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Tackling a tough northerner

GEOTECHNICS OF ROADS

The West Yorkshire town of Bingley was once world famous for its advanced canal locks.Today, another kind of skilled transport engineering is tackling its traffic snarl-ups. Gareth Beazant reports.

Bingley in West Yorkshire was once a quaint village at a key point on the Leeds and Liverpool Canal.

Built in the 18th century, the canal included the world-famous Five Rise Locks at Bingley, a feat ofengineering constructed to raise its height by 18m.

Road vehicles now fulfil the role once carried out by canal barges and Bingley is no longer a peaceful village. But advanced engineering is again coming to the rescue, in the form of the A650 Bingley ReliefRoad, which it is hoped will solve the town's severe traffic congestion.

After several years of planning, work began on the route in summer 2001. Due to open this August, the dual carriageway will free Bingley of 60% of the traffic that uses it as a through route from Keighley to Leeds and Bradford.

The £47.9M A650 is one ofthe biggest road schemes in the north ofEngland and has proved to be one ofthe most challenging, with 26 structures and nine bridges along the route.

Amec Specialist Businesses started on site in October 2001 and was due to finish at the end of May. Geotechnical work was programmed not to be continuous.

'Bingley has a reputation of being difficult to pile in, ' says Amec Specialist Businesses senior contracts manager John Hall. 'Ground conditions vary from fine sands to cobbles and boulders, with very rare bands of clay. Past projects in the area have involved a lot ofboulders so we were prepared to encounter them during piling.' An important part ofthe project is in the centre ofthe route, where Ferncliffe Road crosses the new road. Bridge construction required three large retaining walls - one along Ferncliffe Road and two along the A650, each split in two by the bridge. The bridge is supported by 52 bored piles to depths of22m.

All the retaining walls involved huge amounts ofsoil nailing and could set a precedent for future soil nailing contracts. Amec's design and build project is using Arup as designer and it is hoped that work will help improve UK soil nailing standards.

'The use ofsoil nailing on highways is quite novel. There is a TRL guidance from 1993 on how to specify rather than a specification, ' Hall explains.

'This project is hoping to form the British Standard or code of practice for soil nailing with Arup and Ciria developing the scheme.

'The soil nails are being installed in very hard ground conditions so the specification for soil nails will be very stringent, ' he adds.

Excavation and soil nailing was carried out in benches. After excavating the top bench, the exposed 80° slopes were covered with temporary steel mesh and sprayed with a minimum of 75mm ofshotcrete.

Up to four Amec rigs drilled the holes for the soil nails, angled 15° down, using air flush. Water flush was ruled out because ofthe soil's cohesionless nature.

A total of2,419 Dwyidag galvanised soil nails were installed 9m into the excavated slopes, in up to seven rows, on a 1.5m triangular grid.

The holes are cased with corrugated plastic and grout injected inside and outside the casing for maximum protection.

'The nails have a very robust specification with a 120-year design life - it's usually about 60 years, ' says Amec Specialist Businesses construction manager David Medcalf.

'Each has three levels ofcorrosion protection where the anchorage code looks for two levels.' To ensure the stability ofthe walls, inclinometers have been used to monitor any movement and the soil nails have been designed to respond to small predicted movements.

Two layers ofmesh were placed on the nailed face and the nuts on the soil nails tightened. A second layer ofshotcrete with a minimum thickness of75mm was sprayed over the top. The next bench was then excavated out and the process repeated until road level was reached.

'The soil nailing has a value of £1.5M, which is still considerably cheaper than an alternative such as a diaphragm wall, ' explains Medcalf.

'The difficulties of the project have been shown by the fairly high content of geotechnical work worth around £3.5M.' Other geotechnical work included installing 600mm diameter piles for a pier for the Keighley Road Bridge, using low headroom tripod rigs.

This bridge required three visits by Amec Specialist Businesses because ofroad traffc management. Work involved installing 900mm diameter CFA piles for the abutments to depths of20m.

By Bingley station a 600mm diameter CFA retaining wall was installed to depths of20m and the £5.5M Cottingley Viaduct needed 150, 600mm diameter CFA piles to depths of24m.

One part ofthe project that gave some cause for concern was during installation ofthe foundations for the Tree Rise footbridge which crosses the road to serve local schools.

'On one side ofthe road there is graveyard with the church on the other side. We thought it was possible that the graveyard may have spread across. Fortunately this was not the case, ' says Hall.

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