Thanks for the responses so far :thumbup:
How substantial are the stoppings to the shafts?, is there a life span to be expected of them as everything fills with water? and are we actually saying really the cap above rock level is the permananent closure of the shaft, in other words is it expected for the fill in the shaft to subside?
As an aside when they abandon major coal workingswhat precautions are taken against ongoing subsidence?
So in Cornwall and metal mining areas where shafts are irregular and intersect workings on the lodes what is done there where it is not possible to fill? Just the last stage I take it?
Stoppings to levels off shafts are substantial masonry walls, deeply keyed into adjacent strata (metres deep), able to withstand the weight of perhaps 10 metres of wet concrete which is the sort of vertical extent of the shaft plug both above and below the landing.
Their life span is, i believe, anticipated to be 'permanant' - presumably translates as designed to last a few centuries.
Yes, there will always be subsidence between the top of the shaft fill and the underside of the cap. hence the massive thickness and considerable diameter of the cap, to prevent subsidence of the cap and any above-cap groundworks into this void. A major cause of shaft fill subsidence is poor quality fill. I remember when a local path crossing a relatively small (about 12 ft x 8 ft) realtively shallow (about 400 ft) colliery shaft collapsed with the 1900s timber cap, revealing a void of about 40 ft - i.e. 10% subsidence due to infillinmg with unsorted colliery waste and no stoppings to the two landings. This 4o ft was infilled with more colliery waste and in a year sank by by a further 8 to 10 ft, indicating both compaction of this fill plus, very likely, further flow of infill into the levels.
By "precautions against ongoing subsidence" I presume you mean the general, widespread settling of the land surface following longwall extraction? Answer - none; it tends to be assumed that in a decade or so wide-area settling will have largely ceased. Actually, this settling is limited by void migration rates. basiclally, when the seam roof collapses, the broken rock stacks unevenly, occupying slightly more volume than the solid rock, Then the next layer falls, again occupying slightly more volume than the solid rock. As the void in the worked out seam migrates upwards through layer after layer of roof collapsing, this increase in volume leaves less and less void until the void ceses to migrate upwards. Void migration occurs for up to around 15 times extracted seam thickness. Where multiple overlying seams have been worked it gets more complicated but there are limits to how far the void can migrate. However, where a numbe rof seams have been extensively worked, the entire ground surface subsides, irrespective of these limiots to void migration. Thus the central part of the Glamorgan coalfield has been calculated to have subsided up to 6 metres over the last 100 years or so - a figure that agrees reasonably closely with the total thickness of coal extracted from the various underlying seams.
Irregular shafts, if still worked immediately prior to capping (and hence accessible to build stoppings and strip out shaft impedimentia) could be filled and capped as per NCB proceedures quoted earlier on this thread. However, older abandoned shafts (whether irregular or regular, and irrespective of mineral worked) are not accessible for these detailed treatments so the options are more basic - either fill (hoping no large voids remain and that not too much fill flows into levels) and cap, or just cap.