Presentation on theme: "Gas Protection- What is needed"— Presentation transcript:
1Gas Protection- What is needed Gas Protection- What is needed? Selected case studies of some common problems and pragmatic solutionsJo Strange MEng, C Eng C Env, MICE, MIEnvSci, SiLCRegional Director, Card Geotechnics Ltd
2IntroductionDespite 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.
3Typical Basic Issues Much guidance- but lack of understanding, Poor SI specification eg installation detailsPoor quality data- data acquired during poor monitoring conditions, missing flow dataLack of site model or understanding of gassing regimeOmitted gas protection measuresOver design or poor design of gas protection measures
4Misinterpretation 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 GroupGroundwater in borehole 6.1mbglShallow 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 wellBUT……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 installationREVISED CONCLUSIONNo gas protection measures required (CS 1 applies)
6Result Reduction in requirements for specific gas protection measures. Therefore…..Cost and construction time saving for Client.
7Interpretation of Anomalous data: Example- Residential Development in vicinity of closed landfill site, HampshireSite 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.
8Cont’dNumerous 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.
9Cont’dGeology confirmed as Wittering Formation of the Bracklesham Beds (typically silt and CLAY) overlying Whitecliff SandStrata dip towards the southGroundwater was at depth, but anecdotal evidence of large local fluctuations in groundwater level.
10Clarification of Ground Gas Regime Gas was migrating from unlicensed extension to landfill- concentrations decreasing with increasing distance from landfillMigration occurred effectively within the Whitecliff SandsGases confined below the Wittering FormationGases 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)
12SolutionConfirmation 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 membranePassively vented under floor voidsMonitoring to confirm gas regime above and below clay mantle.
13ResultLocal Authority agreed gas regime and revised proposed gas protection measuresMonitoring confirmed the different gas regimes above and below the clay mantleDevelopment completed and occupied
14Missing Gas Protection Measures This happens not infrequentlyOccasionally due to architect/structural engineer being unaware of requirement for gas protection measures and nothing shown on construction drawingsMore often due to lack of attention to detail by contractor operative under pressureOften 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!
15Use 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.
16Site ConditionsReview 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.
17Table 5.1–Classification of risk, CIRIA 152, 1995.
18Risk AssessmentRisk assessment carried out using CIRIA 152 fault tree analysis for a range of scenarios e.g. explosion or asphyxiationSite 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
19Figure 5.2–Classification of risk, CIRIA report 152
20Result 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 zonesatisfied local authority.
21Retrofit 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.
22SolutionGiven 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.
23Liquid 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.
24Poor 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 holidayRequest from client to advise on building occupancy. (CARD not involved in investigation or design)
25Site AssessmentInvestigation information indicated 7.05 l/hr per sq metreIdentified 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.
27What 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.
29Solution Provide temporary seals to all service ducts and drainage. Establish regular monitoring regimeDetermine key areas for permanent methane monitoringFalse floorsSwitch roomsCarry out rigorous survey of service ducts and provide internal two phase gas seal.Continue to monitor
30Result Site made safe for occupancy Site management scheme set up with permanent monitoring of critical zonesClient confidence restored