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Troglodyte's delight

UNDERGROUND REMEDIATION

Miners of Bath stone had excavated an underground realm stretching 19ha by the time they downed tools 150 years ago. Andrew Mylius ventures into the darkness.

Light from Rob Narbett's headtorch rakes the ragged, exfoliating roof of Byfield stone mine. 'You can hear vehicles on the roads above you, ' he says, glancing upward.

In the disorientating maze of black tunnels beneath Coombe Down, just outside Bath, it is hard to believe you are so close to the daylight. Yet in places only 2m of rock lies between the bustling streets and the inky darkness.

Narbett, a director of consultant and contractor Hydrock, first entered this troglodyte realm as a research student in 1988, when a team from Bristol University investigated and mapped the labyrinth.

He spent the next two years paying out balls of string as he probed the mines' extent, wriggling through gaps between mounded spoil and the ceiling.

What he and his fellow explorers found sent anxiety levels soaring: Byfield and the adjoining Firs mines extend over at least 19ha. 'Those are the bits we know about, ' comments Narbett, who expects to discover further workings in time.

Stone mining in Coombe Down started in the early 18th century - activity was first recorded in 1742, but goes back further. Huge volumes were extracted for Georgian and Regency developers in Bath, Bristol and London.

By the mid-1800s mining had all but ceased and the mines' existence became largely forgotten.

As Coombe Down grew, people built on top of them.

When a roadworks contractor hacked into a section of the mines with his back-hoe while excavating a trench in 1986, alarm bells started ringing.

Bath City Council (now Bath & North East Somerset Council) feared repeated accidents by contractors working on infrastructure buried in the roadway.

Meanwhile, increasingly heavy traffic on local roads raised concern over the mines' load bearing capacity. It called in the team from Bristol University.

The 18th century miners had carved their way along 4m-8m deep seams beneath a hard stratum of 'bastard rock' little more than 4m below ground level, stripping out 80% of the stone and leaving randomly spaced, irregular pillars supporting the roof.

Narbett observes that miners normally set aside nearly 40% of the rock they are working in to provide roof support: pillars at Coombe Down are relatively small, and the spaces between them large. Wooden props installed by the 18th and 19th century miners have long since perished.

Meanwhile the exposed stone has steadily deteriorated in the subterranean dampness.

'In areas you have pillars that have been crushed - that are taking only residual loads or no load at all, ' Narbett says. Loads have been transferred to adjacent pillars, threatening a domino effect of collapses.

Huge flakes and lumps of rock as big as Transit vans hang perilously from the roof.

Vertical fissures or 'gulls' up to 1.5m wide run through the mine complex. Gulls leave the roof unsupported, posing a big problem for roof stability.

Above sit 600 homes, a church, parks, roads, shops and a post office. It was clear action was needed.

To assess how extensive the remedial works would have to be, it was decided to drive a series of safe access-ways through the complex.

Survey work had also identified hazardous areas in the cavern and these were made a priority.

Hydrock is the contractor for the £1M safe access project and plays a key role in advising designer Parsons Brinckerhoff on what direction the route should take (GE June 2002).

The firm 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.

Hydrock engineer and veteran tunneller Jerry Wilcox says:

'This job is completely different to tunnelling. You have to know the ground much, much better.

'With a tunnel, you know where you are going and advance in a straight line, and you should know what's coming. Here, the ground and workings are 150 years old. Everything's random.' Roof support is entirely through propping. Rock bolting would be impossible as there is simply too little competent ground overhead, says Wilcox.

Where rockfalls have occurred there can be as little as 500mm.

Miners are mainly using wooden posts, beams and wedges for the job. The materials are easy and quick to work with, and provide early warning of an imminent collapse by groaning and creaking before failure.

A 'horsehead' system similar to fore-poling is used to support the roof ahead of the propped and protected area.

Propping the roof is not easy.

'You have to know where the keystones are, ' warns Narbett.

Inadvertently lifting a hanging flake or boulder can bring it crashing down, setting off a chain reaction. 'The miners' skill is in knowing which blocks need to be propped and which don't.' There are already 'roadways' through the mine, used by the original miners to shift stone to lift points or the exits, but these are generally 8m deep canyons.

Propping the roof in these areas would be difficult. The miners are therefore working their way through rubble-filled chambers to either side, creating 2.4m wide by 2.4m high pathways.

In easy ground, a team of four men can make as much as 5m progress a day, moving material by hand and using an electric/ diesel-powered digger, and tipping it into deeper areas or packing it behind shuttering to either side of the access route.

Progress can slow to 1m/day when the miners are clearing their way through floor to ceiling rubble and having to break up boulders with pneumatic picks.

Right now, with extreme caution, the miners are excavating their way through a rubble-filled shaft that could avalanche its contents.

Eighteen months into its contract, and working on four headings, Hydrock has so far driven 1,400m of access route.

Her Majesty's Inspectorate of Mines (HMIM) stipulated that where the new access route passes through particularly hazardous areas, steel supports should be used in place of wood.

A bolted, galvanised system with a 100 year design life has been produced for the job.

Deteriorated pillars are being beefed up with a coat of steel fibre reinforced shotcrete, which helps them resist crushing. Grout-filled bags, 1m in diameter and 2m tall, have been installed to provide permanent, passive support to the roof where pillars have failed.

In the most hazardous areas of the mine, shuttering has been erected and the void filled completely with foamed concrete.

'We've put in 1,000m 3so far, ' says McConnell.

What ultimately happens to the mines is still open to question, says Bath & North East Somerset Council senior project manager Antosh Wislocki. The preferred solution involves backfilling 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 whether this strategy is put into action rests on the outcome of extensive public consultation, planning permission and on demonstrating the technical case to regeneration body English Partnerships and the Office of the Deputy Prime Minister.

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