1. Gas Protection- What is needed
Gas Protection- What is needed? Selected case studies of some common problems and pragmatic solutions
Jo Strange MEng, C Eng C Env, MICE, MIEnvSci, SiLC
Regional Director, Card Geotechnics Ltd

2. Introduction
Despite a plethora of guidance, there are still a number of issues which are commonly seen which lead to either poor or over conservative design of gas protection measures for buildings.
The following case studies identify some of those issues and how they were overcome to provide a pragmatic solution.

3. Typical Basic Issues Much guidance- but lack of understanding,
Poor SI specification eg installation details
Poor quality data- data acquired during poor monitoring conditions, missing flow data
Lack of site model or understanding of gassing regime
Omitted gas protection measures
Over design or poor design of gas protection measures

4. Misinterpretation of Data: Example – Residential Development South East London
Site geology comprises typically non-organic Made Ground (gravelly clay or sand & brick) over Alluvium, River Terrace Deposits and Lambeth Group
Groundwater in borehole 6.1mbgl
Shallow well response zone sealed within MG and Alluvium (Max CH4 =0.1%v/v, Max CO2= 3.4% Max Flow 0.5 l/hr )
Deep well response zone sealed within River Terrace Deposits (9-13mbgl) (Max CO2= 3.9%v/v, Max CH4=3.4% Max Flow 0.1 l/hr

5. …cont’d ORIGINAL CONCLUSION
Gas protection required to Characteristic Situation 2(CS2)_ (CIRIA 665), based on max CO2 in deep well
BUT……
Based on well construction, where is gas source in deep well ?
Gas caused by anaerobic reaction within closed void – typified by CH4/CO2 with no flow, in waterlogged well with no gassing source.
Not a sustained source- gas resulting from borehole installation
REVISED CONCLUSION
No gas protection measures required (CS 1 applies)

6. Result Reduction in requirements for specific gas protection measures.
Therefore…..
Cost and construction time saving for Client.

7. Interpretation of Anomalous data: Example- Residential Development in vicinity of closed landfill site, Hampshire
Site being developed on farm land close to closed landfill with active extraction system.
Farm land between landfill and development to be handed over to local authority as Public Open Space, as part of planning negotiations.
Unlicensed landfilling identified outside landfill boundary.
Local authority required robust reassurance that appropriate gas protection measures were proposed.

8. Cont’d
Numerous investigations undertaken and gas protection measures to CS 3 proposed by developer’s consultant.
Local Authority queried anomalous data, which included:
Extremely high flows on occasions to south of site, with little methane and carbon dioxide, and IN SAME BOREHOLES, negligible flow and elevated methane and carbon dioxide.
Local Authority refused to accept proposed design until anomalies explained.

9. Cont’d
Geology confirmed as Wittering Formation of the Bracklesham Beds (typically silt and CLAY) overlying Whitecliff Sand
Strata dip towards the south
Groundwater was at depth, but anecdotal evidence of large local fluctuations in groundwater level.

10. Clarification of Ground Gas Regime
Gas was migrating from unlicensed extension to landfill- concentrations decreasing with increasing distance from landfill
Migration occurred effectively within the Whitecliff Sands
Gases confined below the Wittering Formation
Gases pressurised by fluctuating groundwater, causing high flow rates, increasing to south of site due to ‘funnelling effect’.
Radiocarbon dating showed gas in Whitecliff Sand to be from a modern anthropogenic source, ie landfill.
Similar testing showed gas in Wittering Clay to south of site from a geological source i.e. derived from biological reactions in the soil. (Gas generation caused when risen groundwater sealed base of monitoring well, hence zero associated flows)

11. Ground Gas Model

12. Solution
Confirmation of a minimum thickness of clay mantle between the base of the foundations and the underlying sand (requiring make up in some areas from a borrow pit on site)
Gas protection measures to CS2 to be installed continuous, robust gas membrane incorporated within the floor slab construction to mitigate any residual gas.
Penetration of the membrane limited by design - service entries were limited. All service penetrations were sealed through the membrane
Passively vented under floor voids
Monitoring to confirm gas regime above and below clay mantle.

