Desiree Willis, Technical Writer, The Robbins Company

Six TBMs, three of them Robbins EPBMs, will be used to excavate a major new drainage tunnel under Mexico City. At 62km (39 mile), the Emisor Oriente sewer will be Mexico's longest tunnel and will drain additional water from the Valley of Mexico. The city is founded on what was once an island in the middle of a lake and is sinking at the rate of about 10cm (4in) per year. Over the last 30 years, the city's critical drainage lines have lost their operating gradients and as a consequence, about 40% of their overall capacity. The city is moving forward with the Emisor Oriente project to remedy the problem.

Contractor Carso Infraestructura y Construcción, S.A.B de C.V. signed a complete contract with The Robbins Company in October 2008 for the three machines, their back-up systems, plus cutting tools and spares. The 8.93m (29.3ft) diameter EPBMs will be shipped to the jobsite by the end of 2009 after design and assembly in the US and China, and are scheduled for an early 2010 launch to excavate nearly 30km (18 miles) of the long tunnel. The EPBMs will bore Lots 3, 4, and 5 of 9.6km (6 miles), 9.7km (6 miles), and 9.5km (5.9 miles) respectively, through geology ranging from sandy clay and gravel, to tuff and pumice.

The machines will be fitted with mixed face cutterheads and interchangeable cutting tools. In soils, the machines will use knife-edge bits that can be changed out with 17in (432mm) disc cutters to work in rock. For muck removal, the Robbins EPBMs will feature 900mm (35in) diameter ribbon-type screw conveyors to safely excavate ground with boulders of up to 600mm (24in) in diameter.

Once tunneling is complete in 2012, the $US1.2 billion project, for the National Water Commission (Conagua), will provide drainage at 150m³/sec (5,300ft³/sec), easing surface subsidence problems and adding drainage capacity for the city's booming population that has doubled over the last 30 years to 19 million.

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Other TBMs headed to Mexico City