Bath's stone miners left behind a tricky engineering challenge when they downed tools 150 years ago.
Andrew Mylius ventured underground to find out more.
You can hear vehicles on the roads above you, ' says Dr Rob Narbett glancing upward, the yellow light of his head-torch raking across the ragged, exfoliating roof of Byfield stone mine. In the twisting, disorientating maze of black tunnels beneath Combe Down, just outside Bath, it is hard to believe they are so close to daylight. Yet in places there is less than 2m of rock between the bustling streets and the inky darkness below.
Stone mining in Combe Down started in the early 18th century.
Huge volumes were extracted to satisfy the voracious appetite of Georgian and Regency developers in Bath, Bristol and London.
Byfield and the adjoining Firs mines extend over an area 19ha and 'those are the bits we know about', comments Narbett.
However, mining had all but ceased by the mid-1800s and the mines were largely forgotten over the following century.
Combe Down grew, and people built on top of them.
Narbett, director of specialist consultant and contractor Hydrock, first entered this troglodyte realm in 1988. Two years previously a roadworks contractor had hacked through into a section of the mines with a backhoe while excavating a trench, starting alarm bells started ringing at Bath City Council (now Bath & North East Somerset Council). Concerned about repeat accidents by contractors working on gas, water and telecoms infrastructure buried in the roadway, and with increasingly heavy traffic on local roads, about the mines' load bearing capacity, the council brought in a team from Bristol University to investigate and map the mines.
Narbett, then a research student at the university, spent the following two years probing and surveying the mines' farthest reaches, wriggling through gaps left between mounded spoil and the ceiling.
Carving their way along 4m to 8m deep seams beneath a hard stratum of 'bastard rock', little more than 4m below ground level, the 18th century miners stripped out 80% of the stone, leaving randomly spaced, irregular pillars supporting the roof.
What Narbett and his fellow explorers found galvanised the council into action.
Miners normally set aside somewhere close to 40% of the rock they are working in to provide roof support but not in Combe Down, Narbett observes: Pillars at Combe Down are relatively small, and the spaces between them large.
Over the years, the mines' subterranean dampness has resulted in steady deterioration of exposed stone. In some areas pillars have been crushed and loads transferred to adjacent pillars, threatening a 'domino effect' series of collapses.
Huge flakes and lumps of rock hang perilously from the roof.
And vertical fissures, or 'gulls', up to 1.5m wide running through the mine complex leave the roof unsupported, posing a major problem for roof stability.
Up in the sunlight above, sit 600 homes, a church, parks, gardens, roads, shops and a post office. It was clear action had to be taken.
The survey identified especially hazardous areas, which were prioritised for immediate stabilisation, and recommended a series of safe access-ways be driven through the complex to determine the extent of remedial works.
This is how, 14 years after his initial explorations of Byfield and Firs mines, Narbett is back.
Hydrock is contractor for the £1M safe roadway project, and Narbett and managing director Dr Brian McConnell's intimate knowledge of the mines is also playing a role in advising designer Parsons Brinckerhoff on the direction the route should take.
Hydrock has recruited a team of Welsh 'free miners' to drive the roadway, picked for their experience in reading complex ground and their knowledge about how best to prop and make it safe.
Roof support is provided entirely by propping. Rock bolting was regarded as impossible, as there is simply too little competent ground overhead.
In the main, the miners are using wooden posts, beams and wedges. Wood is easy and quick to work with, and provides early warning of an imminent collapse, groaning and creaking prior to failure. A 'horsehead' system similar to fore-poling, is used to support the roof ahead of the propped and protected area.
The original roadways, used by the miners to shift stone to lift points or exits, are generally 8m deep canyons which makes propping the roof difficult. The miners are therefore working their way through far shallower, rubble-filled chambers to either side, creating 2.4m wide by 2.4m high pathways. In easy ground, a four-strong team can make as much as 5m progress a day, moving material by hand and using a dual-fuelled electric/diesel-powered backhoe excavator, says McConnell. Rubble is tipped into deeper areas or packed behind shuttering to either side of the access route.
But progress can be slowed to 1m/day when working through floor to ceiling rubble, having to break up boulders with pneumatic picks. Currently, with extreme caution, the miners are excavating their way through a shaft filled with spoil that could avalanche if it is not properly supported, he adds.
Sixteen months into its contract, and working on four headings, Hydrock has so far driven around 1,400m of access route.
Where the new access route passes underneath roads, Her Majesty's Inspectorate of Mines (HMIM) stipulated that steel supports be used in place of wood - and a 3t general weight restriction has been imposed on traffic in one area for fear of overloading weak pillars in the mine below. A bolted, galvanised system with a 100 year design life has been produced for the job.
Deteriorated pillars are being beefed up with steel fibre reinforced shotcrete to resist crushing. And grout-filled bags provide permanent, passive support to the roof where pillars have failed.
In the most hazardous areas of the mine, however, shuttering has been erected and the void filled completely with foamed concrete, as the only sure-fire way of supporting collapse areas. 'We've put in approximately 1000m 3so far, but the total mine void is about 400,000m 3, ' says McConnell.
What ultimately happens to the mines is still open to question, says Bath & North East Somerset Council senior project engineer Antosh Wislocki.
The preferred solution at the moment involves backfilling the mine with foamed concrete at an estimated cost of between £40M and £50M. The access routes will divide the mines into manageable 'sectors' that can be shuttered off. But the decision rests on the outcome of extensive public consultation, obtaining planning permission and on demonstrating the technical case to regeneration and funding agency English Partnerships and the Office of the Deputy Prime Minister.