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Ground gas
– A quick
update
Steve Wilson
www.epg-ltd.co.uk
The next 20 minutes• Latest guidance on ground gas assessment and protection
• CIRIA C735 - Verification and testing of ground gas
protection systems – key reference in BS8485:2015
• CIRIA C748 Guidance on the use of plastic membranes as
VOC barriers – key reference in BS8485: 2015
• BS 8485:2015
CIRIA C735 – verification and integrity
testing• Research Contractor
• Hugh Mallett – Buro Happold
• L Cox neeTaffel –Andureau
• Mick Corban – MEC Environmental
• Steve Wilson – EPG
• Chair – Geoff Card – GB Card and
Partners
C735 – Key points
• Promotes a risk based approach to
verification and the up-skilling of the
construction workforce, verifiers and
regulators
• Provides specifications for integrity
test methods for membranes
• Covers verification of membranes
and venting systems
• Contents of verification report – not
just a “certificate”
• Requires independent verification Courtesy of Doncaster
Membranes Ltd
C735 – Verification plan
• A Verification Plan should be
prepared as an integral part of the
gas protection system at the design
stage and submitted to the local
authority regulator as an integral
part of the remediation strategy.
• The verification plan should address
▫ Gas regime and gas protection
system
▫ Inspection regime
▫ Integrity test programme (if any)
C735 – Integrity tests
• Seams
▫ Air channel
▫ Mechanical
▫ Air lance – normally always required
• Areal
▫ Tracer gas
▫ Smoke
▫ Dielectric porosity
• All have advantage and practical
limitations
• Tracer gas testing is no better or worse
than the other tests
• Risk based decision on need
C735 – tracer gas tests• Standard method statements in the
report – should follow closely
• Need to make sure gas is present
below the whole membrane at all
corners
• Research on integrity testing with
helium and smoke tests
• Channelling of gas or smoke can occur
below the membrane
• A quick squirt of helium or other gas is
not sufficient
• Experienced and trained operators
• Response time of detector?
Comparison of tracer gas and
dielectric• Experiment to see if tracer gas test that does not
follow C737 method (ie making sure all gas reaches
all areas of membrane) is effective
• Independent blind test
• Tracer gas test completed first
• Followed by dielectric test
• Results – dielectric testing found defects that were
not revealed by tracer gas test
• Reasons – sweeping too fast with detector, gas not
evenly distributed below membrane
C735 – Air lancing
• Suitable for all types of
membranes and joints
• It is not a destructive test – will not
damage a good joint (even taped)
• Very good at highlighting poor
workmanship (so some
contractors try and discredit it)
• Specialist equipment is required
to ASTM – D4437 Non destructive
testing of membranes
CIRIA 748 Guidance on membranes
as VOC barriers • Authors
▫ Steve Wilson (EPG)
▫ Steven Abbot (TCNF/ University of
Leeds)
▫ Hugh Mallett (Buro Happold)
• Background
▫ Gas membrane – break VOC
migration pathways
▫ VOCs migrate through polymeric
materials
▫ Key points for BS8485: 2015 –
specification of membranes for
durability
C748 - Key points ( wrt BS8485)
• Specification of membranes for VOC permeation (many of the
factors are also relevant to methane and carbon dioxide)
• Puncture resistance
• Tear strength]
• Minimum thickness for welding
BS8485 Update
• Scoring system changed
• Changes to building definitions
• Provides a definition of a gas membrane
• Reporting requirements at design, construction and
verification stage
• Revised ventilation design guidance
• Allows use of CLAIRE RB 17 approach (provides some
refinement) – more robust than monitoring on many sites
• Allows more detailed risk assessment to override Gas
Screening Value approach (which is very conservative)
• Worked examples
• Cross reference to C735 and BS8576: 2013
BS8485 – Scoring system
• Scoring system has changed
• More detail and requirements for structural barrier (floor slab),
eg water proof construction split into Grade 1 and 2 to
BS8102: 2009
• Always need “pressure relief via a preferential pathway” – but
this may occur by default for small foot print building
• Only one score for a membrane – it must be installed correctly
and verified in accordance with C735
• Membrane must be sufficiently strong to withstand follow on
trades
Outline of approach• Are there any sources of gas?
• Are there any landfills nearby or
below site?
• Is radon protection already
required?
• Allows for air tight building
construction that is required for
Building Regulations
• Considers organic carbon content
of Made Ground
Summary of possible results
• Gas monitoring not necessary and specific protection
measures not required
• Gas monitoring not necessary but protection
measures required – determine using TOC content of
source if more than 1m of Made Ground present
(TOC not required in Alluvial deposits)
• Gas monitoring required (higher risk sources)
RB17 approach - Alluvium
• If Alluvial soils are source of gas provide CS2
protection
• The only reason for gas monitoring in Alluvial soils is
if you want to demonstrate that gas protection
measures are not required.
• Beware monitoring results in Alluvial soils – often
response zones are flooded and dissolved methane
is main source of gas in wells
• Dissolved methane in Alluvial deposits does not pose
a risk to developments
Landfill mining project
• Excavating a
commercial/industrial landfill and
treating all materials for re-use
on site
• Using CLAIRE RB17 approach
to manage the re-use of
materials
• Using waste treated in site to
manufacture fill materials for the
site
• Different gas potential in
different areas
Testing
• Comprehensive characterisation of waste materials and
properties that affect gas generation
• Drum tests (gas generation tests)
• Flux chamber testing
• Surface emission surveys
• Groundwater testing (inc dissolved gases)
• Pumping tests in wells
• Temperature measurements of fill materials in various
locations
Landfill mining – drum tests
Lessons learnt from the project
• This is probably one of the best experiments looking at gas
generation from low to moderate risk sources
• Further validates RB17 approach using TOC data
• On low risk sites dissolved gas in ground or pore water is a
major influence on results
• Gas monitoring in wells that intercept groundwater will give a
very conservative risk assessment
• No need for “worst case” gas monitoring where wells intercept
the source – other indicators show if high risk or not
• In evaluating risks at the surface there is no inherently unsafe
methane concentration in the ground
• The question is – how fast will gas come out of ground?
Worst case conditions for design
• For wells installed in the gas source (eg made ground) you do
not need gas monitoring to tell what worst case gas
concentrations are
• Worst case is 55% CH4 and 45% CO2 at point of generation
• Any difference to that is due to chemical changes and air
ingress in ground (usually indicating lower risk)
• Normally a few readings from wells within a source are
sufficient to allow assessment of risk
• Do not need extended or more frequent monitoring
Worst case conditions
• Flow rates that occur on opening a well are not representative
of flow from the ground
• Use steady state flow rates, not peak values
• BS8576 recommends using steady state values
• You may need to monitor flow in excess of 5 minutes to obtain
steady state results
• Consider leaving gas taps on monitoring wells open between
visits to obtain representative results
Final thoughts
• Verification of installation of gas protection measures is just
as important as the risk assessment and design
• BS 8485:2015 is available – please use it
• Gas monitoring is not always required – there are
alternative and more robust approaches
Thank You
Contact: [email protected]