Building safely on landfill

Contamination of land can be due to a variety of processes. It can occur as a legacy of industrial activity, from landfill operations and from agricultural use. It is also important to note natural sources of contamination, such as radon.

The government has announced that 43 000 additional dwellings per year between 1996 and 2016 will be built in the South-East. Among other things, this means 60% will be built on previously developed (brownfield) land, or through conversions of existing buildings₁.

Of course, brownfield land does not necessarily mean contaminated land₂, but in the light of this proposed building programme and the hazards posed by contaminants, there is a need to ensure that houses constructed on sites affected by contamination are built to the appropriate standards.

The Department of the Environment Transport and the Regions (DETR) has introduced new statutory provisions for contaminated land under Part IIA of the Environmental Protection Act 1990 (EPA). Specifically, Section 57 of the Environment Act 1995 inserts a new Part IIA into the EPA, and this provides a new regulatory framework for the identification and management of contaminated land. This regime is supported by statutory guidance which gives a specific definition of contaminated land.

Good practice for dealing with contaminated land requires a risk-based approach which is described in the Handbook of Model Procedures which has been produced by the Environment Agency and the DETR₃.

Contaminated land
The hazards posed by contaminated land are fairly well understood. They range, for example, from the toxic hazards posed by heavy metals to those of aggressive chemicals such as sulphates and organic solvents.

One of the key contaminants of concern though is landfill gas. This poses both health and safety risks to building occupants.

Landfill gas is produced through the breakdown by micro-organisms of organic material present in landfill sites such as vegetable matter, wood and paper. The principle components of landfill gas are methane, which is flammable, and carbon dioxide, which is a toxic asphyxiant.

The main concern with methane and carbon dioxide is that they can migrate off-site and enter buildings through gaps and cracks in the floor and foundations, potentially building-up to hazardous levels₄.

The development of sites affected by contamination (be it landfill gas or otherwise) is normally regulated through the planning and development control regimes. Where necessary, planning authorities can attach conditions or agreements on planning permission that require remedial measures to deal with the contamination.

When it comes to implementing remedial measures for new buildings on contaminated sites, then Approved Document C of the Building Regulations should be consulted₅. This is supported by a body of technical guidance from the DETR, the Environment Agency and CIRIA. With respect to landfill gas, Approved Document C refers to BRE Report 212₆ for specific guidance.

Technical guidance
BRE Report 212 gives guidance for when gas control measures for buildings are required on the basis of measured concentrations of methane and carbon dioxide in the ground. It describes the two key elements of gas protection which are a gas-resistant barrier across the floor and walls of the building, and a ventilation layer beneath the barrier to disperse gases safely to the atmosphere.

The Report contains a range of suitable constructions which are illustrated with a series of simple sketches. Figures 1 and 2 are typical examples.

When producing the Report it was decided to have only simple sketches (as opposed to detailed technical drawings) since the intention was to explain the principles of protecting structures from gas ingress.

Although the Report also gives specific practical guidance on gas protective measures, (such as installing a gas-resistant membrane and providing ventilation beneath floors) designers and builders have experienced difficulties in constructing them. It was therefore felt that a more practical guide was required.

The production of this guide, Protective measures for housing on gas contaminated land, was managed by the BRE with funding provided by the DETR and the Environment Agency. Drafts of the document were the subject of consultation with builders, product manufacturers, local authorities and consultants who had experience of landfill gas problems.

The main purpose of the guide is to give more detailed practical guidance on constructing commonly used floor types that can be used to exclude landfill gases from dwellings. As the focus is on housing, the guide is mainly concerned with passive protection, where air flow is driven by natural ventilation forces as opposed to mechanical ventilation systems. Each floor construction is described in the same manner.

Annotated drawings show each of the key elements of gas protection and are considerably more detailed than the sketches in BRE Report 212. However, they stop short of being construction plans since they do not, for example, show insulation or dimensions.

Each of the key elements of gas protection (such as membrane, air brick, gravel blanket etc) are numbered to show the order in which they should be constructed. The guide also discusses the general performance of the elements.

The guidance also contains a series of issues/problems that need to be addressed for the specific floor construction. Obviously, there is common ground between a number of constructions and so there is extensive cross-referencing. The guide concentrates on typical construction details, but the permutations for installing gas protective systems are numerous.

Finally, the guide gives a brief discussion of retro-fit measures that can be used on existing housing affected by landfill gas.

  Note that Protective measures for housing on gas contaminated land does not replace BRE Report 212. Although it provides more detailed practical guidance for construction and thereby supplements BRE Report 212, it does not state the conditions (such as gas concentrations and flows in the ground) for which the floor constructions can be used.

In this respect BRE Report 212 should still be consulted. Additional guidance on the relative performance of floor constructions in dispersing gas in the ground is also available₇.

That said, the guide does form a part of the documentation that will eventually replace BRE Report 212. The guide also complements the recently published report by the Environment Agency on the development of housing on land affected by contamination.₈