Gas protection
Gas protection is a technical solution to prevent or to control gas penetration into properties. The control of gas migration is normally achieved by blocking the pathway or removing the source of the gas generation. There are several methods available to achieve the protection of existing / new build properties and also another factor is the use of the property and size (economic factors).
The main components that developer have to be aware of in the U.K are methane (which is flammable at 5% to 15% by volume in air) and carbon dioxide (which is toxic), after two gas explosions in the U.K. in the 1980s Loscoe and Abbeystead. Building Research Establishment, British Standards, the Department of Environment and other regulatory bodies including ones in construction industry developed and published guidance for preventing gas ingress into building of these two gases in the UK. Their production in the environment are associated with coal seams, deposited river silt, sewage, landfill, and peat.
In the case of landfill gas migration, the gas is produced by the organic materials in the waste degrading over time. Creating varying concentration over time (CO2 typically 40% by volume and methane (CH
4) typically 60% by volume), as previously explained this gas can be heaver than air or lighter depending on the concentrations of these gases and air, but will move from an area of high pressure to one at a lower pressure irrespective of its relative density.
To try to prevent landfill gas migration from the site the operator should employ such methods as leachate drains with venting, low permeability geomembranes, slurry trenches, and grout curtains around the landfill site.
New buildings on contaminated land
The development of buildings on contaminated land will require a barrier created either vertically in the instance of homes with basements or horizontally under the base of the house. This can be by means of generally a passive barrier but can sometimes be an active system.
Passive system
This system will utilise a barrier with low permeability either via a synthetic membrane or a natural/engineered clay material. The effectiveness of these barriers are greatly affected by the installation methods and the integrity of the barrier being used. It must installed so it conforms to building regulations and quality assurance standards with guidance from BRE414.
The gas membrane can be put over the over a suspended beam and block floor and covered with a screed the area below being vented by airbricks or can be laid over a venting layer (200mm of granular material or 25mm of voidformer /a geocomposite)
Any penetration requiring passage thought the barrier will need to be sealed. This system will prevent entry of gases into the property and will be combined with an adequate venting system to allow dispersal of the gases to prevent any buildup around or under the property.
Note in the photo two types of heat gun that weld the material together or else material can be connected by using double sided butyl tape. The layer also extends over the exterior brick work.
Passive systems should have an integrity test to ensure the safety of the work. A test gas mixed with air is released under the membrane via a special tool. The entire area is then checked by operators using special air sensors to see if there are any leaks. If the work passes then it should be issued with a certificate of integrity.
Active systems
Due to the cost implications and the requirement for management and control this type of protection is mainly used on commercial properties.
There are two main practical types of Active systems to prevent the ingress of gases into buildings:
- Positive pressurisation: this system uses a small low power fan or pump (PPU - Positive pressurisation unit) to maintain a positive pressure under the building to act as a barrier to gases that are rising through the soil. This also will not draw gas towards it by the venturi effect.
- Forced ventilation under the building: again a small fan or pump is used to force air through a void under the building. This method can be run constantly or set to be activated by a gas sensor when the concentration reaches a certain level. The latter being the main application where this system has been used.
There should be alarms for both systems in the event of power failure to the systems as this will lead to an eventual buildup of gases to a dangerous level. These systems should be assessed by a third party to ensure the quality and effectiveness of the system.
Building near landfill sites
The issue with developing housing around such site is compounded by such issues as the stack effect, that is warm indoor air being less dense than the exterior cold air and so creating an area of lower pressure inside a property, which will encourage the ingress of gas. This gas can concentrate in any subfloor void (i.e. basements or gaps created below the floor slab due to settlement) or even in cupboards. It can also enter the building in any gaps left in the construction of the property or created during settlement.
"Waste management Paper 27" from the U.K. Department of the Environment recommends consultation with the relevant local authority if any proposed development is to be less than 250m from the site boundary that has received waste in the last 30 years.
The 250 m is not a definitive insomuch as landfill gas will not be present outside or conversely present within this distance, due to the varied factors contributing to landfill gas migration. Due to this fact there should always be a thorough site investigation if a development is taking place within the 250 m zone or if there is a suspicion of any hazardous gas being present.
Integrity testing
Both systems should have/require 'Gas integrity testing'.
The NHBC traffic light system (a method of gauging require protection need from results form the site investigation - the higher the risk the more points required in terms of protection to allow domestic construction) will offer a score of 2 points for a foil backed gas membrane installed with integrity testing where an untested gas membrane might result in 0.5 points.
This is both due to the conditions under which gas membranes are installed are often difficult and can adversely compromise the integrity required by the manufacturer/client. The purpose of the test to ensure this integrity to allow the installation to be certified as the method of protection performs correctly.
Gas membrane testing
The test is carried out on the membrane immediately after its installation and prior to it being covered up by the following construction processes. The area below the membrane is temporarily pressurised with a mixture of clean air and a specialist nontoxic and inert tracer gas that is extreme sensitivity to detection. Specialist equipment is then used to trace all leaks within the installation with particular attention being paid to the critical points and junctions formed between the membrane material and other structural elements prior to conducting a detailed sweep of the complete area (see construction principles). Any leaks are identified, sealed and re-tested before the membrane is passed and then the Certificate can be issued.
Active systems
Require a test of the alarm in case of failure of the system/power supply and possible dangerous buildup of gas.
References
- CIRCA 665.
- BS8485.
- Department of the Environment, The control of landfill gas, Waste management paper No 27.
- Department of Environment, Landfill sites development control.
- Guidance on evaluation of development proposals on sites where Methane and Carbon dioxide are present, incorporating Traffic Lights. Rep Ref.10627-R01-(02) Milton Keynes; National House Building Council.