Characteristic Situation 2 and Gas Membrane

[Jammy5]

Hopefully this will be my last big stumbling block and advice would be appreciated.

 

I have noticed in my ground investigation report that it states:-

The results of the ground gas monitoring indicate low levels of ground gas that do not warrant gas protection. However, given the presence of elevated gas concentrations and shallow coal mine workings beneath another nearby development, it is recommended that basic gas protection measures in line with Characteristic Situation 2 (CS2) are included as part of any proposed residential properties to protect any future residents from any ingress of Carbon Dioxide and Methane. The site is not at risk of radon gas and no protection measures are deemed to be required.

 

Earlier in the same report, in amongst the detail it states:-

Therefore, in accordance with CIRIA C665, Table 8.5 the GSV for CO2 equates to a Characteristic Situation 1 (CS1). However, given the previous reports for the opposite site and the presence of shallow nearby coal mine workings, it is recommended that basic gas protection measures in line with Characteristic Situation 2 (CS2) are included within new builds.

 

The report found that the maximum recorded CO2 concentration, which has been converted to a Gas Screening Value (GSV) was 0.0025 l/hr GSV.

It also states that since no concentrations of methane have been recorded, no GSV can be formulated.

 

BS 8485:2015 + A1:2019 has a scoring system for gas protection. The uncertain part is how to interpret the recommendations from the report.

Can I consider my CS by Site Characteristic as CS1, as the L/h is <0.07 when working out my minimum gas protection score?

 

If I have to work with CS2 and a type A building this would give me a score of 3.5 which I wouldn't easily be able to adhere to with having a ground bearing slab design that has already been constructed - I have currently constructed the walls to DPC level and compacted the Type 1 hardcore, ready to add a sand blinding and cover with a gas membrane.

 

What I would like to understand is do I actually need to install ventilation or is the gas membrane without ventilation sufficient in this case. Especially with the report classifying as a CS1 but stating to apply basic gas protection measures in line with CS2. I'm hoping this means take the score as 0.0 (instead of 3.5) but put a membrane down that is suitable for a CS2 situation (without the need for any ventilation as my site is classed as a CS1).

 

Hopefully I'm reading the standard wrong or missing something and someone can offer their opinion on the matter.

 

Thanks in advance. 

 

[Jammy5]

Just having a think on this one as well. Do you have to follow recommendations? It doesn't mandate the need for one.