Current guidance and good practice
Ground gas risk assessment
Hugh Mallett
Buro Happold
Ground gases – Guidance & good practice
Contents
1. Guiding principles
2. Investigation & monitoring
3. Data, risk assessment tools & remedial strategy
4. Case study
5. Conclusions
Ground gases – Guidance & good practice
Guiding Principles
“It’s the economy, stupid”
Ground gases – Guidance & good practice
Guiding Principles
“It’s the Conceptual Site Model, stupid”
Ground gases – Guidance & good practice
Figure 1.
Guiding Principles
The Process of Managing Risks Related to Hazardous Ground Gases
Site Characterisation
Assessment of Risk
Define the context & set the objectives
Determination / Validation
of Remediation
Identify remedial
objectives to mitigate
unacceptable risks
Review data
Carry out Phase One Desk Study
Identify feasible remedial options
Is data reliable?
(e.g. appropriate
response zones,
variable/ unrepresentative
groundwater levels)
Develop initial conceptual site model
& undertake preliminary risk assessment
Phased assessment process
No
Undertake
additional
intrusive
investigations
Yes
No
Has development of model
included site specific factors
that may influence
gas/vapour regime
Has monitoring
been carried out
under varying
conditions likely
to influence the gas
vapour regime?
No
Undertake
additional
intrusive
investigations
and/ or
monitoring
& sampling
Yes
Accords with CLR 11. Set out in C665
Yes
Yes
Yes
Detailed evaluation of remedial
options
Source(s) of
gas(es)/ vapour(s)
identified?
No
Develop a remedial strategy
Yes
Yes
No
Design, implementation &
verification of
remedial measures
No
Extent of
source(s) established?
Yes
Are there potential
unacceptable risks ?
Refine conceptual site model
No
Stage 1: Site characterisation
No
No
Results sufficiently
consistent/ reliable?
Have these factors
identified the
potential presence
of gas/vapour?
No
Illustrates principal stages of assessing
risks presented by LFG
Has sufficient
data been
obtained to allow
the selection/ design
of appropriate
remedial
solutions?
Consider odour & toxicity and
incorporate
into risk assessment as appropriate
Yes
Review/
amend
remedial
strategy
Is post installation/
construction
monitoring required?
Yes
Identify further actions to clarify
potential unacceptable risks
Undertake appropriate risk
assessment modelling. Define gas
regime
Undertake post installation/
construction monitoring
Establish objectives of any
further investigations
Stage 2: Assessment of risk
Green
No
Stage 3: Determination/ validation of
remediation
Amber
Red
Is the monitoring
data acceptable?
Carry out further investigation
[desk based/intrusive/ Monitoring]
Does risk
assessment
demonstrate
corrective action
required?
Yes
Completion/Validation Report
Yes
NO FURTHER
ACTION REQUIRED
NO FURTHER
ACTION REQUIRED
NO FURTHER
ACTION REQUIRED
No
Ground gases – Guidance & good practice
Guiding Principles
Published Guidance
The age of enlightenment ?
CIRIA 149.
DETR [PIT]
BRE 414
CIRIA 149
NHBC.
BS8485
Ground Gas Handbook
Good practice in ground gas risk
assessment
2. Guidance - Investigation & monitoring
Ground gases – Guidance & good practice
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.)
Good practice in ground gas risk
assessment
3. Data, risk assessment tools & remedial
design
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
60
50
Gas Screening Values [GSVs]
Celsius
CH4
30
CO2
Temp
/
Qualifying Criteria
40
O2
%
20
Flow rate measurement
Temp
10
0
Temporal resolution of discrete
28
30
32
34
-10
Day
measurements
Uncertainty
36
38
40
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
Ground gases – Guidance & good practice
Risk assessment
tools
BS8485.
CS
Private
housing
Public
buildings
Commercial
Industrial
1
0
0
0
0
Development Sensitivity
2
3
3
2
1
Determine remedial
3
4
3
2
2
measures scores
4
6
5
4
3
6
5
4
7
6
GSV and CS
5
6
Ground gases – Guidance & good practice
Risk assessment tools
BS8485.
Achieve score by
Combination of;
1.
Venting/ dilution
2.
Barriers
3.
Membranes
4.
Monitoring & detection
5.
Pathway intervention
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?
Ground gases – Guidance & good practice
Risk assessment tools
Site specific modification
Void space
Development type
Footprint
Vulnerable space
Maximum concentrations
Foundation solution
etc
Ground gases – Guidance & good practice
Risk assessment tools
Mathematical models - Source
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 drop
P1.V1=P2.V2 [Boyle’s Law]
4. Gas Sim
Source term; emission; migration;
exposure
Ref CIRIA C655 Appendix 5
Ground gases – Guidance & good practice
Risk assessment tools
Mathematical models - ventilation
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
Good practice in ground gas risk
assessment
3. Case study
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 former
30x > 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
After all of this
investigation, data
collection and complex
risk assessment …
Do not forget the importance of verification
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
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”]
Ground gases – Guidance & good practice
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