is also drawn to BS 10175: 2001, ‘Investigation of potentially contaminated sites, Code of Practice’. The British Standard provides guidance on, and recommendations for, the investigation of land that may be contaminated or land with naturally enhanced concentrations of potentially harmful materials, to determine or manage the ensuing risk. BS 5930: 1999, ‘Code of Practice for Site Investigations’ is also relevant. This CoP deals with the investigation of ground for the purpose of assessing their suitability for the construction of the work. It provides recommendations on certain constraints or problems that can affect a site, such as geotechnical aspects and the legal aspects including the need for licenses or permits.
Buildings, and the materials they are constructed from, are classed as receptors and therefore may be subject to damage if they come into contact with contaminated land. A principal concern is that any attack or damage from ground contaminants may affect the structural integrity or serviceability of the building and present a health and safety threat.
Both natural and human generated ground conditions can be aggressive to structures and services, in view of this the construction design should take account of any ground contaminants that could affect or damage buildings, materials and services. In practice it may be more difficult to assess and manage contamination risks when dealing with new work to existing buildings.
Site analysis and hazard identification are necessary to allow assessment of any contamination risks posed to the construction. The susceptibility of construction to aggressive contaminant attack generally depends on four conditions:
presence of water
contact between materials and contaminants, and
The BRE publication BR 255:1994 'Performance of Building Materials in Contaminated Land' provides detailed guidance on the vulnerability of building materials to the hazards arising from contamination.
The following table identifies some common aggressive substances, their possible effects on building materials and possible mitigation options.
Table 3.2. SUBSTANCE AFFECTING MATERIALS
|Materials||Substance||Possible effect||Mitigation options|
|concrete, mortar, masonry||sulphates (acid & water soluble in soil) pyrites heavy metals||expansion, disintegration deterioration|
|chloride (acid & water soluble in soil)||reduced strength increased permeability reinforcement corrosion|
|inorganic and organic acids (depending on type and concentration)||expansion degradation and corrosion of reinforcement|
|ammonium salts||increased porosity|
|structural steel, metal pipes and services||sulphate, sulphur and sulphide||corrosion|
|inorganic and organic acids||corrosion|
|plastics, rubbers||alkalis, ammonia, chlorine, hydrocarbons, oils||swelling deterioration|
|polythenes||benzene, toluene, phenol||permeation|
The potential for chemical attack often depends on the presence of water as a substance carrier.
Concrete, being an alkaline material is potentially vulnerable to attack from acids.