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What a carve out!

Little can beat the benefits of experienced site teams, as David Hayward witnessed in Scarborough where a skilled excavator driver overcame the problem of a deep road cutting's fractured rock.

Easing traffic flows approaching and through the popular north Yorkshire coastal resort of Scarborough is demanding the use of unusual construction skills won through years of practical experience.

The aim is to provide North Yorkshire County Council with a 4.3km diversion of its A165 road just south of the town. This will relieve traffic through two local villages and service a new 600 vehicle park-and-ride facility for central Scarborough.

Most of the route lies in balanced cut and fill. But where it runs through the housing estates of Osgodby village it reduces to a newly created deep cutting barely 20m wide.

This steep-sided cutting had to be formed entirely through weathered friable sandstone and early options included supporting the slope with a secant piled, or diaphragm, wall. But there were fears over possible leakage of bentonite or concrete from either of the wall designs so a soil nail, mesh and gunite solution was chosen.

As the last of over 700 soil nails is inserted to help stabilise the 9m deep cutting, geotechnical contractor Cementation Foundations Skanska (CFS) project manager Jimmy Lee reflects on the now-resolved difficulties of shaping the 70˚ slopes.

"The earthworks contractor forming the steep batter was facing considerable overbreak in the heavily fractured sandstone," he says. "But by bringing in an excavator driver able to carefully chop away the rock, rather than pull it out, overbreak has been minimised."

This simple yet effective solution contrasts with the design complexities of the short 200m long cutting. Sited on both a vertical and horizontal curve, the alignments of its five rows of nails demanded use of specially created setting-out software.

The road forms part of the council's new £30.5M Scarborough Integrated Transport Scheme. This involves not only the A165 diversion, but also a second park-and-ride facility on the nearby A64. The town's traffic control system is being extended and bus priority measures introduced.

Skanska won the £22M-plus road construction contract last year and brought in its own piling and ground engineering specialist CFS to help stabilise the cutting.

The weak sandstone was still to prove influential as earthworks subcontractor Jakto began trimming back cutting sides with a standard excavator bucket.

Tolerances were tight as the council's then consultant Mouchel called for a minimum 150mm thick gunite and mesh layer over the steep benched slopes. The consultant allowed just a 15mm gap in front of the gunite before a masonry wall dressed the finished cutting.

As large chunks of rock broke away, overbreak measured up to 200mm in places and gaps were filled with sandbags anchored to dowel bars. Arrival of the new skilled excavator driver, able to gently carve out – rather than break away – the sandstone considerably reduced overbreak.

It is, however, the follow-on shotcreting operation, rather than the soil nail installation, that lies in CFS's critical path. Little progress can be made on the road's foundation or sub-base until the gunite, plus masonry wall, are in place.

"To ensure the stability of the rock face we can only open up a maximum 2m height of cutting at any one time," explains Lee.

The 720 up to 6m long nails, encapsulated within a grout-filled plastic sheath, double total nail diameter to 50mm. Preforming this corrosion protection in the factory increases costs by a third but ensures an effective seal and cuts down on site preparation time.

CFS's Casagrande C6 drill rig comes equipped with a down the hole hammer that helps achieve a clean bore. At Scarborough the rig can easily double its programmed 18 nails per day insertion rate and is held back only by the follow-on guniting operation.

An extremely strong grout – containing 1.4t cement per cubic metre – fills the annulus between nail and the 114mm diameter drill hole. This helps ensure that the nail starts working simultaneously with the addition of its square-end plate being bolted to mesh and gunite.

The problematic sandstone with its overbreak challenges has led to extra work and time. And it is here, says Skanska, that the use of its own integrated construction and geotechnical teams proves a major advantage.

The all too common disruptive atmosphere of claim arguments are replaced, says Skanska's project manager Terry Budds, "by positive discussion on how best to minimise delays.

"It helps considerably that we are all the same company in the end. "Everyone gets on with the job and works to resolve the issues instead of wasting time over claim and counter claim."

Click here for A165 diverson route


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