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Engineered Solutions for Ground Gas and Vapour Protection

Measures

By Peter Atchison

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Hazards

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The gas contamination problem

If buildings are constructed on or near contaminated land there is a risk that hazardous gasses may enter the building. They would pose a risk to human health if allowed to concentrate. Gasses which may cause problems include carbon dioxide, methane, radon and hydrocarbon vapours. Common causes of their presence include brownfield sites, landfill and natural occurrence.

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Guidance

RecentlyPublishedGuidance

2007NHBC-Guidanceonevalua5onofdevelopmentproposalsonsiteswheremethaneandcarbonDioxidearepresentReportNo10627-RO1(04)

2007CIRIA-C665AssessingRisksposedbyhazardousgroundgasestobuildings

2007BS8485–CodeofPrac5ceforthecharacterisa5onandremedia5onfromgroundgasinaffecteddevelopments

2008LAGuidetoGroundGas(Groundgashandbook)–Wilson,CardandHaines

2012CIRIAC716remedia5ngandmi5ga5ngrisksfromVola5leorganiccompound(VOC)vapoursfromlandaffectedbycontamina5on

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New guidance

BS8576 Guidance on investigations for ground gas – permanent gases and volatile organic compounds (VOCs)

CIRIA Good Practice on the testing and verification of protection systems for buildings against hazardous ground gases July 2014

BS8485 revised and reissued June 2015

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Monitoring

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‘Spot’ & ‘Continuous’ Sampling

•  Discrete, periodic, static measurements of borehole gas concentrations and flows on a weekly or monthly frequency

•  Ground-gas movement is dynamic, it is influenced by many parameters (e.g. barometric pressure, temperature) and can change over hours or minutes

•  The monitoring frequency should ideally match the frequency of these changes

•  When the frequency of monitoring matches the frequency of ground gas change the monitoring can be considered to be ‘continuous’

CrossSec6onConceptualModel

BS8576requirescrosssec6onsformigra6onofpermanentgases

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Chg – Concentration of a specific hazardous gas expressed as a percentage of total gas volume (%v/v)

q – Total gas flow from a borehole in litres per hour (l/hr) Qhg - Calculated flow rate of a specific hazardous gas from a

borehole reading

GSV = Concentration/100 x flow or

GasScreenValue(GSV) (akahazardousgasflowrateBS8485)

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Principles of Protection

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GuidingPrinciplesBreakingthePollutantLinkage

• Removethesource

• Breakthepathway

• Protectbuildingenvelope

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RemovingtheSource

CIRIA716

Receptor

Migra5nggasplumeinpermeablestrata

Source

Impermeablelayer

Breakingthepathway

• Ground-gasplumemigra5onpreventedby• Installingapreferen5alpathwaytoatmosphere• backedupbyabarrier

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Protecting the Building Envelope

A)  Dilute and disperse (provide preferential pathway to atmosphere) B) Exclude ground-gases

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“Gorebridge homes to be demolished after carbon dioxide gas leak”

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Design of Gas Protection

BS8485 An introduction

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BS8485 application

§  To “qualify” a gas protection design:

§  Refer to Table 4 to select a range of measures which contribute to the required gas protection score

§  Only one element from each sub section

– Ventilation – Floor slabs – Membranes (including installation+ Validation) – Pathway intervention

– Validation / Verification

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NHBC Traffic Lights

§  Green site – Ground gas protection measures not required

§  Amber “1” site – Low to Moderate measures required may consist of barrier installed by ground worker to good standards and involve validation towards the upper end. Ventilation should offer 1 change in 24hrs

§  Amber “2” Site – Moderate to high measures required membranes should be installed by specialist contractor, involve appropriate CQA measures and should be validated, Integrity testing should be considered at the upper end.Ventilation should offer 1 change in 24 hrs as with amber “1”

§  Red site – Residential housebuilding not allowed unless the site characteristic situation can be demonstrably lowered.

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Housing using BS8485

Ground bearing concrete slab •  The slab (0.5-1.5pts) is poured directly on to

the gas barrier, (2.0pts) below which is a venting layer. Ventilation paths must be provided from the venting layer to atmosphere (0-2.5pts)

Suspended beam and block floors •  Gas barrier (2.0pts) is laid over the beam &

Block floor (0pts) and covered with a screed. The sub floor is ventilated by air bricks (2.5pts)

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Active (Passive Assisted)

§  Constantperformanceirrespec5veofwindspeed

§  Dilu5on/dispersionbasedonairchanges

§  Singlegeosynthe5cbarriercomponent

§  Provableviaprobesystem§  Flexible/upgradeable§  Costeffec5vew.r.t.well

designedpassivesystem

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Components of Protection Systems

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Ventilation & Ancillary Products

•  Void former –  Made from cuspated High

Density Polyethylene (HDPE).

