Surface stability can be adversely affected by mineworkings to extract minerals such as coal, and by natural dissolution of minerals such as gypsum
Mineral Safeguarded Areas
A Mineral Safeguarding Area (MSA) is an area of known mineral resources that is of sufficient economic or conservation value to warrant protection for generations to come. The purpose of MSAs is not to preclude automatically other forms of development, but to make sure that mineral resources are adequately and effectively considered in land-use planning decisions.
Specialist guidance on Mineral Safeguarding "A Guide to Mineral Safeguarding in England" has been produced by The Coal Authority and the British Geological Survey.
Paragraph 143 of the National Planning Policy Framework (NPPF) requires Local Authorities, when preparing Local Plans to define MSAs and adopt appropriate policies to ensure that minerals resources are not needlessly sterilised by non-mineral development.
Prior Extraction of Coal
Prior extraction of coal is encouraged by both the Coal Authority and Planning Authorities, largely because a potential mineral resource will not be sterilised by the development.
Where coal is present beneath a site, the Local Authority are likely require the developer to consider the opportunity to recover (extract) the coal. Applicants submitting planning applications may need to demonstrate to the Local Authority that they will extract the coal, unless:
- it can be shown it is not economically viable to do so, or
- it is not environmentally acceptable to do so, or
- the need for the development outweighs the need to extract the coal, or
- the coal will not be sterilised by the development
Extraction is generally considered viable where the overburden above a seam is less than 12 times the coal seam’s thickness.
For the developer/landowner, prior extraction of coal can be attractive from a financial perspective, especially where mineworkings in the seam of interest require grouting, and/or the desired build programme can accommodate the time necessary to ensure settlement of the replaced overburden has reduced to tolerable levels. However, extraction usually increases developer abnormal foundation costs.
Lithos are able to assess the viability of coal extraction in light of the findings of an intrusive mining investigation.
Mining Risk Assessments
In July 2011, the Coal Authority (CA) formalised their requirements in relation to planning applications and introduced some new terminology relating to Coal Mining Development Areas.
The CA have defined specific Coal Mining Development Areas and Low Risk Areas. The former are areas, based upon CA records, where potential land stability and other safety risks associated with former coal mining activities are likely to be greatest. They include, for example, areas of known or suspected shallow coal mining, recorded mine entries and areas of former surface mining. The CA is now a statutory consultee for any planning application within a High Risk Area.
Lithos have completed many desk-based risk assessments, which include an examination of the mining report together with published geological maps, geological technical reports\memoirs, any available borehole logs, and any available mine abandonment plans.
A mining risk assessment would normally be included in the broader geoenvironmental Desk Study (or Phase 1) report required by planning applications for all developments other than minor domestic extensions.
Wherever possible we will avoid recommending further intrusive investigation (rotary boreholes), and also provide pragmatic advice, most notably with respect to a suitable foundation solution.
Where necessary Lithos undertake intrusive investigations to determine whether or not old mineworkings actually pose a significant risk to surface stability.
Elland Flag Workings
The Elland Flags (a Coal Measures rock) comprise a major lower leaf of fine-grained sandstone, about 20m thick, with one or more thin leaves close above.
Mining of the Elland Flags was widespread in the Leeds-Bradford-Halifax area in the late 1800s and early 1900s, but the industry declined rapidly from 1905 onwards.
Exploitation, mainly for roofing slate, began around 1300 with the digging of natural outcrops. By the 1600s quarrying was established, and by about 1850 the stone was further pursued by small drift mines.
Shafts were usually circular and 3m to 5m in diameter. The deepest shafts, at about 45m are located around Idle, but the majority were less than 30m deep; probably due to problems of haulage and groundwater. Constraints of ventilation and available power meant that the larger mines had to have numerous shafts, typically at intervals of 50m to 90m.
As with coal, Lithos assess the potential for flag workings during a Desk Study, and where necessary undertake an intrusive investigation to determine whether or not old mineworkings are present and pose a significant risk to surface stability.
Gypsum is an evaporate mineral formed by precipitation from saline waters in warm, shallow marine environments, and it occurs in a sequence of rocks which extend from Teeside to Nottinghamshire.
Gypsum is soluble, and where it is exposed to flowing groundwater, it can dissolve leading to the formation of solution-widened fissures, which can develop into major underground cavities and caves.
Problems associated with gypsum dissolution in the Ripon area are well documented in a DoE Technical Report. This Report includes a Development Guidance Map, which provides a simple, three-fold division of the Ripon area into Development Control Areas (A, B & C), based on an assessment of subsidence potential. Area C is of greatest concern, being the area where gypsum is present and susceptible to dissolution.
Subsidence arising from gypsum dissolution is an irregular and unpredictable process, and there are very few practical means by which a building can be given complete protection. Consequently, prior to any development in Area C, a site must be subject to robust ground investigation and foundation design constraints. Harrogate Borough Council also requires submission of a ‘Ground Stability Declaration Form’ signed by a ‘Competent Person’; Lithos are able to sign this Form.