Cook Group are true specialists when it comes to designing and installing ground gas membranes. We can help with anything from a CS1 site up to CS7. The NHBC classifications for house building sites are also known as Amber 1, Amber 2 and Red. Confusing? It can be! The solution is simple, contact us for simple easy to understand non bias advice.
What are Ground Gases?
The term ground gas basically means a hazardous gas that is generated below ground, the most common ground gases that can affect us in buildings are methane, carbon dioxide and VOC’s (Volatile Organic Compounds).
There are of course thousands of other ground gases that can cause problems too such as nitrogen, benzene, toluene, sulphur dioxide, chlorobenzene, vinyl chloride, carbon disulphide, trichloroethene, hydrogen, tetrachloroethene, tetrachloromethane, xylene, hydrogen cyanide, methyl mercury, 2-butoxy ethanol, arsenic, 1,1-dichloroethane, methanal, furan, ammonia, dichlorodifluoromethane, 1,2-dichloroethene to name just a few!
Radon is also a ground gas which we talk about in more detail in our separate section – HERE.
Ground gases form in the pores and voids within soil and are much more prevalent on brownfield sites, a brownfield site is basically land that has previously been used for industrial purposes or commercial use.
This land may be contaminated by low levels of hazardous waste or pollution. When this land is built on for domestic or new commercial buildings it is possible that ground gases could rise into these new buildings and if the concentrations of these gases rise to significant levels could cause health issues to the occupants.
Ground gases can also form from old mine workings or even naturally with certain soil types such as peaty soils.
Land fill sites
Former landfill sites are also some of the major culprits for ground gas problems with surrounding properties, particularly those that were in use after the 1960’s. The reason is simple! Landfill sites are exactly what the name suggests, land usually former quarries and mine workings that have been filled with a mixture of household and industrial waste and then covered over. Over time this waste begins to break down and begins to decompose, some of the by products of this process are gases, the type of ground gas from a landfill site depends a lot on what was put in it originally.
How do you test for ground gases?
First of all you have to determine whether there is a problem in the first place, for instance if you have an area of land that is just going to be landscaped and not built on then the majority of naturally ocurring ground gases simply won’t cause any issues that requires any remediation work at all and consequently you won’t bother carrying out the necessary testing.
If you are building houses or commercial units, a business park etc then of course it can be a completely different ball game!
Testing of the ground is mandatory and this is carried out long before the contractors turn up on site.
There are many standards and much guidence written on this subject, but the principle British standard is BS 8576 : 2013. Guidance on investigations for ground gas – Permanent gases and Volatile Organic Compounds (VOCs).
There is also CIRIA C735: Good Practice on the testing and verification of protection systems for buildings against hazardous ground gases July 2014
These standards do not cover Radon, this gas has its own seperate standards / guidance.
The primary way that testing is carried out is that holes are drilled into the ground, pipes inserted, monitoring equipment installed and the information from the equipment is recorded over a period of months. The depth, amount of holes and frequency of monitoring is dependant on the site and its previous uses, for instance an untouched green field site is less likely to be subjected to intensive testing compared to a former petro chemical factory site that is adjacent to a landfill site!
This brief statement simplifies a very difficult and extremely technical task that is carried out on sites all over the UK!
How do you protect a structure against ground gases?
The measures required to protect a building against the effects of ground gases can be very complex, which is why there is so much legislation about the subject. A breif overview is as follows.
First of all it starts with analysing all the data from your testing and bore holes, once you have collated all this information you carry out a risk assessment.
This determines the risk or liklihood of ground gases getting into a structure and that will determine what needs to be done to give the best protection to that building and these might include.
Design the problem out! This might seem like a simple answer, as an example if you have a multi storey block of flats with a large open and well ventilated basement car park, does it really need a gas membrane installing?
If you build a new supermarket for instance the car park could be much bigger than the actual shop, if that car park is covered in tarmac and concrete this could easily force ground gases towards the structure, altering the landscaping, creating ventilation channels from the ground could reduce the problems for the actual structure.
These are basically high capacity fans that are usually installed in conjunction with ground gas membranes to ventilate sub floor voids (the space between the ground and the underside of a ground floor). The fans are basically designed to give regular and consistant air changes to the voids.
The installation of a ground gas membrane to a structure whether it is a domestic dwelling, shop unit, school or a commercial unit such as a factory is one the easiest ways to make the property more resistant to ground gases.
The relevent standard for the installation of ground gas membranes is BS 8485:2015+A1:2019 Code of practice for the design of protective measures for methane and carbon dioxide ground gases for new buildings.