THOUSANDS of litres of sea water were poured into part of a former quarry yesterday as contractors completed the first four segments of the $599 million airport railway cross-harbour tunnel. The banana-shaped concrete tubes - each weighing 11,000 tonnes - have been built in a special casting basin at Shek O in a joint venture between Kumagai Gumi of Japan and Tarmac of Britain. Once the fabrication yard has been flooded - equivalent to filling a 300-metre long, 30-metre wide bath - each unit will be towed to a temporary mooring area at Tseung Kwan O. There they will be fitted with a control tower, pontoons and other equipment ready to be towed through the harbour. The first unit is expected to be placed on the seabed outside Exchange Square at the end of next month. Deputy project manager Bob White said it would take 45 hours to fill the basin, using 190,000 cubic metres of water. He said extra buoyancy tanks had been added inside the tubes as they would dip in the middle because of their curved shape. Roger Bayliss, construction manager for the MTR, said the tunnel consisted of 10 units laid in a boomerang shape between Central and the west Kowloon reclamation. The 1.26 kilometre tunnel is due for completion in March 1998.