Robbins TBMs at Chongqing metro Two 6.4 m-diameter Robbins Main Beam TBMs were launched in winter 2009-2010 beneath Chongqing, a Chinese mega-city of over 35 million people. The two machines are excavating twin 12 km-long tunnels for Line 6 of the Chongqing Metro, and are the first hard rock TBMs to be used in the city. Each machine is boring under sections of very low cover, ranging from 10 to 60 m in sandstone from 12 to 50 MPa UCS. Both High Performance (HP) TBMs started up from a common portal after shop assembly in Chongqing. The first machine was moved through the start chamber on a specially designed walking cradle, and began cutting rock on 11.12.2009. The second machine was launched a month later. As of late-February, 2010 the two machines had excavated 800 m and 420 m respectively, starting under about 20 m of cover. The tunnels are the first new lines to be excavated for the city's metro system, which is planned to include a network of nine rail routes, both above and below ground, and approximately 30 km of tunnels excavated by both TBM and drill/blast. Read the full press release. 10/10. The two Robbins TBMs are boring under low cover of as little as 10 m, and must negotiate curves down to a 400 m radius