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Forth Replacement Crossing: Sinking support

Construction of seabed foundations for a third crossing of the Forth Estuary has just started, close to the iconic road and rail bridges. David Hayward reports.

Computer controlled sinking of one of the largest steel caissons ever incorporated into a bridge foundation is underway this week in Scotland’s Firth of Forth.

A network of powerful pumps, concrete tremie pipes and bentonite tubes - thread inside the 1m wide annulus within the massive double-skinned 1,200t caisson - is helping to lower it 40m down beneath the estuary and into the seabed

With the cylindrical ­structure due to be in place late next month, engineers building the £1.5bn cable-stayed Forth Replacement Crossing must then concentrate on two other similar caisson-sinking operations before the bridge’s most critical, risk prone construction stage is behind them.

Underwater foundations

“Forming our underwater foundations is the most challenging part of the entire bridge build programme,” says Carlo Germani, project director for contracting joint venture Forth Crossing Bridge Constructors (FCBC). “We must manage a host of significant but known risks including seabed geology, deep tidal waters and fast changing weather conditions.”

Construction of approach roads to the 2.7km long, multi-span three tower bridge has been underway for nearly a year, but caisson sinking for the crossing’s southern cable tower is the start of the first major marine operation.

Located 700m upstream of the Forth Road Bridge, the new crossing’s southern tower will be founded on sandstone bedrock 20m into the seabed and through a similar depth of water.

Tenderers had the option of using piled foundations, but FCBC - a joint venture of Hochtief Solutions, Dragados, American Bridge International and Morrison Construction - designed the main northern and southern cable towers to be founded on vast seabed concrete bases formed within the permanent steel caissons. The central tower will sit on conveniently located Beamer Rock island with its concrete base formed within a sheet piled cofferdam.

“Forming our underwater foundations is the most challenging part of the entire bridge build programme”

Carlo Germani, FCB

This contractor design, plus FCBC’s decision to construct all offshore structures using a two dozen strong fleet of floating plant, rather than build temporary road access into the estuary, were major factors in the winning £790M main tender coming in some £400M below client Transport Scotland’s initial upper estimate (see box).

The first two caissons arrived by barge at nearby Rosyth Port last month following a seven day sea tow from Polish steel fabricator Crist, based near Gdansk. The largest - 30m tall and 32m in diameter - was then towed on its semi-submersible barge back out into the Firth and anchored temporarily in sheltered water upstream of the bridge site.

Here it was hooked up to an adjoining barge-mounted shear leg crane while ballast tanks in its own pontoon were slowly flooded allowing the caisson to float free as the semi-submersible was partially sunk beneath it. The 400m3 volume annulus in the double-skinned caisson is topped with a temporary steel plate allowing the air within it to provide sufficient buoyancy for the structure to have an effective weight of just 500t, against its normal weight of 1,200t.

Towed 3km downstream

With support from the crane, the caisson was then towed 3km downstream to the southern tower site where GPS helped position it to 200mm accuracy.

“We can cater for a tilt of up to 3.5° during installation but we only get one shot at its horizontal position,” says FCBC project manager for the caissons Ralf Wiegand. “My major concern is not the five hour initial lowering operation but finding the seabed rock unexpectedly high or uneven.”

It is now that the array of pipes in the annulus comes into play as first air is replaced by water allowing the sinking process to start. As the caisson lands on the seabed 20m down, it will continue sinking a few metres through soft upper alluvial deposits, overlying glacial till.

“We do not know exactly how far it will sink under its own weight,” Wiegland explains. “But we can accurately control movement using a combination of water or concrete in the annulus followed by excavating inside the caisson.”

With the caisson stable on the seabed, the perimeter wall must be heightened with an 11m tall single-skin caisson extension bolted around it to keep the top above water level.

