Current guidance and good practiceGround 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

“It’s the economy, stupid”

Guiding Principles

Ground gases – Guidance & good practice

“It’s the Conceptual Site Model, stupid”

Guiding Principles

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

Good practice in ground gas risk assessment

2. Guidance - Investigation & monitoring

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

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

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

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 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