Current guidance and good practice Ground gas risk assessment

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Current guidance and good practiceGround gas
risk assessment

• Hugh Mallett
• Buro Happold

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Ground gases Guidance good practice

• Contents
• Guiding principles
• Investigation monitoring
• Data, risk assessment tools remedial strategy
• Case study
• Conclusions

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Ground gases Guidance good practice
Guiding Principles

• Its the economy, stupid

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Ground gases Guidance good practice
Guiding Principles

Its the Conceptual Site Model, stupid
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Ground gases Guidance good practice
Guiding Principles
Phased assessment process Accords with CLR 11.
Set out in C665 Illustrates principal stages of
assessing risks presented by LFG Stage 1 Site
characterisation Stage 2 Assessment of
risk Stage 3 Determination/ validation of
remediation
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Ground gases Guidance good practice
Published Guidance The age of enlightenment ?
Guiding Principles
CIRIA 149. DETR PIT BRE 414 CIRIA
149 NHBC. BS8485 Ground Gas Handbook
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Good practice in ground gas risk assessment

• 2. Guidance - Investigation monitoring

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Ground gases Guidance good practice
Phase 1 - Desk Study Objectives Information
sources Initial conceptual site model Preliminary
risk assessment
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Ground gases Guidance good practice
Phase 2 - Intrusive investigation Objectives Explo
ration techniques intrusive non
intrusive Spacing Design of response
zone Phasing
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Ground gases Guidance good practice

• Phase 2 - Monitoring
• Monitoring frequency/ period
• Well defined protocol
• Correct kit
• Falling / low pressure
• Comprehensive data
• Monitoring pro forma
• Intelligent recording

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Ground gases Guidance good practice
Phase 2 - Sampling Sample analysis Lab
data confirms field results type
source Sampling protocol) Lab techniques (GC,
ICAMS, carbon dating etc.)
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Good practice in ground gas risk assessment

• 3. Data, risk assessment tools remedial design

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Ground gases Guidance good practice
Data assessment Reliability Temporal
variability Consistency Source identified Source
extent defined
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Ground gases Guidance good practice
Data assessment Defined thresholds Gas Screening
Values GSVs Qualifying Criteria Flow rate
measurement Temporal resolution of discrete
measurements Uncertainty
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Ground gases Guidance good practice

• Risk assessment tools
• Guidance
• GQRA CIRIA 552
• Modified Wilson CardCIRIA C665
• NHBC
• BS8485
• Inconsistency confusion?

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Ground gases Guidance good practice

• Risk assessment tools
• Guidance
• GuidanceAdvice / information aimed at
  resolving a problem, difficulty
• CommandmentDivine formal order or instruction
  to obey
• Sensible application of guidance by experienced
  professionals
• Challenge the data to make informed judgement

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Ground gases Guidance good practice

• Risk assessment tools
• CIRIA C665 NHBC
• Calculate GSVs
• Determine Characteristic Situation
• Limiting values
• Consider modifying factors
• Robustness of data
• Source characterisation
• Flow rates
• Development type
• Generic scope protection measures

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Ground gases Guidance good practice

• Risk assessment tools
• BS8485.
• GSV and CS
• Development Sensitivity
• Determine remedial measures scores

CS Private housing Public buildings Commercial Industrial
1 0 0 0 0
2 3 3 2 1
3 4 3 2 2
4 6 5 4 3
5 6 5 4
6 7 6
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Ground gases Guidance good practice

• Risk assessment tools
• BS8485.

• Achieve score by
• Combination of
• Venting/ dilution
• Barriers
• Membranes
• Monitoring detection
• Pathway intervention

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Ground gases Guidance good practice
Risk assessment tools BS8485.

