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The immersed tube tunnel will be formed by 12 pre-cast reinforced concrete units, each weighing 35,000 tonnes and each of the size of a football pitch, measuring 113.5 metres long, 33.4 metres wide and 8.5 metres high.
"This is a major milestone in the project, and comes two months after the first four tunnel units were completed and towed from the casting basin at Shek O to a temporary mooring area near Tseung Kwan O. There, additional works were carried out to prepare them for the lowering and joining operation," said Mr John Mundy, the Project Manager of Nishimatsu Kumagai Joint Venture, which is responsible for design and construction of the works.
The concrete unit was towed by four tug boats early Wednesday (March 15) morning from Tseung Kwan O through the harbour to Sai Ying Pun. On arrival, it was manoeuvred into position near the Sai Ying Pun ventilation building, and attached to anchors to hold it in position.
On completion of the preparatory work, the lowering process began at about 3 pm yesterday afternoon (Thursday) by pumping 800 tonnes of sea water into tanks inside the unit. The unit was supported from two steel pontoons by cables attached to winches. It was then lowered in stages into the pre-dredged trench and onto its foundation pads.
The precise depth and position of the unit was checked at each stage by a team of engineers inside the control tower and by surveyors on land with divers being co- ordinated to confirm the underwater positions.
By 7 am this morning (Friday), the massive 35,000-tonne unit was resting on foundation pads at the bottom of the harbour.
This first unit has now been successfully secured in position against the connection structure. This is a short section of tunnel which has already been built on the seaward side of the ventilation building at Sai Ying Pun.
Mr Mundy said the lowering of the other three concrete units, now berthed at Tseung Kwan O, will take place over the next two months.