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Scottish cut and thrust: Queensferry Crossing

One of the UK’s largest road embankments is contributing to major cost savings on Scotland’s new Queensferry Crossing. David Hayward reports from the Firth of Forth.

Master plan

The new crossing, marked by pier caissons, will join the existing Forth road and rail bridges

Rising from the turbulent waters of the Firth of Forth, the lower sections of the three concrete bridge towers for Scotland’s largest infrastructure project are becoming increasingly visible.

They offer commuters on the adjacent Forth Road Bridge their first recognisable sight of the new Queensferry Crossing. And it is only now that most will realise the purpose of the vast cylindrical steel caisson foundations that, for much of the last year, they have watched being lowered slowly into the water.

Buzzing around these three marine sites, an armada of barges, pontoons and tugs continues to feed the structures with concrete - last year to form thick plugs and tower bases deep inside the caissons, but now for the initial lifts of the three towers themselves.

Yet observant local residents will have noticed that, either side of their future cable-stayed crossing, new road links from the bridge site to the existing motorway network are fast becoming reality on both banks of the estuary.

Ferrytoll embankment ground improvement

Ferrytoll embankment ground improvement

As innovative in design and construction as the new bridge itself, this 6.4km of new and upgraded motorway link road contributed significantly to the overall project’s surprisingly keen £790M tender bid three years ago from Forth Crossing Bridge Constructors (FCBC) - a joint venture of Hochtief Solutions, Dragados, American Bridge International and Morrison Construction.

This initial length of road is routed across a complex mix of hard, soft, weak and reclaimed ground with local contamination - hence the initial piled viaduct design

At £260M cheaper than its rival bid, FCBC’s value engineered price was due largely to the contractor’s decision to service the marine tower sites using floating plant instead of constructing temporary road bunds out from both shorelines. But an impressive £40M has also been slashed from ­client Transport Scotland’s original upper estimate by ­savings on the new road links.

Now half complete, this £109M package of largely raised motorway both sides of the bridge boasts half a dozen different ground treatment techniques, plus considerable brownie points for sustainable construction, as the contractor swapped the client’s suggested piled viaduct designs for a single, record height ­embankment.

To create the initial 300m of elevated motorway, sweeping off the bridge’s northern end, the client had proposed a multi-span piled viaduct. Instead, FCBC designed a much shorter 100m viaduct, carrying the approach motorway over a local road and dedicated bus lane, with the remainder running on a single 200m long embankment, up to 25m high.

Considerable challenge

Reducing construction costs for this section by £30M has, though, given FCBC’s engineers a considerable technical challenge.

“This initial length of road is routed across a complex mix of hard, soft, weak and reclaimed ground with local contamination - hence the initial piled viaduct design,” explains FCBC head of network connections Ross Glendinning.

“By opting for one large embankment we must first treat the ground beneath with a wide variety of strengthening techniques (see diagram).

The contractor has developed a varied geotechnical design, which is working well and has saved us all time and money

The route crosses reclaimed riverside - now a marsh created with material excavated a century ago to form a nearby dockyard. It then sweeps over an old quarry, loosely infilled with boulders, and a landfill site containing domestic waste.

Ground treatment for the new Ferrytoll embankment ranges from the relatively conventional dig and replace, surcharging, sand and band drains, to more complex trench mixing and controlled modulus columns (CMC). These last two techniques were needed mainly in the weak reclaimed riverside area lying beneath an adjacent smaller second embankment that carries a realigned B-road and new bus lane.

Preparation

Controlled modulus columns being formed to strengthen the ground beneath the embankment

Trench mixing involves a specialised machine with a long vertical chainsaw that cuts an 8m deep trench wall the full width of the formation. Ground within this 400mm wide trench is not removed, but agitated and then injected with a cement-lime mixture that artificially dehydrates the soil, stabilising the full height wall.

The operation is repeated with trenches at 1m to 3m centres. The effect of this parallel wall pattern is to strengthen the entire footprint of the embankment to be laid above, without having to remove and replace the weak reclaimed ground between walls. This ground is left insitu and undisturbed.

