Verification of Ground-Gas Protection Measures John Naylor Technical Director
Engineering Failure? Negligence? Liability? In the case of development, whether the hazardous gas source is natural or man made, it should be considered an ‘ engineering failure ’ where such gases are found to be elevated to levels which may cause damage to property or adversely affect the health and wellbeing of users of that development. Loscoe
Engineering failure may result from: • Insufficient characterisation of the ground gas regime • Inappropriate design and specification of gas protection measures • Poor installation practice • Lack of provision of future maintenance
Looks all right to me ! Bill
Key Documents
CIRIA C735 - Verification “The process of demonstrating that the risk has been reduced to meet remediation criteria and objectives based on a quantitative assessment of remediation performance” • All installations should have some form of verification • Verification should be proportionate to: • A) the risks from the site, • B) the experience of the installation operatives • Verification should be conducted by suitably qualified and Independent practitioners. • Introduces a Verification Plan
Prerequisites The verification process includes an understanding of the gas regime and conceptual site model and technical understanding of the design and materials specification requirements in order to: • Competently produce a suitable verification plan for all aspects of the gas protection system • Competently implement the verification plan
Verification Plan Verification Plan should include: • Site summary • Health & Safety considerations • Design / specification compliance • Define Contents of Final Verification Report • Acceptable installation methods • Installer / verifier competencies • Inspection / testing regime • Non-conformance resolution
• NOS’s VR 612 and 613 describe Competencies the process • NVQ level 2 qualification in gas • Installation operatives membrane installation has been developed and offered by National Construction College (NCC) • Up-skilling course for installation operatives available via the British Geomembrane • Verification practitioners Association ? • Section 3.2 C735 • NOS VR764 has been set
Independence INDEPENDENT verification helps eliminate: • potential conflicts of interest • cost control measures or timing issues that impair quality Need to be a THIRD PARTY WHO should contract the verification work? – The contractor carrying out the installation? – The main or sub contractor ? – The overall client (e.g. main builder)?
C735 Decision Structure • Annex 1 provides example tables A1 to A3 (and notes!) for what levels of inspection and integrity testing may be appropriate • Professional Judgement taking into account: − The gas regime − Number of buildings − Complexity of design − Expertise of installation crew − And other site specific factors….
C735 Decision Structure CIRIA C735 Figure 3.1
C735 Annex 1 Example 2 Situation B Amber 2 site with 150mm vented void Installer Verifier Visual inspection of first 10 plots and after placement of reinforcement (unless protected) with air lance testing to ASTM D4437 General Builder / Subsequent inspections with air lance testing to ASTM D4437 at approximately 1 in 20 plots (I would suggest more!) Groundworker Installer to provide evidence for all plots (sign off sheets and photographs) Qualified and Visual inspection of first 5 plots and after placement of reinforcement experienced (unless protected) with air lance testing to ASTM D4437 installer (at least 1 Subsequent inspections with air lance testing to ASTM D4437 member of the at approximately 1 in 20 plots installation crew Installer to provide evidence for all plots (sign off sheets and NVQ L2) photographs)
Programming • Needs careful planning and communication • Verification Plan • One of the most difficult issues with verification is being present to inspect and test at key stages • Commonly it is a last minute phone call by the site manager requesting your presence as the concrete is about to be poured • You pretty much always find defects – have the repair crew ready
SUBGRADE / SUB BASE No sharp objects Rolled and compacted as necessary No Standing water • Dusted Aggregate • Venting Layer • Geotextile
Clear Void Watchpoints: a. Should have minimum 150mm clear void b. Should have cross ventilation through sub-floor sleeper walls (typically x3 area of external ventilation) c. Ensure vents are above ground and not obstructed d. Sometimes a barrier or concrete oversite used to reduce gas permeability to void
Gas Resistant Membranes
Proprietary Membrane Materials Performance based on: a. Polymers used (non – recycled), interlays b. Thickness (typically 1200g 0.3mm to 3200g 0.8mm) c. Reinforcement (scrims) d. Inclusion of an aluminium core Remember: • Materials selection on design requirements • Need good installation • Construction sites are not benign environments
Jointing and Detailing • Jointing can be achieved by either tape or welding methods • Professionals tend to weld – its quicker, requires less product and typically achieves a very high standard of joint • In either case PRESSURE, and in many cases HEAT, needs to be applied to create an effective bond
How it shouldn’t be done: What do we do about internal walls?
Any offcuts?
Follow on trade damage Other trades? - Lack of understanding of importance of gas protection - Lack of care - Lack of site management - Timing of validation? Solutions - Tool box talks - Protection of membrane - Use of a ‘skid’ Time and cost implications?
Verification Testing • Can be applied to ALL areas of the Gas Protection System • Visual Inspection and Pick Testing through to Integrity Testing and Gas Monitoring • It should be used proportionally and appropriately • There are a number of different test methods currently available • Each are appropriate for certain situations • Further information is included in CIRIA C735 guidance
Mechanical Point Stress Testing (Pick Test)
Air Lance Testing • Only recognized test method for both taped and welded seams • Minimum 345kPa pressure must be applied during the testing • Operator directs the pressurized air at the seam to be tested (not more than 50mm away) • Operator observes for defects
Tracer Gas Testing
Has it worked? Verification by Subfloor Void To test is best! Monitoring 25.0 Methane Carbon Dioxide Oxygen 20.0 Concentration (% v/v) 15.0 10.0 5.0 0.0 10 Jan 12 13 Jan 12 16 Jan 12
3 rd Party Verification Reports
In Conclusion • Fit for purpose Gas Protection Measures relies on: – Good data and assessment – Good design and specification – Good quality workmanship – Appropriate ongoing maintenance • Standards exist and can be applied • Commitment from professionals to raise the standard Verification forms the vital piece of the risk management process
2017 TRAINING PROGRAMME Cambridge: 16 May – 17 May Swansea: 20 June – 21 June Derby: 11 July – 12 July York: 19 September – 20 September Chester: 17 October – 18 October London: 7 November – 8 November Company Technical Talks & Project Specific Discussion – contact info@ggs-uk.com
Thank you! Any questions? John Naylor Technical Director Tel: 0161 232 7465 Mob: 07856 244 224 Email: john.naylor@ggs-uk.com
Recommend
More recommend
