The "artistic" skills of a single excavator operator have helped turn a problem into a solvable challenge on the steep sides of a road cutting close to North Yorkshire's exposed coastline.
The cutting marks the only restricted zone on North Yorkshire County Council's 4.3km diversion of the coastal A165 just south of Scarborough. Main purpose of the new single carriageway road is to relieve roads feeding into two local villages and service a new 600 vehicle park and ride facility for the popular coastal resort.
Most of the route lies in balanced cut and fill. But where it runs through the housing estates of Osgodby village it passes through a newly created deep cutting barely 20m wide.
This had to be formed entirely through weathered friable sandstone. Early options included supporting the slope with a secant piled, or diaphragm, wall. But there were fears of possible bentonite or concrete leakage from either of the wall designs, so a soil nail, mesh and gunite solution was chosen.
As the last of over 700 soil nails are inserted to help stabilise the 9m deep cutting, 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."
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 a new, specially 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 soil nail installation, that lies on CFS's critical path. Little progress can be made on the road's foundation or sub base until the gunite, plus masonry walls, 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, are encapsulated within a grout filled plastic sheath which doubles total nail diameter to 50mm. Preforming this grout 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 which 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 1,400kg cement per cubic metre – fills the annulus between nail and the 114mm diameter drill hole. This helps ensure that the nail starts working when its square end plate is bolted to mesh and gunite.
The problematic sandstone with its overbreak challenges has led to extra work and time. And it is here, says Skanka that the use of its own integrated construction and geotechnical teams proves a major advantage.
The all too common disruptive atmosphere of claims 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," he adds. "Everyone gets on with the job and works to resolve the issues instead of wasting time over claim and counter claim."