Verification of Ground-Gas Protection Measures John Naylor - - PowerPoint PPT Presentation

Verification of Ground-Gas Protection Measures

John Naylor

Technical Director

Engineering Failure? Negligence? Liability?

Loscoe

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.

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

• 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

“The process of demonstrating that the risk has been reduced to meet remediation criteria and objectives based on a quantitative assessment of remediation performance”

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

Competencies

• Installation operatives

?

• NOS’s VR 612 and 613 describe

the process

• NVQ level 2 qualification in gas

membrane installation has been developed and offered by National Construction College (NCC)

• Up-skilling course for

installation operatives available via the British Geomembrane Association

• Verification practitioners
• 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

General Builder / Groundworker Visual inspection of first 10 plots and after placement of reinforcement (unless protected) with air lance testing to ASTM D4437 Subsequent inspections with air lance testing to ASTM D4437 at approximately 1 in 20 plots (I would suggest more!) Installer to provide evidence for all plots (sign off sheets and photographs) Qualified and experienced installer (at least 1 member of the installation crew NVQ L2) Visual inspection of first 5 plots and after placement of reinforcement (unless protected) with air lance testing to ASTM D4437 Subsequent inspections with air lance testing to ASTM D4437 at approximately 1 in 20 plots Installer to provide evidence for all plots (sign off sheets and 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

No sharp objects Rolled and compacted as necessary No Standing water

• Dusted Aggregate
• Venting Layer
• Geotextile

SUBGRADE / SUB BASE

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
• versite 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

• 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?

Other trades?

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? To test is best!

0.0 5.0 10.0 15.0 20.0 25.0 10 Jan 12 13 Jan 12 16 Jan 12 Concentration (% v/v) Methane Carbon Dioxide Oxygen

Verification by Subfloor Void Monitoring

3rd 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

• f the risk management process

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John Naylor Technical Director Tel: 0161 232 7465 Mob: 07856 244 224 Email: john.naylor@ggs-uk.com

Thank you!

Any questions?