This evening I attended a Western Counties IStructE talk in Bristol by Dr McConnell on the Combe Down stone mines project (started 2000 ish) in which the mines are pumped with concrete. It was a well presented, interesting and often a humorous talk, I won’t do it justice with this quick write up.

The village - the read lines show the extent of the mines.

Underneath Combe Down, a village outside Bath better described as a suburb of Bath given the urban sprawl, are 42 acres of dangerously unstable disused stone mines. The situation is such that there is central government funding (heritage fund) to the tune of £154m. This far over shadows the Thermae Bath Spa fiasco which, according to a political pamphlete, cost B&NES residents £197.77 each. That project could not have had more media coverage or public outcry. This project on the other hand has “only made local when news when there is nothing else to report” and the speaker also suggests that “only bad news makes news and that our project is going really well.” Yet here we have a vast sum of funding sunk into Bath and only for the “600 to 700 homes, a few schools and a church” most of which are young in terms of Bath. One audience member thought it cheaper to knock the lot down, fill in the hole and start again! Interestingly I think the speaker said that option was considered.

There were suggestions, from the audience, that something sinister has been afoot. (I loathe conspiracy theories.) Allegedly B&NES have kept this project low profile, and there is a history of local council blunders blah blah yawn.

The mines at Combe Down, as well as many others locally, are due to the entrepreneural achievements of post man come stone miner Ralph Allen.

I once went on a terrific tour of a large stone mine near Box during which I learnt about the ways they mined the stone with large hand saws. I also pocketed two discarded stones now kept on my desk, one a paper weight, the other an oversized pen holder with the groove left by the saw used to place the pen.

When Brunel was tunnelling the Box Tunnel better quality stone was found so Ralph moved his operations that way.

The mines at Combe Down are rediculously dodgy. Not only are they very shallow, today they would be open pit mined, but the percentage of plan area mined is 85%. Depending on depth the figure is usually limited to 50% (rock mechanics). I should say that the mining done is called “room and pillar” as rooms are cut out and around pillars of stone which are left to hold up the roof. It is said that in parts the mine roof is a mere 6 inches thick. Generally the thickness is so slight that most engineers find it hard to believe it is still standing.

The speaker said he could hear traffic and, in places, conversations and foot fall from houses above as he walked around the caves. Apparently the miners left a top layer of stone simply because it was too hard to cut, which they named barstard stone presumably because it was stubborn.

The speaker suggested that it is only because this barstard stone layer is stronger rock that Combe Down has yet to cave in. The largest room is “12 metres across and only 1.5m thick at its crown.”

In places the Combe Down mine is two storeys. ‘Wow’ I thought. Furthermore the pillars are not on top of one another - “clearly designed by an architect” an audience member quipped.

The image below shows one of the stone pillars that has been tapered by local builders robbing stone after the mine was closed. Can this get any worse?!

Well yes. Slender pillars show signs of stress induced delamination, you know - the sort of failure typical of concrete cubes in compression tests. There are “numerous [roof] collapses within the mines, as much as 200 tonnes of stone” in a single collapse.

Not too many geological aspects were covered but enough to make me think I will enjoy my next placement at Buro Happold this summer in their ground engineering group.

If left alone would Combe Down mines collapse? The general public often suffer from garden wall syndrome: it has been here 200 years therefore it will be here for another 200 years. Perhaps some residents of Combe Down will remember a tram that ’sank’ in the beginning of the 20th century. Clearly someone in central government thinks a catastrophic collapse could have happened any time. The speaker said it was just a matter of time.

Of all the other options considered to stabilise the mines filling it in with foam concrete from under ground was chosen. A common method is drill and grout (above ground) as happened under homes somewhere in Reading recently. This method was deemed to be “bloody messy and the reinstatement cost of the gardens too high.” And surely would cause unwanted public attention an audience member insisted.

Apparently ages ago the conservative party in Bath wanted to fill the mines with PFA (pulverised **** ash I think). There are severe environmental issues with this option, some would say, due to the heavy metals and seepage into the water system. The houses in Combe Down are likely to sit on thermolite blocks (for insulation) which are about 60% PFA.

So they had to send people into the mines to pour foam concrete. As it happens lots of other parties wanted to go down too: archaeologists, bat people, hydrologists and probably a retired man called Norman. But it is far too dangerous (only by today’s standards) and the solution came from loads of Welsh miners who created miles of road ways for people and tiny plant. Due to all the pillars almost all road ways are bendy which is a significant design headache.

This marked the beginning of the Combe Down stone mines project back in 2000 (ish) although there have been numerous site investigations and monitoring for years previous.

If you lived above a big void in rock would you want anyone messing around down there? Remember that as the land owner you own what goes to the centre of the Earth. Most owners were only too happy to have the serious problem fixed, something they could never afford, but a few forced the local council to invoke the mineral rights act. One pub had already paid for walls and columns to be built under itself.

The miles of interconnecting safe road ways reduces the mines into numerous manageable sections. Shuttering is erected and that area is flooded with foam concrete in 1m lifts. To ensure the foam concrete gets to the roof underside breather pipes are installed which let the otherwise trapped air escape. When foam concrete comes out stop pumping.

Foam concrete sounds like an oxymoron to me. It is concrete without aggregate (so just cement, sand, water) and with an additive that makes bubbles which expands the mix 3 fold by volume. The base mix is milkshake thick and very cement rich (half tonne per cubic metre). The speaker said the Combe Down site uses the most onsite cement in the UK presently and the most foam concrete worldwide. So much is used that they have taken over a public field to batch produce concrete with four or five deliveries a day of materials. The foam concrete is a mere 1N/mm² in strength, although strong enough for the job. I did not understand how foam concrete is porous yet impermeable.

*yawn*

About 40% of the mine is filled, North Road and work site completed, with about two or three years to go. They are pumping concrete as fast as they can, about 1000m³ a day (that is 333m³ base mix).

There are three or four species of bats in the mine. When asked how the contractors would work alongside the bats during tendering the speaker said “I don’t think they will do much work” such was his care for the bats. Seriously though, they have done lots for the bats even building them a breeding house which has proved too successful as the bat population has doubled since the contractors started work. Quite what will happen to the bats when the mine is completely full nobody knows. Go to another mine? I would.

An interesting thought occurred to me during the talk. All the buildings built with Bath stone taken from the mine, now being replaced with concrete, may as well have been built with concrete if it were not for the time distortion. Not as pretty but better for the environment? Talking of the environment, has anyone else heard of menstrual cups (they are reusable)?

Furthermore there are numerous sites around Bath and Bristol like Combe Down. Across the road is Fox Hill which has government buildings sitting on 240 acres of mines. There are plenty of mines around Box, also Red Cliffe in Bristol.

I could go on about impermeable concrete layer in the strata and the proposed drainage system, the houses in quarries, the management of the workers and the importance of flexible management on this site, using discard rock, public consultations, issue with the clogging, parts of the mine possibly becoming a tourist attraction…. but am tired. Night night.