13. Result
Local Authority agreed gas regime and revised proposed gas protection measures
Monitoring confirmed the different gas regimes above and below the clay mantle
Development completed and occupied

14. Missing Gas Protection Measures
This happens not infrequently
Occasionally due to architect/structural engineer being unaware of requirement for gas protection measures and nothing shown on construction drawings
More often due to lack of attention to detail by contractor operative under pressure
Often missed when no requirement for environmental engineer to be present on site to observe/verify incorporation of gas protection measure.
Solution usually depends on when the error is found!

15. Use of Risk Assessment: Example: Mixed residential/commercial development, East London
Site located on ex-industrial site, with a buried oil tank.
Limited gas monitoring pre-construction indicated worst case CS4 conditions, strictly requiring a gas resistant membrane and underslab passive ventilation.
During construction, the passive ventilation was omitted.
CARD commissioned to undertake a risk assessment to confirm whether development was safe without ventilation for submission to Planning Authority.

16. Site Conditions
Review of data showed that gas concentrations assessed for individual blocks generally fell within CS1. Data for one block fell within CS2. (Higher category was due to limited time frame for monitoring.)
Since design of development, remediation works completed to remove buried fuel tank and surrounding impacted soil in location where worst gas conditions measured.
Up to 2m of Made Ground had been removed from site during archaeological dig and replaced with granular fill.

17. Table 5.1–Classification of risk, CIRIA 152, 1995.

18. Risk Assessment
Risk assessment carried out using CIRIA 152 fault tree analysis for a range of scenarios e.g. explosion or asphyxiation
Site zoned according to location specific worst case data.
Account taken of changes to ground conditions.
Account taken of construction methodologies ie piled raft foundations, membrane installed and building dimensions

19. Figure 5.2–Classification of risk, CIRIA report 152

20. Result the highest risk of explosion is in the smallest confined space
the highest risk of asphyxiation relates to the long term exposure within the small bedroom.
All of the scenarios calculated probability of risk scenario as not exceeding 1 in 1 million (i.e. 1x10-6), (the accepted tolerable level.)
Able to reduce required gas protection to use of waterproofing membrane only in worst case zone
satisfied local authority.

21. Retrofit of Gas Protection Measures: Example- Commercial building, Portsmouth
Due to technical problems encountered during construction of the lift pits, the membrane was not installed prior to casting the concrete.
This came to light when verification inspections were requested by the contractor.
It was not possible to install gas resistant membranes on the inside of the lift pit due to the lack of spatial tolerances.

22. Solution
Given the site constraints, a liquid membrane solution was recommended, (Tecroc Liquid Vapour Membrane, LVM)
Four coats of the LVM (equivalent to specified sheet membrane) was applied to all internal surfaces of the lift pit, tying into the gas membrane at ground level, to achieve the required gas permeability standards.
This approach was approved by Portsmouth City Council Contaminated Land team.

23. Liquid membrane – In situ
A clever touch:
The LVM comes in two colours, black and white, so that each layer can be clearly seen as it covered by the next layer in the alternative colour.

24. Poor design: Example- Commercial development in Hampshire
Building constructed adjacent to landfill site incorporating ‘gas protection measures’ was at commissioning stage.
Gas concentration of between 25% and 113% of LEL in switch cupboards after New Year holiday
Request from client to advise on building occupancy. (CARD not involved in investigation or design)

25. Site Assessment
Investigation information indicated 7.05 l/hr per sq metre
Identified concentrations of 47.7% methane and 24% carbon dioxide.
Concentrations changing over tidal cycle.
Design used Somtube connected to manifold and cowls on chimneys on one side.
Inlet at low level on side nearest to landfill.

26. Design drawing

27. What did we find?
Design did not permit the dilution approach under low flow or no flow conditions.
Identified approximately 150 unsealed service entry points.
Degree of protection relied on quality of pre cast slab and membrane.

28. Not quite a diluting ventilation layer….

29. Solution Provide temporary seals to all service ducts and drainage.
Establish regular monitoring regime
Determine key areas for permanent methane monitoring
False floors
Switch rooms
Carry out rigorous survey of service ducts and provide internal two phase gas seal.
Continue to monitor

30. Result Site made safe for occupancy
Site management scheme set up with permanent monitoring of critical zones
Client confidence restored

31. Thank you for listening