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Yangtse challenge

China

Construction of a major crossing of the Yangtse has engendered friendly competition between the contractors.

Cable spinning is now in preparation for the giant Runyang river crossing in eastern China, destined to be one of the world's most dramatic crossings when it completes in three years' time.

Stretching some 5km across the Yangtse river estuary, the crossing will include China's biggest suspension bridge, with a 1,490m main span and a respectable 406m span cable stay bridge, as well as some 2.5km of elevated road running across riverside flats and marshes.

The two main bridges cross a pair of channels in the Yangtse formed by the small mid-river Shiye island, which is currently bursting with construction equipment, batching plants and 1,000 accommodation units belonging to the biggest contractor on the project. This is the Navigation Engineering Bureau No 2 of the China Harbour Group, which is building the northern anchorage for the suspension bridge. It will also build the northern tower and the cable stay bridge across the narrower north channel.

The Group's No 3 bureau is working on the south tower in conjunction with local contractor, Jiangsu Provincial Communications Engineering Corporation, while the second Highway Engineering Bureau of the China Road & Bridge Group is responsible for the south anchorage and work on the superstructure.

So far attention has mainly focused on the foundations and concreting for the twin columns of the two main towers, which reached their full 220m heights at the end of October 2002.

For the towers there was something of a race, not only because of competition between the two bureaux which are part of the same overall group but also because the No 2 Bureau had elected to use imported western formwork technology, namely SKE 100 self-climbing formwork units from Doka. The Austrian firm supplied these via a Shanghai based company, DEC Doka.

Ten units were called for on each tower column, the columns being 12.5m by 6m wide at their base, tapering to 9.5m by 6m at the top. The wall thickness for the hollow box section columns varies from 3m to 1m.

The investment paid off with the northern tower overtaking the southern despite its four month's head start. The climbing formwork allowed a five day cycle for each 4m high step, one on each side.

Bureau No 3 and its partner used conventional scaffolding, partly for cost reasons and partly to take advantage of the large amount of labour available on the majority of Chinese construction projects.

The north tower was founded on bored piles, some 32 in total sunk 60m down through the river alluvium to a rockhead.

The 1.2m diameter piles were formed from a barge after which the tops were surrounded by a steel cofferdam for the casting of a 12m thick pilecap from which the tower columns rose.

The two columns which form the towers are linked by three cross members at 41m, 142m and 199m above the river level.

While work on the towers was in progress, Bureau No 2 was also been working on the northern cable anchorage.

Designer, Jiangsu Provincial Communications Planning & Design Institute, and its advisers, including Chinese bridge expert Wang Jianyao, chose a single concrete block gravity anchorage for both cables from the bridge; these splay out to anchor points on 12 concrete layers. Each of these is 3.2m thick and formed within concrete box walls. The whole honeycomb will eventually be given additional mass with an infill of sand.

Some Doka units were used alongside traditional formwork for the walls of the box. These were constructed top down, with considerable internal bracing because of soft ground conditions and substantial water flow problems from the nearby river.

The anchorage is 69m wide by 51m long and required an excavation descending to 45m, the deepest so far in China.

Cable spinning will use preformed cable system favoured in China. Cable is being imported from Japan where each of 184 main cable strands will be made up from 127 galvanised wires 5.3mm in diameter.

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