A specialised coal mining consultancy service
The Coal Authority*, as Statutory Consultees to local planning boards, now have a regime which defines whether a property falls into a ‘High Risk Area’ of possibly being affected by coal mining legacy. Their records suggest that the zones of ‘High Risk’ equates to around 15% of the respective Coalfield. Should a property fall into such a zone then the local planning board will automatically consult the Coal Authority who will request that a Coal Mining Risk Assessment (CMRA) is undertaken.
The purpose of a CMRA is:
Present a desk-based review of all available information on the coal mining issues which are relevant to the application site;
Use that information to identify and assess the risks to the proposed development from coal mining legacy, including the cumulative impact of issues;
Set out appropriate mitigation measures to address the coal mining legacy issues affecting the site, including any further works that may be necessary; and Demonstrate to the Local Planning Authority that the application site is, or can be made, safe and stable to meet the requirements of national planning policy with regard to development on unstable land.
In a nutshell, shallow coal and associated features can pose a serious risk to development. This risk needs to be competently researched by a professional Mining Engineer, who has the capability to fully access and interpret the array of information required to compile a report, the expertise of how to evaluate the risks on site, and sound engineering knowledge of how to mitigate against the risks to ensure safe construction.
For a CMRA Lyons CMC will closely follow the guidelines as provided by the Coal Authority themselves (see: https://www.gov.uk/guidance/planning-applications-coal-mining-risk-assessments) along with best practice for the industry as defined in the Construction Industry Research and Information Association (CIRIA) Special Publication 32* document ‘Construction Over Abandoned Mine workings.’
One of the main risk to assess is the likelihood of historic shallow mining voids that have the potential to migrate (over a long period of time in most cases) to the surface, thus causing instability. The CMRA will consider the nature of the specific coal seam in question, namely its predicted thickness and quality (in the Yorkshire region there are up to around 50 different seams, some known to be worked via underground methods more than others), and decide whether an intrusive investigation is necessary and to what degree. The aim being to determine an educated and conclusive opinion regarding land stability and/or whether further works are required to safeguard the proposed development. Archived records of Abandoned Mines, Geological Sheets, Historical information and past site investigations will be reviewed in every CMRA undertaken by Lyons CMC. Competent research in many instances will limit the required investigations, and in some cases eliminate the need for any further works altogether, thus saving developers and/or land owners both time and money. Indeed, in our experience of the cases which require a site investigation for shallow coal workings, only around 10% discover unstable ground that will require further treatment works and/or revised foundation design considerations.
Around three quarters of the risk assessments will conclude that some degree of intrusive investigation (such as boreholes) will be required to confirm the depth and nature of the shallow coal. In cases of small developments, although site investigation analysis generally only needs to be ‘ear marked’ for in order to gain planning consent, it may be desirable to initiate proceedings for a site investigation in the first instance to save time and money. Lyons CMC can operate on this basis if required, offering an all in one rate for both the risk assessment and site investigation. BUT always give us a call first!… we may discover that the risk assessment itself has a strong chance of concluding that a site investigation is not necessary, in which case just the Coal Mining Risk Assessment alone would of course be the sensible approach.
A CMRA report would generally cost £195. Some instances may see an increase on this cost, for example when a larger amount of information requires researching, or for additional site assistance work if required etc. All we would need to carry out the work is a description of the proposed development and a good location plan in the region of 1:2500 scale.
For small scale intrusive site investigations (i.e. a single detached dwelling), costs would generally range between £1K and £2K (depending on the required drilling depth, as determined by the risk assessment), we would carry out a minimum of 4 boreholes just outside each corner of the proposed curtilage, and possible one or two holes within. We would factor into that price the cost of gaining the required license to undertake the work from the Coal Authority, which would need to be carried out in the first instance as it usually takes around four weeks to be processed (expedited option available). Should the site fall into the worse case scenario (around 10% of cases ), where unstable ground is proved, then conclusions would be drawn as follows:
a) Ground stabilisations works necessary – such as grout injection of voids;
b) Revised foundation design necessary only;
c) A combination of above.
Mine Entries – pose a different concern to development. We are aware of 1000’s of both recent and historic mine entries across our coalfields. The Coal Authority applies a general rule that for every shaft we know of, there is an additional shaft we don’t know of. This means that ‘uncharted’ mine entries are very often discovered during property development in former coalfield areas, particularly where workable coal is at shallow depth. The most successful site investigation work for mine entries involves a ‘site scrape’ of the superficial soils or made ground to expose natural ground. However, in cases of substantial depths of such material then boreholes may be required. The Coal Authority are normally owners of a coal mine entry unless it has historically been transferred to the land owner by deed.
Web links – Coal Authority: https://www.gov.uk/government/organisations/the-coal-authority
Ciria Special Publication 32 – We follow defined process and principles for our service based around CIRIA publication 32.