Robbins EPB assembled at Mexico jobsiteMexico City officials have celebrated the erection of the country's largest ever TBM with a cutterhead turning ceremony. Next week, the 10.2 m-diameter Robbins EPB will begin driving a 7.7 km-long tunnel for metro line 12, the city's first new route in ten years.The TBM was assembled for the ICA Consortium at the jobsite in a 17 m-deep launch shaft. Crews assembled the machine in a concrete cradle at the shaft bottom, using gantry cranes to lower in components including the forward and rear shields, cutterhead, and screw conveyor. Testing of all sub-assemblies is currently underway. This is the first-ever EPB TBM to be assembled at the jobsite using Onsite First Time Assembly (OFTA). The Robbins-developed process allows TBMs to be built initially on location, rather than in a manufacturing facility. This eliminates in-shop assembly and disassembly time and reduces shipping costs because smaller components are involved. The small launch shaft, approximately 34 m-long by 14 m-wide, is located in one of the most densely populated areas of the city. The TBM will excavate the first 70 m of tunnel using umbilical cables connected to back-up gantries on the surface. The gantries will be lowered into the shaft in sequence as the machine bores forward and creates the necessary space. Ground conditions in Mexico City are unique, requiring extensive vibration monitoring. Layers of clay, sand, and boulders up to 800 mm in diameter are expected, as the area is part of a drained lake. The machine is uniquely designed to fit the conditions, utilizing a two-stage, 1,200 mm-diameter ribbon-type screw conveyor to handle the large boulders. The machine will also feature active articulation, used to prevent deformation of the segment rings on curves as small as 250 m radius. Line 12 is scheduled to go into operation in 2011. The 24 km-long route will pass through 22 new stations between Tlahuac and Mixcoac neighbourhoods. The Mexico City metro is one of the world's largest, with over 200 km of rail and nearly 4 million daily passengers. For more click mx/11 and visit www.robbinstbm.com. 04/10.