Just downstream from Thailand's capital Bangkok the River Chao Phrya makes a huge sweeping curve, doubling back to form a distinct peninsula. At the narrowest point of the isthmus a mass of construction is under way, mostly for the final stage of an industrial ring road.
The six-lane highway will serve the heavy freight traffic from the city's port which sits on the slow deep water of the river loop.
To complete a 'box' around the port area the road needs to cross the river, which means two bridges are needed, one with a 328m central span, the second downstream - spanning 398m.
The road box also allows the route to connect with a long distance motorway coming in as an elevated road from the west of the country. This junction sits between the two bridges, so all three elements are under construction at once.
'If anything, the junction is the most interesting element of all, ' says Knut Nielsen of Norwegian designer Norconsult, which began work on the project in the late 1990s. 'It brings the two motorways together with some extraordinarily sweeping curved slip-roads which will be up to 70m high.' An interesting feature of the junction, he says, is the virtual elimination of expansion joints on its high climbing curves; the size and curved form allows expansion and contraction with movement taken up in the curves themselves.
'There are also some very slender columns to support them, ' adds Nielsen.
Curves are extended because gradients for the bridges are limited to 3.5%. Experience on an earlier bridge suggested that a greater than 5% slope can cause old and worn truck engines to wheeze and struggle. Keeping the link between the bridges at height helps avoid too many climbs.
However, the two bridges are already the dominant forms visually. Like previous cable stays in the city, the shape and final appearance of both has been created by specialist architect Ron Yee Associates of London.
The firm has worked with Norconsult on several bridges worldwide.
'Both will have tapering diamond-shaped pylons, ' says Nielsen. This form also reduces the land take for the foundations, which are all in a relatively crowded semi-urban area.
Both bridges are being constructed by a Japanese led consortium of Taisei Corporation, Nishimatsu Construction, NKK Corporation and Sino-Thai Engineering (TNNS), which won two roughly equal value contracts of Baht 2,900M ($71M).
A third contract, worth the same amount, sees another Japanese-Thai group - Kajima, Tokyu Construction and Unique Engineering - building the interchange complex. This excludes the top connector road which is shared by the two bridge contracts.
Schedules are tight, says TNNS construction manager Toshio Ichihashi: 'We have 34 months in the original scheme.' Client the Department for Rural Roads has allowed some extension because of delays in acquiring land.
The st technical issue has been weakness of the ground, says Ichihashi. Nearly all Bangkok is built on around 20m of toothpastelike marine clay which overlies a firmer sand layer. There is a second sand layer beneath that.
Piling is needed for every piece of significant temporary work as well as for the permanent structures. Around 10,000t of temporary steelwork will be needed.
Piles must also be reasonably strong because of the high levels being worked, Ichihashi adds.
'A 50m-high support tower can cause substantial bending moments.' Major towers are used to keep up the falsework for the back spans, which need piles of 1.2m diameter and 55m deep.
The permanent foundation for the main pylons requires even bigger piles - 70m deep and 1.5m diameter. The bigger south bridge uses 45 of these for each main pylon: those on the north use 40.
Siam Tone was subcontractor for permanent groundworks using a modern sonic tested pile and a polymer modified bentonite to support the bore. Thai Bauer did the temporary works. A 5m thick pile cap went on top - it has a stepped profile to be added later for architectural neatness - and for this a low heat concrete mix with a high pulverised fuel ash (PFA) content was used.
Peri slipforms were used for the tapered towers. Because of the demand for high accuracy Ichihashi says he put his best Filipino workers onto this section.
He says his Thai workforce is less experienced, and he worries about operation of a huge formwork gantry which will be used for the two bridges' all-concrete backspan decks.
The gantry is used to form cantilevering 'wings' either side of the main deck box.
'The deck is very wide, ' says Ichihashi. 'It is never less than 35m and widens out to around 50m to include the arrival of slip roads at the intermediate junction'.
These changes in the deck width mean the gantry has variable extensions for casting the variations in the curve and shape of the deck. Each side cantilevers from 8.5m to 15m.
While that is under way the tower top with the cable anchorages must be made.
This is the most demanding job of all, with steel guide tubes and back plates being set in concrete to millimetre accuracy.
'The tower will move when loads are added from the cables: There is concrete shrinkage and even the sun on one side of the tower is a problem, ' says Ichihashi. 'This makes surveying very difficult.' He plans to use a preassembled rigid work-frame for the operation, combined 'with a quite old fashioned method - the plumb line', although of course the contractor will also use the full range of modern equipment.
Main span deck elements are being pre-fabricated elsewhere in the country. They are composite steel and concrete for lightness - the heavier, purely concrete, back spans act as counterweights - and will be barged to site for lifting.
The heaviest will weigh around 450t.
Meanwhile the dramatic curves of the interchange are taking shape. For this work contractor KTU is using at least three heavy movable scaffold systems from FreyssinetThailand, the largest weighing 2,000t. These have to be adjustable in three dimensions to follow the complex curves and inclinations of the interchange ramps and slip roads.