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BATH'S NATURE TRIAL

TUNELLING - Beautiful Bath in Somerset was built from locally mined oolitic limestone. But the once the stone was moved out, bats moved in, calling for conservation measures before stabilisation work begins. Damon Schünmann reports.

'The city of Bath is a world heritage site and it's constructed from Bath stone which had to come from somewhere and this is one site, ' says contractor Ritchies business development manager David Gibson, referring to the mines at the village of Combe Down, which are now in need of conservation.

But at the Grey Gables Mine, both Lesser and Greater Horseshoe Bats have taken up residence, and since both are European protected species, a multidisciplinary plan was needed to create a safe habitat. The mines also sit over a water source protection zone and the Combe Down Mines lie above a Grade I aquifer - rock in which water can be stored and pass through that is then suitable for use as drinking water.

The conservation work, now under way, forms part of a greater stabilisation project for the 23.7ha of mines in the area. Central government has allocated £154.6M of funding for the main project, due to begin in April next year although emergency stabilisation on the Firs and Byeld mines has begun.

'Grey Gables is a room and pillar mine and there has been pillar robbing, also known as skimming, which reduces their size. Because of the mines' shallowness [an average 5m from the surface], and an extraction of almost 80%, their long term stability was in question. After the accidental breakthrough by a local service contractor there has been a programme of investigation and emergency works, ' says Gibson.

'The bat mitigation work is an interesting part of the main project as we have three disciplines with geotechnics, civil engineering and mining all wrapped up in one job.' SES Contracting is bringing mining experience to the project with Ritchies providing engineering and project management skills on the £1.35M contract for client Bath and North East Somerset Council. Donaldson Associates is the designer.

The scheme involves the creation of a 'bat flyway', connecting the Grey Gables and Mount Pleasant mines through virgin ground, with a 2.4m internal diameter precast concrete tube placed in a cut and cover trench. SES Contracting is tunnelling into the mines from both ends of the cut and cover section. Once nished, the bats will be able to travel between the two mines.

'We are dealing with what the old miners have left us, including their calculations written on the mine walls, and what Mother Nature has left us, ' says Gibson.

The completion of a 2.5m diameter vertical shaft at the north west extreme of Grey Gables will also create a ventilation through-ow along a route running from here to a shaft in Mount Pleasant mine.

Site workers will reconstruct another existing shaft in Grey Gables, which is capped with a manhole, as a bat incubator.

The bat-occupied part of Grey Gables is currently blocked off from the portion which will be connected by the bat yway to Mount Pleasant. A previous contractor installed this barrier - built as stabilisation that runs under and along the line of Shaft Road - that Ritchies believes comprises foamed as well as dry mix C30 concrete. So SES Contracting must work towards this barrier, stabilising the route as it goes.

'It's important we don't touch any of the existing robbed pillars so we must nd a route through that avoids them, ' says SES Contracting site manager John McMurdo. The miners started tunnelling into Grey Gables with a Webster 2000 roadheader, but soon found their path blocked by pillars.

'Once into the mine we had to change to an Eimco 627 loader shovel because the Webster was too long and could not avoid the pillars as the route requires some 90 Co turns, ' he says.

'One of the biggest challenges is dealing with the fractured ground here, ' says Ritchies project manager Julian North. 'We are on the edge of a valley and after the last ice age there has been cracking as the limestone tries to travel down the valley.

This creates tension joints that inll with clay, locally known as gulls, and it is very difficult to predict where they are in the solid rock.

'The last thing we want to do is put our cut and cover trench along the line of one of these gulls. It would create stabilisation issues for the trench that is quite deep - at the Mount Pleasant end it's 13m deep and 9m at Grey Gables. We used boreholes and ground radar to map the area and see where existing pillars are to get the best idea for the tunnel route, ' he adds.

Work preparing the outside edges of the two mines, where the pipe will connect them, was critical in such fractured ground. Scott Wilson Mining is the engineering supervisor and provides mine management.

'It's not often we get civil engineering and mining coming together and so this one comes under mining regulations, ' says the company's mine manager Mike Davies.

'It's interesting to see the civils work such as stabilising the ends of the excavation and that's given us experience in that side of things.

So we have been able to improve the [face stabilisation at the] Mount Pleasant end having done Grey Gables rst.' This reinforcement at Mount Pleasant will allow site workers to begin tunnelling the 18m needed to get into the mine.

Two rows of vertical steel dowels extend down across what will be the tunnel's crown, while horizontal spile bars have been installed 6m into the face above the crown, to lock the mass together. This will support the excavation tunnel until the miners can install their own horsehead system that includes an overhead sliding beam that extends forwards into the tunnel as it progresses.

Vertical 50mm diameter Dywidag Gewi dowel bars at 0.5m centres support the sides of the cut and cover trench, toeing 2m into the ground.

In addition, support is provided by whaling beams along the sides of the excavation.

Rockfall netting placed at the bottom of the batter above the trench and steel reinforcement meshing fastened against the face by rockbolts plates provide extra protection against movement of the fractured rock.

Piling work to support a 100t crane that will service the project included the installation of reinforced concrete pile caps supported by ductile iron tubular driven piles driven to refusal at between 5m and 12m.

Work to create the bat habitat is due to complete in late autumn.

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