The accuracy of a shaft position depends on the source of the information. Shafts shown on Ordnance Survey plans are usually within 6 metres of their true position, but shafts shown only on old mining plans may be up to 20 metres out of position, or may not exist at all if they were drawn on the mineplan as proposed shafts or boreholes. Some shafts shown only on early Geological Survey plans have been found to be ponds or areas of colliery waste tips which had the appearance a shaft depression to the surveying geologist.
Shafts can be located by excavating trenches into virgin ground, or by drilling a grid of holes in a pattern radiating from the assumed position if the shaft has been covered with thick fill deposits subsequent to abandonment, or where the disturbance to the ground has to be minimised. Once the shaft is located, it is drilled to its full depth to verify the condition of the infill.
Disused mineshafts can be treated by capping with reinforced concrete, ideally at rockhead, with a square cap measuring twice the shaft diameter. Shaft caps are mostly 5.0m square because most old shallow shafts are 2.5m in diameter. If rockhead is too deep to reach by excavation, the shaft is capped on firm ground and the filling is grouted by drilling the shaft with casing and pumping a cementitious grout to consolidate the shaft filling.
Shafts under existing buildings
Although all sites in mining areas are now investigated prior to development, this has not always been the case and there are houses and factories which have been built over old shafts. As it only became mandatory for solicitors to obtain a Coal Authority search in mid 1989, there are many properties which are now known to be built over shafts but which were purchased by the present owners without this being known.
In the case of lead mining shafts, there is no central authority with shaft details, and it is possible that purchasing solicitors from outside the locality may have been unaware of the possible presence of mineshafts.
As the true position of a mineshaft may be some distance from its indicated position, some dwellings have become unsaleable because of a mineshaft shown in close proximity which cannot be located or investigated because it lies outside the dwelling curtilage, or shown under the property in which case even if located cannot be treated without demolishing the existing building.