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Current guidance and good practice Ground gas risk assessment Hugh Mallett Buro Happold.

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Presentation on theme: "Current guidance and good practice Ground gas risk assessment Hugh Mallett Buro Happold."— Presentation transcript:

1 Current guidance and good practice Ground gas risk assessment Hugh Mallett Buro Happold

2 Ground gases – Guidance & good practice Contents 1.Guiding principles 2.Investigation & monitoring 3.Data, risk assessment tools & remedial strategy 4.Case study 5.Conclusions

3 Ground gases – Guidance & good practice “It’s the economy, stupid” Guiding Principles

4 Ground gases – Guidance & good practice “It’s the Conceptual Site Model, stupid” Guiding Principles

5 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

6 CIRIA 149. DETR [PIT] BRE 414 CIRIA 149 NHBC. BS8485 Ground Gas Handbook Published Guidance The age of enlightenment ? Ground gases – Guidance & good practice Guiding Principles

7 Good practice in ground gas risk assessment 2. Guidance - Investigation & monitoring

8 Phase 1 - Desk Study Objectives Information sources Initial conceptual site model Preliminary risk assessment Ground gases – Guidance & good practice

9 Phase 2 - Intrusive investigation Objectives Exploration techniques [intrusive & non intrusive] Spacing Design of response zone Phasing Ground gases – Guidance & good practice

10 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

11 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

12 Good practice in ground gas risk assessment 3. Data, risk assessment tools & remedial design

13 Ground gases – Guidance & good practice Data assessment Reliability Temporal variability Consistency Source identified Source extent defined

14 Ground gases – Guidance & good practice Data assessment Defined thresholds Gas Screening Values [GSVs] Qualifying Criteria Flow rate measurement Temporal resolution of discrete measurements Uncertainty

15 Ground gases – Guidance & good practice Risk assessment tools Guidance GQRA [CIRIA 552] Modified Wilson & Card [CIRIA C665] NHBC BS8485 Inconsistency & confusion?

16 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

17 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

18 Risk assessment tools BS8485. GSV and CS Development Sensitivity Determine remedial measures scores CS Private housing Public buildings CommercialIndustrial Ground gases – Guidance & good practice

19 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

20 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 tools BS8485. What else can we do?

21 Ground gases – Guidance & good practice Void space Development type Footprint Vulnerable space Maximum concentrations Foundation solution etc Risk assessment tools Site specific modification

22 Ground gases – Guidance & good practice 1.EA Gas Generation [Q=10MT/8760 m 3 /hr] 2.Surface emission (Peckson) [Borehole emission rate l/hr/10=surface emission rate l/m 2 /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 Risk assessment tools Mathematical models - Source

23 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 tools Mathematical models - ventilation

24 Good practice in ground gas risk assessment 3. Case study

25 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

26 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

27 Ground gases – Guidance & good practice Gas Protection measures 1.CIRIA – Slab, membrane +venting 2.BS 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

28 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

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

30 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

31 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”]

32 References 1.CIRIA Protecting development from methane 2.DETR [PIT] Passive venting of soil gases beneath buildings 3.BRE Protective measures for housing on gas contaminated land 4.CIRIA C Assessing risks posed by hazardous ground gases to buildings 5.NHBC Guidance on evaluation of development proposals in sites where methane and carbon dioxide are present 6.BS Code of practice for the characterisation and remediation from ground gas in affected developments 7.Wilson Card & Haines The Ground Gas Handbook Ground gases – Guidance & good practice


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