David
If the system provides the background ventilation rates or ventilation areas required in the Building Regulations they are more than adequate to deal with gas emissions from the ground if the site has been assessed using the correct application of the screening approach in BS8485 (which is more than just the points system and covers the points you have made as well as Sarah and Karen). The Characteristic Situation approach is a very conservative screening approach. Experience on numerous sites over 20 years has shown that the car park venting is more than adequate to deal with ground gas emissions in accordance with the scoring system in BS8485.
The screening approach is also only used to define the SCOPE of any gas protection measures - any system (ventilated void, membrane or car park venting) requires detailed design.
The factors you have stated are all very valid but should also apply to any other form of protection proposed under the points system (gas membranes, vented voids, etc).
Unfortunately the points system is now being seen as the only thing that matters and it is being used by unqualified people to "design" gas protection systems. I have also recently seen someone claim points for active venting in a basement waterproofing cavity and also the points for pressure relief which is completely wrong.
Steve
Steve Wilson, Technical Director
The Environmental Protection Group Ltd
Tel 07971 277869
www.epg-ltd.co.uk
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-----Original Message-----
From: Contaminated Land Management Discussion List [mailto:[log in to unmask]] On Behalf Of David E Jackson
Sent: 02 June 2017 09:45
To: [log in to unmask]
Subject: Re: BS 8485:2015 Car exhaust venting for gas protection
Dear List,
Thanks to Sarah and Karen for their comments.
It seems increasingly apparent that there is way-more to this, than can be reflected in the arbitrary scoring system offered by BS8485 (based on compliance with Building Control Part F).
It seems that vehicle exhaust venting systems referred to in Building Control Part F are NOT designed for ground-gas (CH4/CO2) protection and that the sensor types, sensor locations, venting rates, system operations and maintenance systems and the underpinning risk assessments proposed under the BC scheme are designed specifically for CO management, only.
Clearly such systems can make a useful contribution, but LA-CLOs need to be satisfied that they have been adapted and modified to monitor and manage ground-gas specific risk scenarios, and not simply assume a 4pt score (BS8485) is appropriate!
Best wishes to all,
David E Jackson
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