TRENCHLESS TECHNOLOGY
SAVING A CITY FROM SINKING
Mexico City’s gigantic sewerage project could help stop the city sinking. German trenchless specialist Herrenknecht explains how its Earth Pressure Balance Shields are helping
M
exico City is subsiding and its sewage system is over capacity. During rainstorms like that of 4 February this year, the sewage system can break down within hours. That
morning, rescue workers had to use boats to navigate the streets and evacuate trapped residents, while 28 people lost their lives. On the approach to the city airport’s terminal building is a huge
billboard which announces that the Federal Government of Mexico is working on a solution; it reads, “The Emisor Oriente tunnel, the largest water project in the world, will double the capacity of the drainage system and reduce flooding in the Valley of Mexico.” Flooding in Mexico City has increased over the past ten years. This is because subsidence has robbed the Emisor Central main sewage duct of about 40% of its original capacity of 170m3/s. “Most of the city is built on swampy ground,” says geologist Federico
Mosser. “Since this swampy ground is used as a source of groundwater, it is becoming ever more compact and causing the city to subside.”
Ancient dykes
In fact, Mexico City is built on the site of a former lake. Tenochtitlán, the capital of the Aztec Empire, was situated on an island criss-crossed by canals. The Aztecs managed the water surrounding their capital with a sophisticated system of dykes. The system was destroyed by the Spanish conquistadors who drained the lake and built on the resulting land. Four devastating floods occurred in 1555, 1579, 1604 and 1629, but it was not until centuries later that a solution was sought for the problem. In 1900, the first network of sewage ducts, the Gran Canal, was opened. Despite this, there was another huge flood in 1969. At the time, construction work on the Emisor Central had been under way for two years; it was completed in 1975. Since the 19th century, more water has been removed from the
swampy reservoirs beneath the city than could be replaced by rainfall. About 70% of the water needs of the city’s 20 million inhabitants are met from groundwater, and that is why the city is sinking. The records of the National Water Commission (Conagua) show that Mexico City was 1.9m above the water level of the former Aztec lake 100 years ago. Today, the city is 10m below that level. “This subsidence means the Emisor Central sewage duct no longer has the same gradient as before,” says Jesús Medecigo, project manager at Conagua. “In some places it even rises and we have to use pumps to remove the sewage.” This is why construction began on the Emisor Oriente tunnel in mid 2008. With a diameter of 7m, it will feed up to 150m3/s along a
16 Water & Wastewater Treatment May 2010
Above: the
Earth Pressure Balance Shield S-497. