Current Guidence and Good Practice - Hugh Mallett
-
Upload
shingkeong -
Category
Documents
-
view
20 -
download
1
description
Transcript of Current Guidence and Good Practice - Hugh Mallett
Ground gases – Guidance & good practice
Contents
1. Guiding principles
2. Investigation & monitoring
3. Data, risk assessment tools & remedial strategy
4. Case study
5. Conclusions
Undertake additionalintrusive
investigationsand/ or
monitoring & sampling
No
Review data
Is data reliable? (e.g. appropriate response zones,
variable/ unrepresentative groundwater levels)
Has monitoring been carried out
under varying conditions likely
to influence the gas vapour regime?
Results sufficientlyconsistent/ reliable?
Source(s) ofgas(es)/ vapour(s)
identified?
Extent of source(s) established?
Refine conceptual site model
Consider odour & toxicity and incorporate
into risk assessment as appropriate
Undertake appropriate risk assessment modelling. Define gas
regime
Does risk assessmentdemonstrate
corrective actionrequired?
No
No
No
No
Yes
No
Yes
Yes
Yes
Yes
Yes
Amber Red
NO FURTHER ACTION REQUIRED
Green
Define the context & set the objectives
Carry out Phase One Desk Study
Develop initial conceptual site model& undertake preliminary risk assessment
Has development of modelincluded site specific factors
that may influence gas/vapour regime
Are there potential unacceptable risks?
Identify further actions to clarify potential unacceptable risks
Establish objectives of any further investigations
Carry out further investigation[desk based/intrusive/ Monitoring]
NO FURTHER ACTION REQUIRED
Yes
No
Yes
Identify remedial objectives to mitigate
unacceptable risks
Identify feasible remedial options
Has sufficientdata been
obtained to allowthe selection/ design
of appropriateremedial
solutions?
Undertake additional intrusive
investigations
Detailed evaluation of remedial options
Develop a remedial strategy
Design, implementation & verification of
remedial measures
Is post installation/ construction
monitoring required?
Undertake post installation/construction monitoring
Is the monitoringdata acceptable?
Yes
No
No
Yes
Yes
NO FURTHER ACTION REQUIRED
Site Characterisation Assessment of Risk Determination / Validation of Remediation
Have these factors identified the
potential presence of gas/vapour?
Yes
No
No
Completion/Validation Report
Figure 1. The Process of Managing Risks Related to Hazardous Ground Gases
No
Review/amend
remedial strategy
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
Ground gases – Guidance & good practice
Guiding Principles
CIRIA 149.
DETR [PIT]
BRE 414
CIRIA 149
NHBC.
BS8485
Ground Gas Handbook
Published GuidanceThe age of enlightenment ?
Ground gases – Guidance & good practice
Guiding Principles
Phase 1 - Desk Study
Objectives
Information sources
Initial conceptual site model
Preliminary risk assessment
Ground gases – Guidance & good practice
Phase 2 - Intrusive investigation
Objectives
Exploration techniques
[intrusive & non intrusive]
Spacing
Design of response zone
Phasing
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
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.)
Ground gases – Guidance & good practice
Ground gases – Guidance & good practice
Data assessment
Reliability
Temporal variability
Consistency
Source identified
Source extent defined
Ground gases – Guidance & good practice
Data assessment
Defined thresholds
Gas Screening Values [GSVs]
Qualifying Criteria
Flow rate measurement
Temporal resolution of discrete
measurements
Uncertainty
-10
0
10
20
30
40
50
60
28 30 32 34 36 38 40
Day
%
/
Cel
siu
s CH4
CO2
Temp
O2
Temp
Ground gases – Guidance & good practice
Risk assessment tools
Guidance
GQRA [CIRIA 552]
Modified Wilson & Card
[CIRIA C665]
NHBC
BS8485
Inconsistency & confusion?
Ground gases – Guidance & good practice
Risk assessment tools
Guidance
Guidance:“Advice / information aimed at resolving a problem, difficulty”
Commandment:“Divine formal order or instruction to obey”
Sensible application of guidance by experienced professionals
Challenge the data to make informed judgement
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
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
Ground gases – Guidance & good practice
Risk assessment toolsBS8485.
Achieve score by
Combination of;
1. Venting/ dilution
2. Barriers
3. Membranes
4. Monitoring & detection
5. Pathway intervention
Ground gases – Guidance & good practice
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
Ground gases – Guidance & good practice
Risk assessment toolsBS8485.
What else can we do?
Ground gases – Guidance & good practice
Void space
Development type
Footprint
Vulnerable space
Maximum concentrations
Foundation solution
etc
Risk assessment toolsSite specific modification
Ground gases – Guidance & good practice
1. EA Gas Generation[Q=10MT/8760 m3/hr]
2. Surface emission (Peckson)[Borehole emission rate l/hr/10=surface emission rate l/m2/hr]
3. Atmospheric pressure dropP1.V1=P2.V2 [Boyle’s Law]
4. Gas SimSource term; emission; migration; exposure
Ref CIRIA C655 Appendix 5
Risk assessment toolsMathematical models - Source
Ground gases – Guidance & good practice
Ventilation capacity of void space
BS5925:1991 Building Regulations
Ventilation capacity of void former
Ventilation capacity of active system
Darcy’s Law
Ground Gas Handbook
Risk assessment toolsMathematical models - ventilation
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
Ground gases – Guidance & good practice
But
• Very limited data set
• 50% readings > 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
Ground gases – Guidance & good practice
Gas Protection measures
1.CIRIA – Slab, membrane +venting
2.BS 8485. Score 3.5 against target 2.
3.Check ventilation
a) Required flow 60m3/hr[Darcy – Equil Conc <1%]
b) Req’d ventilation area 5+70mm2/m
[B Regs 1500mm2/m]
Min spacing of vents = 45m
Risk “dead spots”
Design spacing 25m
c) Flow thro void former30x > req’d flow.
4. Peer review
5. Scrutiny by LPA expert
Ground gases – Guidance & good practice
Series of Electricity Sub Stations
1.GSV 2.2 (methane) - CS2/3
2.Standard GRP enclosure
3.Massive concrete slab (part)
4.Service entries in side wall
5.Ventilation for cooling
6.Spark suppression
7.GSV & emission rate
8.>>Building Regs ventilation rate
9.Time to reach 5% CH4 in still conditions
10.Ventilation requirement vs ventilation achieved by standard construction
No specific additional gas protection
Approved by LPA expert
Ground gases – Guidance & good practice
Do not forget the importance of verification
After all of this investigation, data collection and complex risk assessment …
Ground gases – Guidance & good practice
Verification
The QA / inspection processContractor employed specialist supplier and installerCheck sheet systemPrior to concrete pourEach section Inspected & Signed off by:
InstallerBuro HappoldMain Contractor
Check Sheets and photographic record – Appendix in verification report
Ground gases – Guidance & good practice
Concluding comments
1. Understanding CSM is critical
2. Reliable data – key component
3. Generic models – site specific modification
4. Combine systems to achieve BS “score”
5. Math tools – aid understanding/ increase confidence. A tool to help you make the decision.
6. Site verification
7. Application requires thought[“The strength of the Guidance is that it still relies upon judgement”]
References
1. CIRIA 149. 1995. Protecting development from methane
2. DETR [PIT] 1997. Passive venting of soil gases beneath buildings
3. BRE 414. 2001. Protective measures for housing on gas contaminated land
4. CIRIA C665. 2007. Assessing risks posed by hazardous ground gases to
buildings
5. NHBC. 2007. Guidance on evaluation of development proposals in sites where
methane and carbon dioxide are present
6. BS8485. 2007. Code of practice for the characterisation and remediation from
ground gas in affected developments
7. Wilson Card & Haines. 2008. The Ground Gas Handbook
Ground gases – Guidance & good practice