CMC is a relatively new technique in the UK that achieves a similar degree of strengthening and offers equally sustainable construction. A conventional high torque piling rig uses a specially designed soil displacement auger to drill 360mm diameter holes to a tight 2m grid.

Concrete is pumped down through the hollow auger into the 14m deep holes as the auger is extracted, creating structural columns.

A tensioned geotextile membrane and a thin layer of granular fill are then laid on the ground, allowing loading from the embankment above to be transferred and dispersed down through the 4,500 columns.

Economic option

These columns again leave soft ground undisturbed, and are much more economic than the alternative precast piling often earmarked for particularly weak areas.

Ground strengthening is now complete, and about two thirds of the main 160m wide embankment laid over the top. Towering up to 25m, it is claimed to be one of the UK’s highest artificially created embankments, and is being formed conventionally in maximum 800mm layers.

Fill for both embankments is sourced locally. A 20m high dolerite hill lies to the north near the motorway trace and could have been left alone, but FCBC opted to remove it by blasting to provide a valuable 120,000m3 of fill for the main embankment.

The remaining 380,000m3 of fill is also won from near the bridge site. This material is recycled from a vast stockpile of spent oil shale, the discarded by-product of the area’s once booming shale oil industry, sited near the estuary’s southern shoreline. It is then trucked across the Forth Road Bridge to the northern embankment.

“The contractor has developed a varied geotechnical design, which is working well and has saved us all time and money,” says Transport Scotland roads and infrastructure manager Steven Brown. “Use in the embankment of non-primary aggregates won locally has offered a good sustainable solution.”

Our environmental approval process is modelled on a hybrid bill similar to the one used to oversee the Channel Tunnel project

South of the bridge, the main challenge in constructing the 3km link road to the existing motorway network centres on minimising disturbance to the small town of South Queensferry. The closest section passes just 50m from housing, and the road profile has been designed specifically to economically balance cut and fill. This allows a semi urban stretch to be formed within a sound-muffling 8m deep cutting topped with acoustic barriers.

Noise, dust and vibration are closely monitored round the clock at both road sites, in common with most major construction schemes.

But the Queensferry Crossing project faces particularly strict restraints - not just contractual but legally binding through the project’s parliamentary Act.

Abbreviations like PCNV and IFC dominate the working day for Glendinning and his fellow engineers. Any construction activity likely to generate even minor disturbance has to be approved up to 72 days in advance by a 20-strong external liaison group, formed of key stakeholders, national environmental bodies and local councils.

To this committee Glendinning must submit for approval a detailed Plan for Control of Noise and Vibration (PCNV) and from which he hopefully receives an Issue for Construction (IFC).

“Our environmental approval process is modelled on a hybrid bill similar to the one used to oversee the Channel Tunnel project, forcing me to plan detailed construction activities up to six months in advance,” says Glendinning. “For major events we have to provide around 20 bespoke documents, triggering a significant administration exercise.”

Yet, despite the constraints - such as vibration limits twice as strict as the industry norm - Glendinning, with fingers discreetly crossed, claims the number of complaints in his in-tray since road construction began 30 months ago remains just a handful.

Bridge Towers

Bridge Towers

Bridge Towers

The initial sections of all three bridge towers are now emerging from their estuarial foundations.

These are formed of two vast concrete-plugged cylindrical double skinned steel caissons sitting in the river bed for the north and south towers, with the central tower founded in a cofferdam cut into the mid-estuary Beamer Rock island.

The hollow, elliptical tapering towers are being cast in 4m lifts, with concrete ferried out on barge-mounted mixers from the contractor’s batching plant on the northern shore.

At up to 210m in height, the towers will be the tallest on any UK bridge and, when construction is sufficiently advanced, they will start supporting sections of deck erected as balanced cantilevers out from either side of each tower.

The original plan was to lift these barge-mounted deck sections from the water using two gantry cranes sitting on the carriageway above and operating sequentially from the three towers.

Early in the project, however, contractor FCBC proposed an accelerated programme involving six erection gantries working simultaneously from all three towers.

This will help ensure the overall contract remains on schedule for the bridge opening to traffic in late 2016.

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