Q&A with Steve Wilson, Technical Director,
The Environmental Protection Group Ltd (EPG)
Following on from our recent webinar Mine Gas Risk Assessment: Best Practice
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Q. If shallow workings are grouted how is there still a pathway for mine gas?
A. Grouting has been used in the past to reduce gas migration from coal mine workings. The pathway should be reduced or removed because the volume available for gas to accumulate is reduced and the pathway area is reduced. However there may still be residual voids and the risk of this is greater around the perimeter of the grouting. It requires specific verification which would include more test wells than normal after completion of the grouting works and in some cases surface emissions surveys at falling atmospheric pressure over the completed site. The grouting should also extend well beyond the building perimeter.
Q. The high risk site example indicated workings had ceased around 1870. Does BS5930 not indicate that such age of workings are lower risk?
A. The older the workings the less risk there is with respect to methane desorption. There is negligible risk of this from workings over 100 years old. However oxidation of coal can still occur and generate carbon dioxide. I am not aware of any advice on the age workings in BS5930.
Q. What is the best approach to understanding how compression of ground material at a site will impact mine gas behaviour
A. There is no standard approach. The rate of compression has to be assessed and whether the ground collapse is likely to change migration pathways.
Q. Do you use the Coal Authorities Gas Data at all?
A. We have not used it to date, because they have not had any data for the sites we have requested it for. We do ask for it though where it may be useful. The Coal Authority is a valuable resource that we should all make more use of.
Variations in groundwater can push gas out of shallow workings in some cases but it is site specific and should not just be assumed to occur. It is however an important consideration that should be assessed for all sites (even with shafts into deep workings). It will depend on the magnitude of the water level variation in relation to the seam level and the likely gas generation/recharge rate in between water level fluctuations.
Q. Would we see similar effects in unlined landfill sites situated on permeable deposits? And your guidance to not rely only on gas monitoring data?
A. You can see similar effects on unlined landfill sites but it usually not significant if there are no open pathways. The void space for gas to accumulate is a lot smaller and the permeability of the pathway is lower than flow from mineworkings via fractured rock or shafts, etc
Q. Does the water table need to be intercepted to understand influence on gas migration, as some consultants only undertake investigations above the water table in permeable deposits and near a gas source?
A. Gas monitoring wells should always be installed in the unsaturated zone above the groundwater, otherwise both the gas concentrations and flow rates can be unrepresentative of the surrounding ground.
Q. Can we read your expert witness report for Gorebridge?
A. No it is confidential
Q. Could you give the title of the RSK report which noted 0% incidents associated with Deep mine workings?
A. Prevalence of CO2 from disused mineral mines and the implications for residential buildings: research. It is available here.
Q. Do you ever have in issues with benzene release?
A. I have not come across this from mineworkings.
Q. How do you define deep and shallow workings?
A. Shallow is less than 30m. Deep is greater than 150m. There is a grey area in between that is difficult to define but we are trying to provide more definitive guidance on that.
Q. In what conditions does coal self heat, then self combust?
A. Coal self heats when it is exposed to a sufficient supply of oxygen. It also depends on the quality of the coal (higher quality coals are less likely to self heat). There are some seams that are known to be more susceptible to self heating and combustion than others. From the moment that coal is exposed to air, it is subject to low temperature oxidation (weathering) by atmospheric oxygen. This process is exothermic and if the heat cannot dissipate quickly enough then self heating and ultimately combustion occurs
Q. What tools do you use to model gas risk?
A. We do not use any tools as such. Most of what we do is apply good practice with an understanding of the generation and migration pathways with sound application of judgement. Most of our time is spent at the desk study/preliminary risk assessment stage and assessment of data (looking for trends and relationships with other factors) and data quality. We do use in house models but these are generally adjusted on a site specific basis to suit the migration pathway and building construction.
Q. Steve do you stand by your title of BEST practice or is what you have shared GOOD practice - ie what everybody should be doing all the time?
A. A good point - it should be good practice because this is what a competent risk assessor should be considering for mine gas assessment.
