WORK on the $8 billion Western Harbour Crossing took a major step forward after the first batch of immersed tunnel units were floated out from the Shek O casting yard near Stanley. Each of the four massive tube segments was towed by four tugs and guided by a flotilla of smaller work boats through the Tathong Channel to a mooring area at Tseung Kwan O. During the next three weeks each will be fitted with a control tower, buoyancy tanks and other equipment before being towed through the harbour and sunk at Sai Ying Pun next month. Sandy Hone, deputy project manager with Nishimatsu-Kumagai Gumi joint venture, said the tanks were needed as part of the sinking operation because despite weighing 33,000 tonnes each sealed unit floats naturally. Extra ballast has to be added to ensure the 113-metre long, 8.5-metre high unit can sink to the bottom of a specially prepared trench on the seabed. Mr Hone said the project was going 'very well'. 'We towed the first and second units out last week and the third went on Monday,' he said. The fourth came yesterday. Twelve units will be fabricated for a 1.4-kilometre three-lane dual carriageway tunnel between Hong Kong Island and Kowloon as part of the Route 3 highway.