Three Robbins EPBMs to Improve Mexico City DrainageMexico City, founded on what was once an island in the middle of a lake, is sinking at the rate of 10 cm (4 in) per year. Over the last 30 years, the city's critical drainage lines have lost their slope, resulting in a 40% reduction in overall capacity. To remedy the problem the city is moving forward on the 62 km (39 mi) long Emisor Oriente sewer tunnel to drain additional water from the Valley of Mexico. The tunnel will be constructed using a total of six TBMs, including three 8.93 m (29.3 ft) diameter Robbins EPBMs, to be shipped to the jobsite by the end of 2009. Carso Infraestructura y Construcción, S.A.B de C.V. signed a complete contract with The Robbins Company for the supply of three machines on October 26, 2008. Additional supply will include the back-up systems, cutting tools, and spares. The three machines are slated for an early 2010 launch after design and assembly in the U.S. and China. The Robbins EPBMs will bore Lots 3, 4, and 5 of the Emisor Oriente project in geology ranging from sandy clay and gravel to tuff and pumice. The tunnel lengths will be 9.6 km (6.0 mi), 9.7 km (6.0 mi), and 9.5 km (5.9 mi), respectively. Mixed face cutterheads will be utilized on the machines, with interchangeable cutting tools depending on the ground conditions. In soil the machines will utilize knife-edge bits, which can be changed out with 17-inch (432 mm) disc cutters in rock. For muck removal, the Robbins EPBMs will feature 900 mm (35 in) diameter ribbon-type screw conveyors to safely excavate ground with boulders, which are predicted to be up to 600 mm (24 in) in diameter. Once tunneling is complete in 2012, the USD $1.2 billion project, for the National Water Commission (Conagua), will be the country's longest tunnel. The finished tunnel will provide drainage at 150 cubic meters of water per second (5,300 ft3/sec), easing problems due to both surface subsidence and the city's booming population, which has doubled over the last 30 years to 19 million. 50/08.