what was pillar robbing

Photograph:

🔗Minllyn-Slate-Mine-User-Album-Image-015[linkphoto]Minllyn-Slate-Mine-User-Album-Image-015[/linkphoto][/link]

Pillar robbing was a practice whereby in stone and slate mines the pillars that should have been left to support the roof...

pillar chamber pillar chamber pillar chamber

...were either removed totally or made dangerously thin for financial gain, i.e. pillars meant leaving good rock behind; it the result of pillar robbing was a far higher risk of collapse due to large unsupported areas of roof. Officially frowned upon, a blind eye was sometimes turned to it.

A couple of pics here in Croesor, Ch1E which has been substantially pillar-robbed resulting in several chambers being joined, and a huge collapse. The remains of what should have been a solid pillar of rock are clearly visible.

🔗Croesor-Rhosydd-Through-Trip-23-07-2006-Image-006[linkphoto]Croesor-Rhosydd-Through-Trip-23-07-2006-Image-006[/linkphoto][/link]

🔗Croesor-Rhosydd-Through-Trip-23-07-2006-Image-005[linkphoto]Croesor-Rhosydd-Through-Trip-23-07-2006-Image-005[/linkphoto][/link]

---

If i remember correctly, Llechwedd origionally worked to 35' of chamber then 35' of wall or pillar and so on. The floor above and below would have their chambers under and over the pillar of the floor between so the end result would look like a chess board, however, the slate mines don't seem too of adhered to the recommended safety margins!
This method leaves 50% of good rock underground, so you can see why pillars were robbed as the good rock starts to dwindle.

There is a classic example of pillar recovery (robbing) in Mount Wellington mine, where in one of the stopes a tunnel 8 feet by 8 has been driven through the centre of two pillars.
Most times only resorted to when the mine is closing or going bust, and is then known, down here at least, as 'Picking the eyes out of the mine'

---

The floor above and below would have their chambers under and over the pillar of the floor between so the end result would look like a chess board, however, the slate mines don't seem too of adhered to the recommended safety margins!

I'm sure the Surveyor dummy on the tour says that it was his job to ensure that the walls on each floor were in alignment.

It isn't always a dodgy practice - the eventual robbing of pillars is usually planned to occur at some time.

In slate workings the relatively low extraction rates shouldn't require pillars to be removed until the later stages - although I remember once having to use some persuasion to stop a mine owner robbing a pillar of good metal at the outbye end of a level to save on development costs - and his lease allowed pillars to be removed only when all else was worked out! :devil:

In coal the robbery was part of the planned working in bord and pillar. In "working in the whole" headings would be driven out to the district boundary taking about 40% of the coal. Pillars were then split or worked off in lifts (sometimes this was being done in places while development continued) back to the main roads, taking as much as possible while still bearing in mind the effects of subsidence. The low relative value of coal compared to metal ores or slate and therefore higher bulk extraction was the principle reason for this.

There are several examples of pillar robbing in Blaenau, done much more recently! I am not referring to untopping.

Talking of pillar robbing, the investigation into the Crandall Canyon disaster in the US has now been published. That was pillar robbing on a stupidly large scale, with what seen to me to be very wide roadways, by UK standards, under a lot of rock cover.

http://www.msha.gov/Genwal/CrandallCanyon.asp  Link to MSHA site.

20 feet isn't overly wide in pillar working - the usual change is that pillars are larger with increased depth. 20ft/6m wide headings have been fairly common for many years in British mines - in fact a manager met opposition in Whitehaven between the wars when he wanted to reduce the width with the aim of speeding up development.

The pillar dimensions mentioned in the report don't seem too small either - we had 40 yards square at 1500 ft depth and some were later split (nearer to main roads) or even fully removed without problem. The trouble seems to occur when the pillar extraction is badly phased throwing weight onto the remaining pillars instead of into the waste areas.

Yes, it was a most complex series of events, but the warning signs had been very obvious for some time, which makes it inexcusable.

Another thing which struck me is that the hydraulic props they were using in the failed rescue attempt seemed to me to be very flimsy compared to what I have seen in the UK. And as for what looked like chicken wire to contain bursts from the walls, I would have thought it was pointless.

Robots will be the way forward in coal mining, I think.

That report makes good reading - almost a book. The obvious cause is greed! The pillars being robbed were themselves formed in a barrier pillar left to protect earlier longwall/pillar workings. The barrier was already stressed before the new pillars were formed and they were not large enough for the depth. 😮

The hydraulic props weren't there for primary support, they were being used after problems arose and in the recovery. They seemed to be pressurised to a higher degree than what we used 40 years ago or more. (We used props and bars on longwalls 4-7 ft high and up to 300 yds long prior to introduction of chocks in the '60s. With no shield behind the face it was some experience to see (and feel) the roof breaking into the goaf). 😉

The mesh isn't used to prevent mass movement - it's only to retain the gradually deteriorating roof and sides and (except where those clowns were concerned) is usually quite effective.

Yes, greed does make people do daft things, and stupidity makes others go along with it.

I do wonder if we would see things like that in the UK if we were still seriously into deep mining, considering that the profit motive must dominate the actions of any private mining company.

Regardless of political considerations, nationalised industries don't need to be subject to the same kind of pressure.

Alan