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: stevewilson@epg-ltd.co.uk