The importance of the design, installation and verification of gas protection measures

Transcription

1 The importance of the design, installation and verification of gas protection measures John Naylor Technical Director

2 Engineering Failure? Negligence? Liability?

3 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

4 June 2014

5 The Main Hazardous Gases CO H 2 H 2 S CH 4 CO 2 N 2 VOCs Rn O 2

6 Name: Radon Symbol: Rn Atomic Number: 86 Atomic Mass: Half Life: days Melting Point: C Boiling Point: C Number of Protons: 86 Number of Neutrons: 136 Classification: Noble Gas (SPT) Crystal Structure: Cubic Density: 9.73 g/cm 3 Colour: colourless Odour: odourless Taste: tasteless

7 Methane (CH 4 ) Flammable gas Explosive in air at 5 to 15% v/v Typical H&S criteria: 20% LEL (1% Vol) Main component of natural gas and landfill gas Density: kgm -3 NTP (x0.55 air) Odourless Colourless Tasteless Clacton Explosion Source: BBC

8 1% 2% 5% 8% 10% 20%

9 Ground-Gas Risk Assessment

10 Ground-Gas Measurement

11 Boreholes (Monitoring Wells)

12 Monitoring Instruments

13 Spot & Continuous monitoring Methane Weekly monitoring on these dates shows almost no methane Weekly monitoring on these dates shows falling methane Weekly monitoring on these dates shows rising methane

14 Environmental Correlations Multi-parameter continuous data Identify or eliminate correlations with the environment Identify or rule out ground-gas drivers

15 Differential Pressure Click to edit Master title style

16 Differential Pressure High Low Atmospheric Pressure Turf Topsoil Boulder Clay (very low permeability) Coal Gas Reservoir Sandstone Bedrock Response zones need to be designed too!

17 Ground-Gas Management

18 Entry of ground-gas into properties Usually from ground on which house is built Through pathways, e.g. cracks, service entries Sometimes through groundwater

19 How much gas is bad? Molecule / Compound Methane Carbon Dioxide Benzene Radon Typical hazardous screening value 20% Lower Explosive Limit 2,500ppmv (0.25% v/v) Acceptable average indoor air quality 1,000ppmv (0.1% v/v) Long Term Occupational Exposure Level (8hrs) 1ppmv (0.0001% v/v) 200 Bqm -3 (domestic action level) 3.5ppqv ( % v/v)

20 Active Passive Protecting the Building Envelope Progressively build up fail safe protection measures: A) Exclude ground-gases (Horizontal barriers or membranes) B) Dilute and disperse, provide preferential pathway to atmosphere (Ventilated sub-floor void and membranes NHBC ) C) Active venting systems with barriers CS4 and above D) Active positive pressurisation CS4 and above E) Monitoring and Alarm Systems CS5

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22 Gas resistant membranes meeting ALL of the above requirements HERO or ZERO

23 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

24 SUBGRADE / SUB BASE No sharp objects Rolled and compacted as necessary No Standing water No significant voids Dusted Aggregate Venting Layer Geotextile

25 Proprietary Membrane Materials Performance based on: a. Polymers used (non recycled) 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 Needs good installation Not a benign environment

26 Membrane Gauge (Thickness) millimetres 0.5mm 1.0mm 1.5mm 2.0mm gauge 1200g 2000g 4000g 8000g Based on the application it is being used for NOT the thickness of the reinforcement scrim Too thick - difficult to install, increased cost Too thin can t weld, easy to damage Mass 450g/m 2 LDPE >0.91g/cm 3 HDPE >0.94g/cm 3

27 Gas-Proof Membranes How gas resistant do they need to be? Based on: Room volume of 40m 3, 50 Pa driving pressure, 100% methane, no dilution Wilson, Card and Haines, 2008 Membranes are only as good as their installation!

28 Gas Resistant Membrane Installation

29 Jointing and Detailing Jointing can be achieved by either tape or welding methods (ensure product compatibility) Professionals tend to weld its quicker, requires less product and typically achieves a very high standards of joint In either case PRESSURE, and in many cases HEAT, needs to be applied to achieve an ACCEPTABLE BOND

30 How it shouldn t be done: What do we do about internal walls?

31 Any offcuts?

32 Pass or Fail?

33 Other trades? 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?

34 CIRIA C735 Published 2014 ALL installations should be subject to some form of verification Verification should be proportionate to the risks from the site AND the experience of the installation operatives Verification should be conducted by suitably qualified INDEPENDENT practitioners.

35 C735 Decision Structure Click to edit Master title style CIRIA C735 Figure 3.1

36 Verification by Measurement Click (Sub-floor to edit Master Monitoring) title style It s Best to Test!

37 a) In line with methodologies in Annex 2, 3 and Click to 4 edit of CIRIA Master Good practise title on the style testing and verification of protection systems for buildings against hazardous ground Gases. (Leak Detection) Smoke Testing Tracer Gas Testing Dielectric Testing b) In line with methodologies in ASTM D4437 (Seam Testing) Pick/Probe Test Air Lancing Integrity Testing Pressure Testing There are also others

38 Summary Click to edit Master title style Ground gas can be hazardous Gas protection design needs good data Designs requires detail Installation standards need to be higher Verification is vital Thank You! John Naylor