The Principles and Practices of Gas Protection

Principles and practices of gas
protection
Peter Atchison
www.pageo.co.ukLike pageotechnical on Facebook

Principles of remedial
design & gas protection
• Guiding principles - breaking the ‘pollutant linkage’
• Receptor sensitivity & management
• Protecting the building envelope
• Achieving ventilation
• Gas-proof membranes
• Installation
• New BS8485

What is gas protection design?
Design is using professional knowledge and
experience to analyse risks from site, risks to the
receptor and produce a solution using available
components to reduce the combined risk to an
acceptable level.
Q :Who should conduct design?
A : A suitable qualified person

Design is NOT “detailing”
“Design”
What to use, why, how to achieve a required result
and demonstrating this in theory.
“Detailing”
How a particular component is fitted into a
structure

The Process
DESIGN :Involves choosing appropriate
components to suit site and construction
circumstances. MAY include dilution calculations to
prove performance. Should have justification and
be produced by a suitably qualified person.
DETAILING :is often conducted as part of the
design process but may be accomplished
separately say by using manufacturers standard
details.
Detailing and design ARE NOT THE SAME

Components cost money
The designers permanent quandary is
balancing data with Risk and achieving an
appropriate design risk assessment
commensurate with acceptable risk.
Over – engineering costs the contract money.
Under - engineering costs lives.

BS 8485

“BS8485 2007”
Code of practice for the characterization and
remediation from ground gas in affected
developments.
“BS8485 2015”
Code of practice for the design of protective
measures for methane and carbon dioxide
ground gases for new buildings

BS 8485 2015
MAIN Changes:
1.0 Now includes detail on SI data analysis, site
classification and Conceptual site model.
2.0 Has much more detail on ventilation design and
performance as well as foundations and membranes
in appendices
3.0 Includes appendices on VOC’s and RADON
4.0 Emphasises the IMPORTANCE of validation
5.0 Includes details from the up to date advice documents
issued since 2007
6.0 NOW 84 pages (was 28!) that’s inflation for you!

BS 8485 2015
WHAT HAS NOT CHANGED
1.0 Still relies on Professional judgement
2.0 Still uses a RISK based analysis principle
3.0 Starts with site classification and analysis
4.0 Still considers receptor construction “sensitivity”
5.0 Still uses a points based system to CHECK solution design
6.0 Still covers all types of building
7.0 Still top of the hierarchy of documentary design support
8.0 Replaces 2007 document which is withdrawn

BS 8485 2015
Site Classification
Expectation of CSM AND appropriate SI data
Construction
Expectation of Construction type (often difficult!)
Ventilation
Expectation of qualification of ventilation DESIGN
Membrane
Expectation of membrane installation and
VALIDATION

BS8485-Achieving
Ventilation

Principle of Passive Dilute and
Disperse in Ventilated Void
After CIRIA 149, 1995
Plan
WIND
Zone of
positive
pressure
Zone of
negative
pressure
(suction)

Ventilation protection measures should be one of the following
five types, and points can only be scored for one of these
measures:
a) pressure relief pathway only (no effective dispersal
layer);
b) passive dispersal layer; (calculation provided)
c) active dispersal layer (fan suction);
d) active positive pressurisation (air blanket); and
e) ventilated basement substructure present.

There is a presumption in the standard
that any ventilation design will be
supported by an appropriate
calculation
Details are provided in Appendix “B”
Provability

Dilution Calculations are normally
used to PROVE a design
Passive Gas Dilution Calculation
Measured data : Gas concentration and flow
Data to be used in Dilution calculation? – decision needed with
justification
Assumptions : Wind speeds to be used (normally 3m/s; 1 m/s and 0.3m/s)
Components : Composite, void former, gravel, clear void etc.
Design : How much of what and where? Number of inlets/ outlets &
position.
Then and only then
Calculations can be used to “prove” the design according to identified
parameters. Typically the ability to keep theoretical concentrations below
a particular level given a wind speed

Dilution Calculations
Often completed using spread sheet calculators but can be done
from first principles.
For each gas considered separately:
Site emission rate is calculated from borehole data (using
Pecksen)
For assumed wind speed:
Flow in through inlets, flow out through outlets and flow through
venting media are determined separately
The limiting factor is determined (should be the ventilation media)
Effect of surrounding buildings is presumed (1 no effect 0.4
considerable effect)
Maximum “theoretical” concentration of gas in the sub floor void
is given

Ventilation Products
• Clear void
• Polystyrene block on legs
(vent form)
• Drainage composites
(12-50mm)
• Gravel and pipes
• Gravel beds
Improved
performance

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.

