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Precision blasting helps conserve the landscape

QUARRYING

Surrounded by areas of outstanding natural beauty, Arcow Quarry needed a specialist approach.

Paul Thompson reports

TURN off the A65 at Settle and it is easy to see why this part of the Yorkshire Dales has been designated a National Park.

Even on a grim, drizzly, late spring day, the landscape is powerful.The road winds up Ribblesdale, through part of the backbone of Britain, the Pennines.

It runs parallel to the Settle-Carlisle railway line, famous for its 24-arch Ribblehead viaduct, and the scenery is quite simply stunning.

Strange, then, that an industry which the Government reckons is a terrible blight on the landscape is such a fundamental part of it.

Quarries abound in this part of Yorkshire and all the big-hitters have operations in the area.

But drilling and blasting specialist Ritchies is helping one to keep the landscape as it is.

The Norwich-based company has a deal with Tarmac Northern to stabilise a section of its Arcow Quarry, which digs hard greywacke rock, normally used in dressing road surfaces because of its high polished stone value - a measure of the surfaces' ability to help vehicles resist skidding.

An environmentally listed limestone pavement sits on top of the band of greywacke material at the quarry.

The area is also covered by the Ingleborough Complex Site of Special Scientific Interest and boasts a Special Area of Conservation among the many protection measures slapped on it.

'Certainly it is well protected by environmental legislation, ' agrees Ritchies' contracts manager Glyn Barnes, 'it means we have had to be extremely careful in the work we're carrying out.'

Ritchies, the blasting and geotechnical arm of Edmund Nuttall, has a term maintenance contract with Tarmac Northern but this particular project has been negotiated separately.The aim of the scheme is to stabilise areas of greywacke cliff face around the sides of the quarry and, as a result, the protected limestone pavement on top of it.

On the western face, the geological folds in the material dive down into the floor of the quarry itself and inherent weaknesses in the strata have caused it to begin to fail, with stress fracture lines evident at the top of the quarry.

'The greywacke rock has been deposited in thick layers with weaker material in-between, ' Ritchies' business development director, David Gibson, explains.'These beds have then been folded over time in different directions and by different amounts.'

Tarmac Northern drafted in consultant geotechnical engineer WA Fairhurst to carry out a full investigation on Arcow Quarry's geotechnical and slope stability problems. It assessed the area, including the consequences of failure, and developed two initial options for the problem.

The first involved strengthening the greywacke rock to increase its resistance to failure - but this was rejected following an appraisal of the technical, environmental, cost and health and safety implications.

The second - and preferred option - involved the removal of unstable rock and reprofiling the top of the quarry so that the failure planes in the rock sat perpendicular to the quarry's face.

'There has been a history of instability at the quarry for a number of years and there was the potential for a large scale failure, ' says WA Fairhurst's engineering geologist, Allan Rutherford.'Basically, the solution is to turn the face of the quarry around so that the fissures are at an angle to the face. It is a simple answer to a complex geological problem.'

The work does have its financial benefits, alongside the environmental ones.

Mr Barnes estimates the work will extend the life of the quarry by another two years and there are plans to turn the site into a geological training centre once its working life has been exhausted.

'There are only about five years' life left in the quarry at the moment but the stabilisation work should extend that by another two at least, ' he says.

The overlying limestone pavement in the Ingleborough SSSI complex has been stripped back with grass and sods of earth collected by hand and stored on site in a series of mounds that look not too dissimilar to freshly dug graves.These must be replaced by the end of July on the finished surface so that they can enjoy a significant bedding-in period before the harsh winter weather descends on this particular corner of the Dales.

Following stripping, a series of closely spaced 'pre-split' holes were drilled along the agreed line of extraction down to the floor level of the upper working bench and charged with explosives to help form a fracture plane.

'It ensures that any blasting of the bulk excavation area is limited to the pre-split line, ' explains Mr Barnes.'They are blown immediately before the main bulk charges and help to funnel the blast and material away from the pre-split.'

The extra greywacke material is shifted away and work can then begin on reprofiling the quarry's face, including replacing limestone rubble slopes along the bottom of the overlying limestone pavement.

'The limestone's fragments will darken to their grey, exposed colour, quite quickly, ' adds Mr Barnes.

And by then, day trippers funnelling up Ribblesdale to see the famed viaduct will have no idea of the amount of the work undertaken to protect their views.

Arcow Quarry's geology

THE HUGE unstable blocks of greywacke rock that loom over the Arcow Quarry have been folded from their initial horizontal planes in different directions and by varying amounts.

What is left is a jumbled mess of cracks and fissures throughout the deposit.

The instability of these cracked blocks has been exacerbated by the fact that the thick layers of greywacke have been laid between thinner layers of weaker material.Water trickling through joints in the limestone pavement above the deposits further weakened the natural structure.

Such a confused and folded rock formation has meant that the quarry suffers from stability problems from three differing types of rock failure - planer, toppling and wedge failure - and the huge cracks opening up at the top of the western face of the quarry show the severity of the situation (pictured).

'These mechanisms are not static and, if the work is not carried out, faces will continue to fail, ' says Ritchies business development manager David Gibson.'This could result in multiple bench failure, possibly extending beyond the boundaries of the quarry and cause significant environmental damage.'

Holme Park Quarry

DRIVE 40 km or so west of the Arcow Quarry, crossing from Yorkshire into Lancashire and finally dropping into Cumbria, and Ritchies is involved in more pre-split and blasting work around an important nature reserve.

Aggregate Industries'Holme Park Quarry, south of Kendal, Cumbria, is in the rather bizarre position of having a Limestone Pavement National Nature Reserve within its confines.

Leased to and managed by English Nature, the reserve forms an island in the centre of the quarry itself and was created when Aggregate Industries won planning approval for the extension and deepening of the quarry in August 2000.

Ritchies was brought in to help trim back the edges of the quarry, so that none of the important limestone pavement would be lost during blasting.

This involved drilling pre-split holes along a line, set out and agreed with English Nature and the Mineral Planning Authority, through the underlying rock at 1 m centres and packing each hole with 110 kg/m of explosives.

The pre-split forms a fracture plane along the agreed extraction line and ensures that disturbance to rock beyond the line is minimal.

Pre-split charges are then detonated a fraction of a second before the bulk charges bring down the rest of the face to be felled.

'The better the rock, the easier it is to ensure the pre-split works exactly how it is supposed to, ' explains Ritchies' contracts engineer Steve Lashley.Mr Lashley has control over blasting at a dozen or so quarries across the north of England and as far afield as the Isle of Man.'The rock here is pretty good, so the quality of pre-split is high, ' he adds.

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