In January 2010, Mexico City officials celebrated the completion of the country’s largest ever TBM with a cutterhead turning ceremony.  The 10.2-m (33.5-ft) diameter Robbins EPB will begin excavation in the first week of February.  The 7.7-km (4.8-mi) long tunnel for the Mexico City Metro represents the first new route in 10 years and will service thousands of passengers daily.   

The giant machine for the ICA Consortium (ICA, CARSO, and Alstom) was assembled 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.  

The machine 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—a process that eliminates in-shop assembly and disassembly time as well as costs for shipping larger, partially assembled components.  “With proper project management and fit up of components, OFTA can save about 70 to 80 percent of the time required for a similar assembly at a shop,” said Ismail Benamar, Tunnel Manager for ICA.   
 
The small launch shaft, approximately 34 m long by 14 m wide, is located in one of the most densely urban areas of the city.  Machine launch in the tight space will require the TBM to excavate the first 70 m of tunnel using umbilical cables connected to back-up gantries on the surface.  Gantries will be lowered into the shaft successively as the machine bores forward.  

Ground conditions in Mexico City are unique, requiring extensive vibration monitoring. Layers of clay, sand, and boulders up to 800 mm (30 in.) in diameter are expected, as the area is part of a drained lake bed. The machine is uniquely designed to fit the conditions, utilizing a two-stage, 1,200-mm (4-ft) 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 (820 ft).

In 2007, the Mexican Federal District announced plans to build Line 12 of the Mexico City Metro.  Due to go online in 2011, the 24 km (15 mi) long route will pass through 22 new stations between Tlahuac and Mixcoac neighborhoods. The Mexico City metro is one of the world’s largest, with more than 200 km (125 mi) of rail and nearly 4 million daily passengers.

Desiree Willis is a technical writer at Robbins Co.

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