–  High strength and crush

resistance. –  Available as fully wrapped or

single-sided geotextile.

Principle of Passive Dilute and Disperse in Ventilated Void

After CIRIA 149, 1995

Plan

WIND

Zoneofposi5vepressure

Zoneofnega5vepressure(suc5on)

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Gas-Proof Membranes

High quality membranes are only as good as their installation!

Wilson, Card and Haines, 2008

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Thethreemostimportantelementsofmembraneefficiency

• INSTALLATION• INSTALLATION• INSTALLATION

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Detailing is IMPORTANT

•  Penetra6onsforservices,andjunc6ons,mustbesealed

•  Preformed‘TopHat’unitsandadhesivemembranesec6onsareusedforthispurpose

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How to do it well!

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And How not to!

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The good!

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The bad!

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And The Ugly!

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Attention to detail

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NVQ/SVQ Structure

•  NVQlevel2“qualifica5oningasmembraneinstalla5onavailablesince2012

•  BasedonworktotheNOSstandards•  Twomethodsofadainment:

1.   OSAT(onsitetrainingandassessment)aimedatexis5ng“experienced”opera5ves

2.   SUP(SpecialistUp-skillingProgram)aimedatlessexperiencedopera5veswhotakea5daycoursefollowedby6monthassessment

RESULTinNVQlevel2adainmentand“Blue”CSCScardUKCG(MainContractorsandhousebuilders)increasinglyrequiringappropriatequalifica5onsonsite

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Sealing Techniques

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Validation &

Verification

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Validation and Verification

Presumptions: ALL installations should be subject to some form of

validation Validation should be proportionate to the risks from the

site AND the experience of the installation operatives Validation should be conducted by suitably qualified

INDEPENDENT practitioners. In the event of appropriate self validation this should be

AUDITED by a suitably qualified independent auditor.

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Validation / Verification

– Increasinglybeingspecifiedaspartofawelldesignedsolu5on– Mayconsistofavarietyoforcombina5onofprac5cesandshouldbespecifiedappropriatelyaccordingtorisk– Thereisno“Oneoff”Valida5on!Mayconsistof:– Checkforcompliancewithspecifica5on(Includingmaterials,qualifica5onsofinstalla5onopera5vesandposi5onofcomponents.)– Visualinspec5onofwork– Closephysicalinspec5onofworkincluding“picktes5ng”– Seamandsealtes5ngusingtestequipment– Fullinstalla5onintegritytes5ngusinggas,smokeordielectrictes5ngtechniques.– CIRIAProjectcurrentlyunderwaytoofferguidance–wasduetobepublished2013!…..availablespring–summer2014!

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WHY Verification?

a.  Join6ngtapenotcon6nuous

a.  Cornernotsealedorweldedtomainbodyofmembrane

b.  Thiswasinstalledbyaselfproclaimedspecialistinstaller

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WHY Verification?

a.   Scaffoldingpolepushedstraightthroughmembrane

b.   Lackofeduca6onforothertrades

a.   CreasestraightthroughJoint(knownafishmouth)

b.   Par6cularproblem2000gcentreandtrifoldmembranes

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WHY Verification?

Followontradedamage- Lackofunderstandingofimportanceofgasprotec5on- Lackofcare- Lackofsitemanagement- Timingofvalida5on?

Solu6ons- Toolboxtalks- Protec5onofmembrane- Useofa‘skid’

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In Conclusion

v Muchhasbeenaccomplishedoverthelast10yearsv Muchiss6lltobedonev Qualitydesignreliesonqualitydata.v Goodqualitymaterialsneedgoodqualityworkmanshiptoperform

v Standardsexistandcanbeappliedv Valida6onandverifica6onformpartofaqualitydesign

v Socouldyoubea“validator”………………..?

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Spot The difference!

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Thank You And remember ………………..

“Beayards5ckofquality.Somepeoplearen'tusedtoanenvironmentwhereexcellenceisexpected.”SteveJobs(1955-2011)

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Thank You

Any questions? ByPeterAtchison

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