Urban EPB tunneling in difficult geology below downtown Mexico City

The unique aspects of soft ground TBM tunneling in urban environments can be challenging, particularly for low cover tunnels. At 10.2 m in diameter, Mexico's largest EPB is excavating the capital's first new metro line in ten years. The 7.7 km tunnel route travels under cover ranging from 7.5 m to 20 m, and within meters of sensitive structures including building foundations and a highway overpass. In addition to the project location, the ground conditions are some of the most difficult in the world. Modern Mexico City was founded in 1524, on what was once an island in the middle of Lake Texcoco. Ground conditions along the route of the city's new line consist of soft clay interspersed with sand, gravels, significant ground water, and large boulders up to 800 mm in diameter. This paper will address the unique challenges of urban tunneling in a downtown area, as well as the design of the EPB TBM needed for excavation in challenging geology. The current status of the project and any problems encountered to date will also be presented.