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A cut above the rest

Hong Kong When Stonecutters Bridge opens in 2008 it will be one of the longest cable stay bridges in the world.Ruby Kitching reports from Hong Kong.

When most people think of Hong Kong, their first thought is usually of an international business centre and a sea of high rise buildings twinkling in the night sky. But Hong Kong is also one the world's busiest ports, handling 220,000 ships annually.

In less than three years, the Hong Kong Highways Department predicts, roads around the port will become choked.

So it comes as no surprise that a new 7.6km long trunk road serving the port is planned to be completed by 2008. The main feature of the road will be the 1,596m long, £185M Stonecutters Bridge, connecting Tsing Yi in the west and Stonecutters Island in the east across the entrance to the Rambler Channel port area. Construction began in May last year and will continue until 2008.

A colossal main span of 1,018m is needed to maintain a 900m wide navigation channel - both during construction and operation. Once complete, Stonecutters will be one of the longest span cable stay bridges in the world, surpassed only by the Chongming and Sutong crossings in China.

Arup is the consultant and main contractor is MaedaHitachi-Yokogawa-Hsin Chang joint venture. Checking engineer for construction is Surrey-based consultant Tony Gee & Partners.

Arup director Klaus FalbeHansen explains that a cable stay bridge was the obvious choice for the structure.

'Aesthetically the bridge will be fantastic - I'm looking forward to seeing it on postcards when it's built, ' he exclaims.

Nine spans make up the crossing, with four back spans between 70m and 80m long each side of the main span over the channel. The distinctive 300m tall single reinforced concrete towers and the backspan piers are built on reclaimed land. Stainless steel will sheath the top 118m of the towers to maintain the aesthetic requirements of the design (see box).

To maintain lateral stiffness and aerodynamic stability a twin deck design is used.

Box girders 18.5m wide, 3.9m deep, set 14m apart and connected by cross girders every 18m, run outside the towers. Insitu concrete is used for the backspan deck boxes, to counterbalance the lightweight steel boxes of the main span.

Piling for the back span piers on the east side was completed last month. These 2.8m diameter, bored under-reamed piles are 60m to 95m long and contain 50mm diameter reinforcement bars placed 'as close together as possible, ' says Arup associate director Fergal Whyte. Since most of the land around the Rambler Channel is reclaimed, the piles are constructed down to bedrock. Pile caps are now complete and last month saw construction start on the first back span pier, and excavation begin for the foundations of the tower on the east side.

'It'll take six months to excavate the 36m by 47m by 8m deep tower pile cap alone, ' says Whyte.

The tower pile caps are adjacent to the seawall and prevent the reclaimed land around the entrance of Rambler Channel from being washed away. 'We built a cofferdam using sheet piles, and put in three levels of struts to keep the excavation open. The main reason it's taken so long is that the reclaimed land is very permeable. The sand contains no fi nes so we have had to do a lot of pumping to keep it dry, ' adds Falbe-Hansen. Concrete will be poured in 1m sections using fl ked ice to keep the curing temperature below 70ºC.

Lower construction will begin in August, but contractors are trialling the jump form shuttering system to ensure the correct profi can be maintained. The base of the tower is 18m by 24m with 2m thick walls, morphing into a 14m diameter circular cross section at deck level which tapers to 7m diameter at the top.

The erection sequence for the backspan will involve building 70m long reinforced concrete piers using jump form shutters, which will be monolithically connected to the deck. The concrete deck sections will be built off temporary precast concrete columns as they will be too heavy to be self supporting until the cable stays are in place.

Falbe-Hansen scoffs at the description 'temporary', given that these columns require piles up to 60m long and will support the deck for two years until the weight is transferred to the towers. When the deck sections are complete, they will be pre-stressed - first transversely and then longitudinally.

Steel deck sections in the central span will be assembled from the towers incrementally, cantilevering until they meet in the middle. A gantry connected to the tower will lift the first steel section into position from barges.

When the back span piers and deck are up and the towers completed, the main deck sections will be lifted into position and connected to the cable stays. At this point, the back span deck cables will be jacked and the decks will lift off the temporary columns.

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