The 20,000K was an estimate by a capping contractor. Would have been a third of our conversion budget gone in a hole in the ground. Where we saved money was by hiring a jcb and driver to do the excavation with a capping professional standing by. He also drew up a schematic for the cap/plug that satisfied the building regs guy from the council. A professional plug done diy.

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Many thanks, that's really useful to know, as we're having a 'specialist' do us a quote, which we think will be rather costly and was thinking of doing the same as you. Fortunately for us, the backfilled hole isn't near a building, it's at the side of a field and we may even get away with simple fencing. The most it would effect is a timber framed outbuilding, so I don't think building control will be that interested in this instance. Once again, many thanks. Phil

When Kerrier were plugging lots of shafts it was found there really was a need to reaccess some around Dolcoath to investigate areas concern, however the plugs were so solid that this proved impractical. Richard Hocking of the County Council proposed using polystyrene cylinders within the concrete plug, which could be dug out if the need ever arose. Somewhere I have the engineering drawings for these. Apparently this would actually work out cheaper than the cost for the same volume of concrete (?). I believe CCC used these on sites around Carn Brea Mines.

Now that sounds like an even better solution. Our mining report (they dug a lot of holes and did a lot of drilling) found just this one backfilled shaft, and as it's not impacting the main dwelling, going overboard with concrete seems a little too much. I like the idea of a removable plug. I shall do some more exploring. We do have another shaft, much bigger, open at the top but surrounded by a 1.5m high wall, but I have other plans for that one 🙂

Ali, What they did do in certain instances, including Wheal Leeds was do the base of a batcastle and then slab over the top instead. So, they'd put in a couple of beams, put boards over the beams, sit concrete rings on the boards/beams, fill the outer bit with concrete and then board over the top and put a 15cm thickness of concrete.

From what I've seen, it seems that the circular batcastles (think the couple of new ones at Consols) are used for smaller shafts and plugs with identity sleepers are used for choked shafts. (Think Peevor and Consols).

The planning report on Cornwall's Biopark runs through the mathematical design of shaft plugs in great detail, but I think in practice, they just put some beams in, then whack a F-Load of concrete on top, having previously coned the sides out. I had an old carnon chap tell me that they used a depth of concrete 3x the shorter dimension of the shaft as a rough guide.

Doing a decent and well designed shaft cap need not be stupidly expensive, or overkill. I have a good mate who would be able to advise in the SW.

It seems to make sense having a polystyrene core. I'd personally use something like 2 part expanding foam (PU).

Give me a shout if you are interested in capping in Cornwall. It's not an impossible challenge. I know a good mining engineer who is currently not doing mining engineering and would probably come around and have a look gratis.