ion area, about 23 metres (75 feet) below the waterline, when steel poles used as part of a dam collapsed, said Kathy Jones, spokeswoman for construction firm Transfield. Two workers were seriously injured, one suffering head injuries, said an ambulance spokesman. "I think we're very, very fortunate not to lose the lives of all those people in the tunnel," he said. Initial reports by police and ambulance officials said a barge had collided with an underwater tunnel, causing it to collapse, trapping 12 construction workers "It was not a tunnel collapse. The coffer dam gave way and water and dirt flooded the area," said Jones. The coffer dam, a watertight enclosure pumped dry to permit work below the waterline, juts about 20 metres (65 feet) into the Cooks Rivers. The light rail tunnel from the centre of the city to the airport will run under the Cooks River near the airport in Sydney's southern suburbs. The accident sparked a major emergency alert with police, rescue, ambulance and fire brigade squads arriving at the scene within minutes.