Gas protection
Encyclopedia
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 ( typically 40% by volume and methane 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.
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 build up 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 the check by operators using special air sensors to see if there are any leaks. If the work passes then is should be issued with a certificate of integrity.
There are two main practical types of Active systems to prevent the ingress of gases into buildings:
There should be alarms for both systems in the event of power failure to the systems as this will lead to an eventual build up of gases to a dangerous level. These systems should be asses by a third party to ensure the quality and effectiveness of the system.
, 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 (ie 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.
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 a 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.
The main components that developer have to be aware of in the U.K are methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
(which is flammable at 5% to 15% by volume in air) and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
(which is toxic), after two gas explosions in the U.K. in the 1980s Loscoe
Loscoe
Loscoe is a small village near Heanor in Derbyshire, England.The research for Highfield House dates back to 1650 or possibly as early as 1630. This may now be the oldest surviving house in Loscoe as many houses in the village were demolished due to subsidence.-1986 landfill gas explosion:Loscoe was...
and Abbeystead
Abbeystead
Abbeystead is a small, picturesque village located in the Forest of Bowland Area of Outstanding Natural Beauty, in Lancashire, England. Abbeystead lies close to the Trough of Bowland but even in medieval times, was considered part of Wyresdale rather than within the domain of that powerful local...
. Building Research Establishment
Building Research Establishment
The Building Research Establishment is a former UK government establishment that carries out research, consultancy and testing for the construction and built environment sectors in the United Kingdom...
, British Standards, the Department of Environment
Secretary of State for the Environment
The Secretary of State for the Environment was a UK cabinet position, responsible for the Department of the Environment . This was created by Edward Heath as a combination of the Ministry of Housing and Local Government, the Ministry of Transport and the Ministry of Public Building and Works on 15...
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
Sewage
Sewage is water-carried waste, in solution or suspension, that is intended to be removed from a community. Also known as wastewater, it is more than 99% water and is characterized by volume or rate of flow, physical condition, chemical constituents and the bacteriological organisms that it contains...
, landfill
Landfill
A landfill site , is a site for the disposal of waste materials by burial and is the oldest form of waste treatment...
, and peat
Peat
Peat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...
.
In the case of landfill gas migration
Landfill gas migration
Landfill gas migration is a complex process of the gas moving from the site of original deposition to other places via diffusion. Usually, the gas moves from areas of high concentration to areas of low gas concentration around a landfill. The process is also affected by the permeability of the...
, the gas is produced by the organic materials in the waste degrading over time. Creating varying concentration over time ( typically 40% by volume and methane 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
Leachate
Leachate is any liquid that, in passing through matter, extracts solutes, suspended solids or any other component of the material through which it has passed....
drains with venting, low permeability geomembranes
Geomembranes
Geomembranes are a kind of geosynthetic material made up of an impermeable membranes. Their use includes lining canals, ponds and also waste containment.-Composition:Geomembranes are made of various materials...
, 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
Geocomposite
The basic philosophy behind geocomposite materials is to combine the best features of different materials in such a way that specific applications are addressed in the optimal manner and at minimum cost. Thus, the benefit/cost ratio is maximized. Such geocomposites will generally be geosynthetic...
)
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 build up around or under the property.
Note in the photo two types of heat gun
Heat gun
A heat gun is a device used to emit a stream of hot air. They are superficially similar in shape and construction to a hair dryer, though they run at much higher temperatures...
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 the check by operators using special air sensors to see if there are any leaks. If the work passes then is 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 effectVenturi effectThe Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The Venturi effect is named after Giovanni Battista Venturi , an Italian physicist.-Background:...
. - 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 as 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 build up of gases to a dangerous level. These systems should be asses 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 effectStack effect
Stack effect is the movement of air into and out of buildings, chimneys, flue gas stacks, or other containers, and is driven by buoyancy. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. The result is either a positive or...
, 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 (ie 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
Landfill gas migration
Landfill gas migration is a complex process of the gas moving from the site of original deposition to other places via diffusion. Usually, the gas moves from areas of high concentration to areas of low gas concentration around a landfill. The process is also affected by the permeability of the...
. 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 a 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.