Two 17.6-meter-dia tunnel boring machines (TBMs) are now driving a 4.2-kilometer-long highway link between Hong Kong's Lantau Island, site of the territory's international airport, and the New Territories.The second TBM set off on June 15, two months behind the first, on twin drives of Tuen Mun-Chek Lap Kok road tunnel.

The Dragages-Bouygues joint venture of France is building the tunnel under a contract valued at nearly $2.4 billion. According to Bouygues, the TBMs, supplied by Germany's Herrenknecht A.G., are the world's largest and can map the rock face in real time plus remotely detect cutterhead damage, reducing manual inspections under compressed air.

Working some 50 m below sea level, the TBMs are slated to complete by end of next year.