“My major concern is not the five hour initial lowering operation, but finding the seabed rock unexpectedly high or uneven”

Ralf Wiegand, FCBC

This steel ring is arriving on one of two follow-up barges journeying from Gdansk. Its role is solely to form a watertight cofferdam and the extension will be removed when lower levels of the tower itself have been concreted.

The caisson will then be 41m tall and barge-mounted grabs will start a six week operation to excavate 10,000m3 of seabed alluvial and glacial deposits from within its open base.

With concrete filling the annulus and high pressure water jets operating beneath the caisson’s lower cutting edge and bentonite injected outside its walls to reduce skin friction, the vast tube will sink further through the soft ground to settle on the sandstone and mudstone bedrock.

Jet grout seal

Its toe into the rock will be sealed with jet grouting until, over eight days early in October, a continuous 16,000m3 underwater concrete pour, tremied down from barges, will form a 25m thick solid plug over the bedrock. Dewatering above the plug will allow the reinforced concrete tower base to be cast in the dry before lower sections of the concrete tower are formed in 4m strips and jump lifts using self-climbing formwork.

Engineers will overlap foundation positioning with a similar operation for the northern tower, centring on a slightly smaller 23m tall caisson. Less challenging should be the central tower foundation, completed largely in the dry on Beamer Rock.

There, a 30m long rectangular platform will be blasted into the dolerite. Then a novel 6m deep sheet piled cofferdam, pre-formed of piling sections supported on precast concrete bases will be placed. Into this, a 15,000m3 reinforced concrete base for the tower will be cast.

“We are building the world’s longest three tower cable-stayed bridge,” says Transport Scotland project director David Climie. “But above the waterline it is all proven technology and construction will be firmly under our control.”


Keeping the costs down


“The Forth Replacement Crossing - open 2016” declares a large sign beside the A90, a few kilometres south of the Forth Road Bridge. A few years ago such a firm proclamation could have proved risky but, in the nearby site offices, only confidence rules.

The four-nation bridge contractor Forth Crossing Bridge Contractors has gathered together an impressive international array of experienced engineers. The surprisingly keen £790M tender price, over a third less then Transport Scotland’s own upper estimate, encouraged everyone to go largely for only proven technology. And - of equally paramount importance - any delay could cause a significant proportion of Scotland’s travelling public to soon start complaining.

Public consultation about the bridge’s future name is due later this year, although its working title explains all.

Weakened suspension cables, failed expansion joints and mounting maintenance combined to warn of a premature economic end to the existing 48 year old bridge - the only road crossing of the Forth directly serving Scotland’s capital.

“Under existing traffic loading the bridge could be forced to close to heavy good vehicles as soon as 2017,” claims Transport Scotland project director David Climie. “Our replacement crossing will allow most of the current bridge’s annual 24M vehicle flow to be diverted, leaving it just for buses, taxis, cyclists and pedestrians plus a lot lighter loading.”

It is hoped this reduced load, and the possibility to then carry out major repairs, will recover much of the bridge’s remaining 50 year design life.

“High winds forced the bridge to close to all traffic three times over the last year, resulting in serious transport disruption across the whole region,” recalls Climie. “Our bridge should never have to close completely due to wind.”

State of the art 3.4m high transparent, open louvred vinyl windshielding barriers, either side of the 42m wide deck, should dampen and divert even the highest crosswinds. And the hard shoulder will, if needed, accommodate diverted buses.

Little else though is novel - except perhaps the 146m overlap of cable stays in both main spans, designed to stiffen the deck and help support the unanchored central cable tower.

The key to the contractor’s lower than expected tender price is - claims FCBC project director Carlo Germani - the adoption of floating plant to build virtually all the over water structures.

“Temporary works are minimised, with no road bunds needed into the estuary, plus less dredging and the elimination of construction congestion,” he says.

Large, more economic concrete pad foundations instead of piling; narrower composite steel and concrete deck sections reducing steel transport costs from the Chinese fabrication yard, plus an intensive year long pre-award competitive dialogue discussion period, all helped FCBC offer a price £260M below its nearest competitor.



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