Construction of seabed foundations is underway for a third crossing of the Forth Estuary near 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 took place in Scotland’s Forth Estuary in June.
A network of powerful pumps, concrete tremie pipes and bentonite tubes threaded inside the 1m wide annulus in the massive doubleskinned 1,200t caisson helped to lower it 40m down beneath the estuary and into the seabed.
With the cylindrical structure programmed to be fully positioned as GE went to press, engineers building the £1.5bn cable-stayed Forth Replacement Crossing are
now concentrating on two other similar caisson-sinking operations before they can claim that the bridge’s most critical, risk-prone construction stage is behind them.
“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 multi-span three-tower bridge has been underway for nearly a year, but caisson sinking for the crossing’s southern cable tower marks the start of the first major marine operation.
Located 700m upstream of the existing Forth Road Bridge, the new crossing’s southern tower will be founded on sandstone bedrock 20m below the seabed and through a similar depth of water.
Tenderers had the option of 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 third tower will sit on the conveniently located exposed Beamer Rock island with its concrete base formed within a sheet piled cofferdam.
The case for caissons
This contractor-developed 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 below).
The first two caissons arrived by barge at nearby Rosyth Port in May following a seven-day sea tow from Polish steel fabricator Crist, based near Gdansk. The largest - 30m tall and 32m diameter - was then towed on its semi-submersible barge back out into the estuary and anchored temporarily in sheltered water upstream of the bridge site.
Here it was hooked up to an adjoining barge-mounted shearleg 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 doubleskinned 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.
With support from the crane, the caisson was then towed 3km downstream to the southern tower site where a GPS guidance system positioned it to 200mm accuracy.
“We can cater for a tilt of up to 3.5 degrees 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.”
“Forming our underwater foundations is the most challenging part of the entire bridge build programme.”
It is at this point that the array of pipes in the annulus come into play as first air is replaced by water allowing the sinking process to start.
As the caisson landed on the seabed 20m down, soft upper alluvial deposits, overlying glacial till, allowed it to continue sinking a few metres.
“We do not know exactly how the caissons will sink under their 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.”
Before this excavation starts, and with the caisson stable on the seabed, the perimeter walls must be heightened with an 11m tall singleskin caisson extension bolted around it to keep the top above water level.
This steel ring is arrived on one of the 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 extended caisson is 41m tall and barge-mounted grabs were then used to complete the six-week operation to excavate 10,000m3 of seabed alluvial and glacial deposits from within its open base.
With concrete now filling the annulus, plus 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 was expected sink further through the total 20m of soft ground to settle on the sandstone and mudstone bedrock beneath.
Its toe into the rock is sealed with jet grouting until, over eight days in early October, a continuous 16,000m3 underwater concrete pour, tremied down from barges, will form a 25m thick solid plug over the bedrock.
Dewatering above this plug will allow the reinforced concrete tower base to be cast in the dry before lower sections of the tapering concrete tower are formed in 4m strip and jump lifts using selfclimbing formwork.
Manoeuvring logistics for the marine fleet of crane barges, tugs and materials pontoons remain critical while engineers 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.
The 125-year continuous operation of the rock’s only inhabitant, Beamer lighthouse, was curtailed last year when the bridge team removed it stone by stone - hopefully for resurrection nearby - to allow a 30m long rectangular platform to be blasted into the dolerite.
Onto this will be placed a novel sheet-piled cofferdam, preformed of piling sections supported on precast concrete bases.
Into this 6m deep cofferdam will be cast a 15,000m3 reinforced concrete base for the tower. By then, say both client and contractor, risks should drop significantly.
“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.”
Winds of change across the forth
The large sign beside the A90, a few kilometres south of the Forth Road Bridge, declares bravely: The Forth Replacement Crossing - open 2016. 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 FCBC has gathered an impressive international array of experienced engineers.
The surprisingly keen £790M tender price, more than a third less than Transport Scotland’s own upper estimate, encouraged everyone to go largely for only proven technology. And - equally important - any delay could cause a significant proportion of Scotland’s travelling public to soon start complaining.
Public consultation on the bridge’s future name is due later this year, though its working title actually explains all.
Weakened suspension cables, failed expansion joints and mounting maintenance combine to warn of a premature 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 goods 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, adding: “Our bridge should never have to close completely due to wind.”
State-of-the-art 3.4m high transparent, open louvered 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 lower-thanexpected tender price is, says FCBC project director Carlo Germani, largely the adoption of floating plant to build virtually all the over water structures.
“Temporary works is minimised, with no road bunds needed into the estuary, plus less dredging and construction congestion eliminated,” he says.
Large concrete pad foundations instead of piling; narrower composite steel and concrete deck sections reducing steel transport costs from their Chinese fabrication yard, plus an intensive year-long pre-award client-tenderer discussion period all helped FCBC offer a price £260M below its nearest rival.