• Tables 2 3 - arbitrary
• Consider moderating factors
• (Annex B)
• Location of source
• Consistency of source
• Risk rating of gas regime
• Pathway permeability
• Foundation conditions
• Complexity of substructure
• Validation of installed measures
• Maintenance requirements
• End use sensitivity
• Use BS scores sensibly

What else can we do?
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Ground gases Guidance good practice
Risk assessment tools Site specific modification
Void space Development type Footprint Vulnerable
space Maximum concentrations Foundation solution
etc
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Ground gases Guidance good practice
Risk assessment tools Mathematical models - Source

• EA Gas GenerationQ10MT/8760 m3/hr
• Surface emission (Peckson)Borehole emission
  rate l/hr/10surface emission rate l/m2/hr
• Atmospheric pressure dropP1.V1P2.V2 Boyles
  Law
• Gas SimSource term emission migration
  exposure
• Ref CIRIA C655 Appendix 5

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Ground gases Guidance good practice
Risk assessment tools Mathematical models -
ventilation
Ventilation capacity of void space BS59251991
Building Regulations Ventilation capacity of
void former Ventilation capacity of active
system Darcys Law Ground Gas Handbook
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Good practice in ground gas risk assessment

• 3. Case study

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Ground gases Guidance good practice

• Site in East London
• 7m Made Ground
• 13 standpipes
• Majority 2 readings
• Elevated gas 50 occasions
• CH4 7 CO2 12 Flow 1l/hr
• GSV 0.05 CS1

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Ground gases Guidance good practice

• But
• Very limited data set
• 50 readings gt limiting criteria
• Gas more elevated in east ?
• Thickness/ nature of MG
• Sensitivity of development
• Determine Design classification as CS2 (west
  zone) and CS3 to east

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Ground gases Guidance good practice

• Gas Protection measures
• CIRIA Slab, membrane venting
• BS 8485. Score 3.5 against target 2.
• Check ventilation
• a) Required flow 60m3/hrDarcy Equil Conc
  lt1
• b) Reqd ventilation area 570mm2/m
• B Regs 1500mm2/m
• Min spacing of vents 45m
• Risk dead spots
• Design spacing 25m
• c) Flow thro void former30x gt reqd flow.
• 4. Peer review
• 5. Scrutiny by LPA expert

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Ground gases Guidance good practice

• Series of Electricity Sub Stations
• GSV 2.2 (methane) - CS2/3
• Standard GRP enclosure
• Massive concrete slab (part)
• Service entries in side wall
• Ventilation for cooling
• Spark suppression
• GSV emission rate
• gtgtBuilding Regs ventilation rate
• Time to reach 5 CH4 in still conditions
• Ventilation requirement vs ventilation achieved
  by standard construction
• No specific additional gas protection
• Approved by LPA expert

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Ground gases Guidance good practice
After all of this investigation, data collection
and complex risk assessment
Do not forget the importance of verification
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Ground gases Guidance good practice

• Verification

The QA / inspection process Contractor employed
specialist supplier and installer Check sheet
system Prior to concrete pour Each section
Inspected Signed off by Installer Buro
Happold Main Contractor Check Sheets and
photographic record Appendix in verification
report
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Ground gases Guidance good practice

• Concluding comments
• Understanding CSM is critical
• Reliable data key component
• Generic models site specific modification
• Combine systems to achieve BS score
• Math tools aid understanding/ increase
  confidence. A tool to help you make the decision.
• Site verification
• Application requires thoughtThe strength of
  the Guidance is that it still relies upon
  judgement

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Ground gases Guidance good practice

• References
• CIRIA 149. 1995. Protecting development from
  methane
• DETR PIT 1997. Passive venting of soil gases
  beneath buildings
• BRE 414. 2001. Protective measures for housing
  on gas contaminated land
• CIRIA C665. 2007. Assessing risks posed by
  hazardous ground gases to buildings
• NHBC. 2007. Guidance on evaluation of development
  proposals in sites where methane and carbon
  dioxide are present
• BS8485. 2007. Code of practice for the
  characterisation and remediation from ground gas
  in affected developments
• Wilson Card Haines. 2008. The Ground Gas
  Handbook