I’ve written a few times in this space about Bertha, a tunnel boring machine currently taking a break from chewing a bypass for traffic dozens of feet under Seattle. This week, I’m shifting south to cast a light for readers on a TBM project in Mexico City.

When finished, the Eastern Discharge Tunnel, which has been called "one of the largest wastewater tunnels in the world," will stretch 38 miles from Mexico City to water treatment plants in the neighboring state of Hidalgo. The $1-plus billion project, years in the making and expected to open in 2016, is impressive any way you look at it.

Background of the TEO

The Eastern Discharge Tunnel — officially el Túnel Emisor Oriente — will help drain the city, which has almost 9 million people, but nearly 20 million people if you throw in the suburbs. Hundreds of years ago, the city sat at the center of a lake and was accessible by canoe or causeway. The lake was drained long ago, but the city still deals with the topography you’d expect when you live in a drained waterway. Rains can prove problematic.

Rains from above form part of the problem. Sinking from below adds to it. Groundwater overexploitation has led to sinking in parts of the city. In fact, some areas sink almost 4 inches (10 centimeters) annually. Imagine the havoc that can wreak on infrastructure. It also makes the bowl shape of the city even deeper, making flooding more likely when it does rain.

A Failing Grade

Part of the issue with the sinking of the land is that it has changed the grade of the Grand Canal, which used to drain the city. The Emisor Central, which opened in 1975, was designed to help, but the slope of the Grand Canal has gotten so extreme the slope cannot effectively be overcome with pumps and water back ups into the Emisor Central. The backup is serious business, because the Emisor Central was designed to be dry part of the year for maintenance. In fact, most maintenance on the flooded Emisor Central was deferred from 1995-2008.

The new Eastern Discharge Tunnel will give the Emisor Central an assist.

The TEO Project

The TEO tunnel is being built by Conauga, the Comisión Nacional del Agua in Mexico. The TBM machines on large parts of the project are from The Robbins Company, a United States firm, and Herrenknecht, a German company.

Find more about the tunnel here. It will have a finished diameter of about 23 feet (7 meters), and range in depth from about 98 feet (30 meters) to about 492 feet (150 meters). It will have a drainage capacity of 150 cubic meters per second. That should help quite a bit during the rainy season, from June through October, when much of the city’s annual 34 inches in annual rainfall arrives.

I find this project interesting and researching it gave me a chance to practice some of the Spanish I’ve been learning (though I link to only English sources so I don’t put off my English-speaking readers). I hope you’ve enjoyed reading about it.

Stay safe out there, drillers.