Hi just wondering with abandoned coal mines, Is it the law to collapse the coal face once it closes down or is it still possible the coal face will be up?

Regards,
tonto125

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Long wall faces will tend to collapse, although the roadways "may" remain as they are supported. There are however many pictures of main roadways moving / collapsing due to the weight on them even in working mines.
Pillar and stall workings often remain many years after closure. There are several mines that have been opencast which revealed old pillar and stall workings. These were shallow so not much weight on the roof.
Plus of course most mines flood, fill with Methane (fire damp) or black damp. Non of which you want to be in!

Not law, but certainly in "modern times" the supports would have been withdrawn leaving only temporary support to allow that process. I've seen an old longwall - 50 years since working - that was still standing with wooden props, slightly closed up but potentially travellable (if you were daft enough to want to!), but normally they tend to close up pretty quickly in most cases.

Not much left supported around Plank Lane and Wigan Flashes, IIRC, 80 feet of coal removed and 50 feet of surface depression?
There's also a figure for Tonypandy Square but probably erroneous as there's no beach in Tonypandy.

Edit: OS bench marks haven't changed so probably erroneous.

The rate of collapse of faces will depend upon a number of factors:

Roof - good sandstone roofs will stand for much longer than poor shales and mudstones. The span of the roof and the degree to which the gob has been stowed will also affect how rapidly it collapses. The amount of propping left in will mak a big difference.

Floor - a solid floor will be far less affected than will a soft clay floor which will squeeze upwards to partly fill the face - in extreme cases the heaving of a floor can completely fill the worked out void.

Style of working - longwall workings will generally close up far faster than room-and-pillar workings and other styles of working that leave a greater proportion of support in pillars relatively evenly distributed across the working areas.

Thickness of seam extracted - a less factor but the thinner the seam extracted, the sooner the collapse of roof will fill the void as the void migrates upwards.

Depth below surface - the greater the weight of overlyng strata, the more workings tend to close up.

Factors influencing subsidence reaching surface - as well as the above factors, the thickness of the extracted void in relation to the depth from surface limits subsidence. The engineering rule of thumb is that 12 to 20 times seam thickness is the maximum distance of void migration towards surface before the void closes up and ceases to migrate. The quality of the strata between seam roof and surface affects the rate of migration and how far the void migrates with sandstones giving low multipliers (say x12) and mudstones and shales higher multiples (to a maximum of x20). If a number of seams have been worked the calculation becomes more complex.

Despite the limits to void migration mentioned above, extensive extraction still lowers the land surface over wide areas. Central Glamorgan sank around 4m over the course of the twentieth century, this being calculated by a Welsh Office study accurately remeasuring bench marks across the region. A knock-on effect of this lowering of the land surface at a rate that was, geologically speaking, very fast, was the significant number of land slips on valley-sides in the early and mid twentieth century when the extraction rate was at its maximum; again there have been studies of this.

Thanks Graigfawr, most enlightening regarding the effects of rock type.

Thanks for the replies see, What do you think the odds are on Blaentillery or Blackbarn coal faces still being up?, Just a curious question

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If you enquire on Welsh Coal Mines Forum, someone there may well have knowledge of the roof and floor conditions in those small mines and be able to give you a reply. There are quite a few members there with experience in small mines who probably have knowledge of conditions in these two mines.

Blackbarn will be under water,and they only drove roadways to reach the take,so no face`s as to speak.

Blaentillery has a lot of Blackdamp,so be warned.

Thanks everyone for the helpful replies, will take into account everything everyone said and this will help a lot on planning my next adventure.

all the Bests,
tonto125

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So Ty Gwyn what do you think on Blaentillery face's still standing?

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The rate of collapse of faces will depend upon a number of factors

how is flooding factored in to calculating the degree of collapse likely? given that flooding of workings happens much faster once pumping is stopped than natural subsidence.
A flooded working will presumably resist total collapse better?

Quite correct: hydrostatic head will partly support the roof - but void migration will still occur; in the case of some rocks, the presence of water may accelerate delamination. Overall, it isa ssumed that all deep workings will be submerged and that void migration will still propagate upwards to its maximum extent (as above: broadly x12 to x20 seam thickness). As migration occurs, the water (or air, in workings above water-table) becomes disseminated in the voids between the fragments of roof as they come to lie randomly on the floor. Ultimately all the seam working void is accounted for in inter-fragment voids and upwards migration ceases. A useful analogy is excavating a given volume of solid rock - say 1m3: after excavation it occupies, say, 1.5m3 due to the spaces between the fragments. Underground, the same effect (after allowing for a degree of compression by the overlying rock) ultimately results in the seam working void plus these inter-fragment voids (whether filled with air or water) filling the void and it ceases migrating upwards.

If you've ever seen the muck resulting from an underground blast, you'll have seen that the muck pile is always much larger than the void formed by the blast - and that the pile is dauntingly large to shovel into trams compared to the volume of ground gained...

Tonto,
When last underground at Blaentillery around 2 years back,some faces were still standing well,others had squeezed badly and some were flat,
The belt drift has squeezed badly,trouble passing the belt structure in places and the height has reduced a fair old bit,mainly due to the last operators way of working.
The main drift where the original longwall faces worked by the Llewellyns when raising 500 ton per week is holding reasonable,due to pillars left alongside drift,only a few places needing repairing,its out of these longwalls on the left hand side that the Blackdamp seeps from,the lower ones are the worse due to the connection to the old Red Ash workings where the water drains to.
On a dull damp day like today the Blackdamp is higher up the drifts nearer the surface,not a place to go into without experience of these working conditions,
After 2yrs i expect the squeezed faces and roadways to be a lot worse due to the fireclay soft bottom.and wet conditions.
The surface layout of this Smallmine was similar to a 60`s NCB Colliery,hence the clearance to handle the output obtained.

Thanks alot Ty Gwyn,

Nice to talk to someone whom has actually been in this mine and know the conditions, this has helped me a lot on determining how far to go in this mine, thanks.

You are definite about at least some of the faces still standing?

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If you've ever seen the muck resulting from an underground blast, you'll have seen that the muck pile is always much larger than the void formed by the blast - and that the pile is dauntingly large to shovel into trams compared to the volume of ground gained...

ah we never had to muck out with shovels, LHD's are better! :lol:
joking aside, I guess we can infer that in coal workings at least, full collapse will still occur, the presence of water will just slow things down a bit..unless the water is trapped by falls at both ends of a tunnel, in which case pressure would build and prevent it

You are definite about at least some of the faces still standing?

Nothing is definite underground after 2 years butty,only the Blackdamp.

Send me your e-mail,i`ll scan a plan i have and post on so you can see the extent of the workings and realize where all the Blackdamp comes from.

The only reason we went down this Colliery was we had permission from the owner,and it was to see if it was a viable proposition to re-open as there is a lot of coal left in the take,sadly it is over 2 % Sulpher and no market in this day and age,so we knocked it on the head.

The volume of water in the finite (i.e. sealed by blockages) water-filled void that you envisage remains the same before, during, and after void migration. The change due to void migration is that the space occupied by the water changes from being a single large void, to being a very large number of very small voids between the collapsed roof fragments.

Water filling workings certainly does help support roofs - pumping out is not uncommonly followed by roof collapses when the support provided by the water is removed.

I understand that many of the old workings from Sheffield city centre heading NE, well below river level, are kept flooded for that very reason. One of the old breweries in the 19th century, situated on top of an old colliery in town, supposedly pumped water up from the mine to use in beer-making. Yum.

Ok Ty Gwyn thanks butt.

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