Active Ventilation
• Constant performance
irrespective of wind speed
• Dilution/dispersion based on
air changes
• Single geosynthetic barrier
component
• Provable via probe system
• Flexible/upgradeable
• Cost effective w.r.t. well
designed passive system

BS 8485
Gas Resistant Membranes

Gas-Proof Membranes
How gas resistant do they need to be?
High quality membranes are only as good as their installation!
Wilson, Card and Haines, 2008

Membrane
Key aspects of membrane materials
1 Robustness (ability to withstand
installation)
2 Permeability to challenge gases
3 Long term durability

Membrane Performance (permeability)
• Most proprietary membranes currently on the market
will meet the permeability requirement in BS8485 of
<40ml/m2/d/atm according to ISO 15105-1
• ALL membranes with aluminium foil >12 microns
should pass this criteria BUT this should be supported
by an independent test certificate from a reputable
laboratory and not taken from supplier specifications.

Gauge abuse
• The EN for measurement uses a 10mm round sample
area to even out imperfections in blown film
• This is fine for simple blown films, BUT can be massively
misleading when measuring reinforced films. For
example if there is a relatively close reinforcing this
measurement can judge overall thickness where the
reinforcing crosses thus massively over-reading
• The answer is to always compare quoted gauge with
mass/unit area:

Membrane gauge quick check
1. Remember DENSITY cannot lie!
2. LDPE IS 0.918 g/cc
HDPE IS 0.94 g/cc
3. Both are nearly 1.00 +/- 10% So a simple way to remember
is:
Take mass per unit area in g/m2 and divide by 1000
4. e.g. a 350g/m2 product will be nominally 0.35mm thick

Proprietary Membrane Materials
There are fewer manufacturers than you would imagine!
• Most so called “proprietary gas barrier membranes are LDPE
Reinforced membranes with an aluminium foil.
• Manufacturers using this equipment are few
SO many materials are exactly the same irrespective of
suppliers claims!
Remember:
• It is the material’s performance which counts
• Suppliers who will NOT declare the manufacturer will
probably not provide original test certificates.
INSIST ON THEM!

Recycled V’s Virgin Polymer
• Recycled films tend to be much weaker and tear easier
• Problems with permeability and consistency
• Heat fusing becomes problematic with recycled materials,
and will require tape jointing

The three most important elements of
membrane efficiency
•INSTALLATION
•INSTALLATION
•INSTALLATION

Detailing is IMPORTANT
• Preformed ‘Top Hat’
units and adhesive
membrane sections
are used for this
purpose
• Penetrations for
services, and junctions,
must be sealed

Improving Standards
▪ In 2008 - Gap identified in the National Qualification
Framework (NQF)by Construction Skills
▪ National Occupational Standards (NOS)developed
during 2009 & issued in Feb 2010 (VR612 & VR613)
▪ “Gas membrane installation for domestic and
industrial/commercial buildings”
▪ NOSs reflect current occupational practice
▪ Become UK standards for installing geomembranes
www.ukstandards.co.uk
Recently revised (2014) and reissued April 2015

NVQ/SVQ Structure
• NVQ level 2 “qualification in gas membrane installation
available since 2012
• Based on work to the NOS standards
• Two methods of attainment:
1. OSAT (on site training and assessment) aimed at existing
“experienced” operatives
2. SUP (Specialist Up-skilling Program) aimed at less experienced
operatives who take a 5 day course followed by 6 month assessment
RESULT in NVQ level 2 attainment and “Blue” CSCS card
UKCG (Main Contractors and house builders) increasingly requiring
appropriate qualifications on site

Sealing Techniques

How to do it well!

And How not to!

The good!

The bad!

And The Ugly!

Attention to detail

Gas Membrane Installation:
Heat Sealing

Looks good from here
Or does it?
Concrete prep on Full Line Out

Investigate
Building Gas Protection

Investigate
Risk

Investigate
Risk
Design

Investigate
Risk
Design
Install

Investigate
Risk
Design
Install
Verify

Thank you
Discussion / Questions?
Principles & Practices of Gas Protection
Peter Atchison

The Principles and Practices of Gas Protection

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