The simple matter of installing Asia’s largest precast unit
It looked straightforward enough – build a big precast unit and install it.
Except that the unit was possibly Asia’s largest ever unit that had to be transported on water from the mainland and installed accurately in the face of wave action.
That was the task facing Paul Lui Yau-chun, a director at Leader Civil Engineering Corporation, a subsidiary of Build King Holdings (0240), on a key contract under the Central-Wan Chai Bypass project.
A joint venture of Gammon Construction and Leader was awarded the contract worth HK$705 million in January 2011 to build the precast unit for the bypass unlike the other parts of the bypass which were to be built in-situ.
The unit’s vital statistics were certainly awe-inspiring.
It weighted 46,000 tonnes, roughly equivalent to the displacement of a US Navy Iowa-class battleship.
A total of 18,000 cubic metres of concrete and 8,000 tonnes of steel reinforcement were used in its construction.
The unit was 125 metres long by 48 metres wide and 12 metres high, with the area of the unit on plan being equal to five standard sized swimming pools.
With the float-and-sink method being adopted, the first order of the day was deciding to actually make one huge precast unit instead of the five separate units as originally envisioned by the consultant AECOM.
The original five units were of such an awkward shape that they were difficult to transport on water.
“When we combined the five units together into one, it then floated steadily and so did not require heavy vessels for installation,” Lui said.
A welcome consequence of this decision was that it then only required one trip instead of five trips, so reducing the inconvenience to other vessels in Victoria Harbour.
To eliminate as far as possible any loading on the MTR Corporation’s existing Tsuen Wan Line tunnel, the unit would rest clear of the tunnel on two rows of 51 bored piles of diameter 2.5 metres on either side of the MTR tunnel.
Work could only be carried out at night after the last train.
Still it was going to be a bit close for comfort though.
Headroom from the top of the Tsuen Wan Line tunnel to the underside of the precast unit was less than five metres while the bored piles would be less than 10 metres away from the side of the MTR tunnel.
To reduce any adverse effect from piling operations on the tunnel, a rotary drilling machine was used, operating from a temporary fixed steel platform installed in the harbour.
Such a method, when compared with traditional piling vessels, drastically reduced the area of water required and ensured more accurate placing of the piles.
After piling, pile head cutting was carried out by saw cutting at a level of -10mPD which resulted in much less noise and dust than with traditional manual cutting of pile heads, not to mention being much safer.
Then it was wait for the right weather and right time to transport the unit from Guishan Island which was located southwest of Lantau Island in mainland waters.
The unit had been constructed in a huge excavation by the shore with only a temporary embankment of land or dyke keeping the sea at bay.
The embankment was then blown up with 18 tonnes of explosives allowing the sea to enter the excavation and float up the precast unit.
After what must have been a nail-biting journey lasting 16 hours by sea, it arrived in the waters off Tseung Kwan O on 15 July 2013.
There the four tugboats were joined by another two tugboats for the final two-hour journey to Wanchai, after waiting for the last cruise liner for the day to sail into Victoria Harbour.
On arrival at the site, immediately west of the Hong Kong Exhibition and Convention Centre, there was the small matter of placing the unit in its final position.
The joint venture had to work to a tolerance of 150mm on either side as per the specification.
Unlike a precast unit placed underwater beneath the waves, part of the unit would be above water during final positioning and thus would be subject to wave action.
For this final step, the joint venture turned to Real Time Kinematic GPS to position the unit.
With four GPS sensors on the unit, it was able to compare in real time the current position of the unit compared with the design position.
With so many factors potentially working against the unit, did the contractor come up of smelling roses?
“When the unit was installed on the piles, the final reading we obtained was a difference of only 20mm on left and right,” Lui noted.
And the response from AECOM?
“The consultant was very happy,” Lui said.