A mix of precast and insitu construction brought cost and time benefits on the new link to Hong Kong's Chek Lap Kok airport.
Angela Tam reports.
As its name implies, the 2.2km Tsing Yi North Coastal Road (TYNCR) will provide an alternative route to Hong Kong's Chek Lap Kok Airport along the northern coast of Tsing Yi Island, bypassing local roads.
It complements two existing expressway links either passing over the Ting Kau Bridge on to Tsing Yi before connecting with the Lantau Link, or diving through the Cheung Tsing Tunnel, which cuts across the heart of the island.
The HK-775.6M (US-99.5M) contract was won by Gammon Construction in February 1999 with an alternative design based on precast segmental construction of the four viaduct sections using a launching girder it was about to decommission from nearby Route 3 Country Park Sector.
For client, Hong Kong's Highways Department (HyD), the precast option promised considerable time and cost savings.
But, insitu construction methods were still on the cards for the easternmost viaduct, where a busy roundabout meant erection of a noise semi-enclosure, 'From the design point of view, the deck of the viaduct has to be as rigid as possible because of the noise enclosure over it, ' HyD chief resident engineer Arun Shah explained.
However since insitu construction would be noisy and disruptive because of the need for formwork, the contractor came up with a compromise solution.
The deck was constructed insitu to guarantee its rigidity, while the noise semi-enclosure itself was precast in the same casting yard in Dongguan, across the border, as the viaduct segments.
Some 655 segments were turned out used short-line match-casting method with four steel moulds and a sophisticated surveying program.
All the beams, slabs and columns for the enclosure were brought to site and erected by mobile cranes at a rate of three bays, 15m in length every five days. This rate of progress would have been impossible insitu.
Says Gammon project manager KF Tam: 'The columns are nearly 6m tall and slender. It would have been very difficult to get homogenous, void-free concrete given the amount of reinforcement in each column. With precasting we were able to lay each column mould on its side, so it was much easier to pour, and we could control the quality.'
The 432m long noise semienclosure will be the longest in Hong Kong. Since there are housing blocks only to the north, it will be left open on the southern side.
Further from the housing estate, traffic noise will be contained by a 7m high, 400m long barrier with a 3m cantilever over a sliproad. The steel framework for the barriers was prefabricated in China before being shipped to site. Cladding, like the enclosure, in sheets of coloured plexiglass should, with help from the open textured bitumen surfacing of the carriageway, reduce noise generated to less than 70dB.
Since the precast segments on TYNCR are bigger than those the launching girder had originally been designed for by Gammon working with Robert Benaim Associates, it was modified by addition of a cable-stay with a mast plus welding of additional flange and web plates to improve stiffness.
The construction sequence began with the loading of the pier segment adjacent to the rear leg of the gantry, followed by the one adjacent to the front leg. The segments in between were then loaded from front to rear, each picked up by a hoist carriage and attached to a single pin before being carried forward, then rotated 90infinity on plan to an intermediate position to await the other segments.
To create room for this rotation, the segments were overlapped one up, one down, until an entire span had been loaded.
They were then moved to about 300mm-500mm forward of their final position and lowered one by one with temporary prestressing to provide a connection to the previous segment.
Epoxy resin was then applied to the contact surfaces. Once the span was joined, permanent prestressing was fed through the hollow box girders, and secured on the piers.
Completing each 38m to 43m span with its 12 to 13 segments took an average of five to seven days. This represented a saving in time and also in materials because the precast structures are lighter, and the columns and foundations required to support them are smaller. On a steep site where the time taken to set up a platform for foundation works took longer than the works themselves, the savings were welcomed.
'The alternative design involved changing the method of prestressing the bridge segments from the traditional internal prestressing method to external prestressing. This resulted in a much lighter section which in turn resulted in slimmer and more aesthetically appealing columns, smaller pile caps and fewer bored piles for the foundations, ' Shah said.
'We estimate that savings of around -6M and a cut of 130 days in the overall construction programme have been achieved.
Another -1.1M is saved on site supervision costs because of the shorter period of construction.'
Unlike other large HyD contracts, all the works were supervised by the department, in-house, Shah explained.
'We went for in-house management, design and construction supervision because of the availability of staff resources at the required times during planning, design and construction stages' he said. 'Had construction of the TYNCR been entrusted to consultants, they would have insisted on a chief resident engineer based full-time on site just for the one project plus many more supporting staff than we have on this site.'
On completion in February, TYNCR will be incorporated into the Tsing Ma Control Area which covers the Lantau Link and Ting Kau Bridge, to form a seamless network of expressways connecting the airport with the rest of the territory.