Q. What technology exists to collect continuous flow data?
A. The two most common instruments are the Gas Sentinel and the Ambisense GasfluX units.
Q. How can standards be improved in GRA be improved?
A. Personally I think that ground gas risk assessment and mitigation design should only be signed of by Chartered Professionals with appropriate experience. Society of Brownfield Risk Assessment accredited risk assessors or similar (SiLC or RoGEP) would be a good start. We really need to start to crack down on the poor assessments and designs in the industry that are completed by people without the required qualifications or experience. There are also too many RIBA accredited CPD courses delivered by people from gas membrane suppliers that stray into gas risk assessment and we need to stop that as well.
Q. Installing within shallow worked coal seam will change the gas regime, can you comment on this please. Also, to play devil's advocate, do shallow installations measure closer to the receptor and therefore no need for deeper wells installed in shallow worked seams?
A. That is a good question and observation. It could change the gas regime because it could introduce oxygen locally and cause oxidation of the coal and increased CO2 which may not occur of the well was not present. We see this in soils so it will occur in workings as well. It is very likely that the well will act as a chimney or shaft from the workings and cause elevated flows. You then have to decide from the CSM if that is representative of what is likely to happen in the surrounding ground. That is why using the CSM to assess the data is so important and why using GSVs or the points system for mitigation design in BS8485 is not applicable to gas from mine workings.
Q. Should a gas risk assessment be completed for grouting works given this could cause a problem on and off a site as a result?
Q. When should a gas risk assessment over the coalfield areas require a quantitative assessment?
A. We do not have a definitive answer to that yet and are working on it. However as a general rule if there are shafts/adits within 50m, shallow workings less than 30m that are not permanently flooded it is likely to be necessary. At the other end of the scale it is not necessary where workings re greater than 150m depth, more than 50m from shafts or where workings are permanently flooded.
Q. Some deep foundations can act as a pathway presumably all mine gas risk assessments take account of this and perhaps exclude some from consideration in some circumstances?
A. That is a good point and is one of the considerations in the assessment process. The main problem is with vibro replacement (stone columns). Piled foundations are not likely to form pathways unless they penetrate a thin confining layer that is trapping large volumes of gas below it. See Wilson and Mortimer, Environmental Geotechnics, 2017. It has been suggested by others that whiplash in driven piles can cause a pathway but whiplash only affects a length that is about 2 or 3 diameters from the top of the pile so is not likely to cause a pathway unless a thin confining layer is breached and gas is trapped below it.
Q. Does investigating gas risk actually increase it?
A. It may do if the wells are not sealed afterwards. All boreholes should be backfilled with grout that is properly mixed and tremied into the hole. The monitoring wells should be decommissioned afterwards by filling the pipe with grout. This still leaves the gravel annulus and so the top of the well should be cut off at 1m or 2m below proposed founding level and the hole should be filled with bentonite cement (this is to seal the gravel annulus). The alternative is to extract the well pipe and gravel and then grout up but this is generally not practical.
Q. Presumably the risk assessment is also based on foundation solutions, i.e. piles?
Yes - see above. It also needs to take account of deep drainage that may penetrate any drift deposits over the coal workings
Q. In relation to an extension within 5m of an untreated mine shaft would your recommend any contaminated land conditions are attached relating to mine gas?
A. A good mining desk study would be appropriate as a first stage and followed up by SI if needed. The position of the shaft on the Coal Authority records is +/- 5m in any direction so it could be under the foundations for the extension. Stability is as much an issue as gas.
Q. Do you believe that any submission in CM areas/geology should be rejected if a CA Consultants Reports is not provided (i.e. it should be Mandatory)?
A. Yes. A competent consultant assessing mine gas risk (or to be honest stability issues as well) should be using the Consultants report.
Q. What is the time frame for measurement in order to produce assessment document, continuous what time frame against portable monitoring?
A. You want at least 3 weeks normally and that will usually give you 4 pressure drops that will allow reltaionships to be identified (if there are any). CLAIRE Technical Bulletin TB17 Ground Gas Monitoring and ‘Worst-Case’ Conditions gives more information monitoring.
Q. Would backfilling a borehole with cement/ bentonite grout over the depth of the superficial soils (where drift is >5m) and plugging the borehole at rockhead provide a suitable seal?
A. This has been common practice in the past in some areas of the UK. However the risk is that the seal fails and the grout sinks in the hole. Some believe that this was a contributory cause to the Gorebridge incident in Scotland. However in my opinion it was a minor contributor (if indeed the seals had actually failed) given the presence of numerous stones columns. Notwithstanding that I would not recommend the practice and would grout up the entire length of any boreholes in Coal Measures.