To comply with the legal requirements of The Coal Authority, their Permit Form has to be obtained, signed, and returned prior to any physical investigation commencing.
Trenching with an excavator can be used to locate very shallow mineworkings and outcrops which are within 6 metres of the ground surface.
Drilling is used to investigate deeper mineworkings. The investigation of shallow coal mineworkings is usually limited to 30 metres below ground level. Only if old workings are unusually large or the ground particularly weak is this 30 metre depth limit exceeded.
Unless exceptionally heavy foundation loads are to be imposed on the ground by a proposed building, drilling is by open-hole techniques whereby rock cuttings from the bit are flushed up the hole and identified as drilling progresses.
Core drilling is used when rock strength needs to be determined by crushing cores in a laboratory.
Research
Plans of coal workings are kept by The Coal Authority at the Mining Records Office in Mansfield on behalf of the Health & Safety Executive and are available for inspection by appointment.
Old topographical surveys, including estate plans and tithe maps as well as the Ordnance Survey, may also show the locations of mineshafts and mining sites.
The Geological Survey has records of mining and has published maps showing coal outcrops together with some mining details since the late nineteenth century.
Geotechnical hazards
The term shallow mineworkings usually refers to those mineworkings which have the potential to cause ground subsidence if the workings were still open.
This subsidence is caused by the collapse of the roof of mining tunnel, with the overlying strata falling downwards to fill the void and thereby forming a crater at ground level if the workings are less than 30 metres below ground level. Subsidence from deeper mining will have ceased by now and is not considered a risk.
Remedial work
To prevent subsidence from shallow mineworkings, it is necessary to infill any open tunnels with a cement based grout.
A grid of boreholes at 6 metre centres is drilled over the bearing area of a proposed building, and grout pumped down the holes and into any voids which might remain.
This remedial work will prevent subsidence, but because the ground may have already been weakened by previous subsidence, reinforced foundations are used for new buildings.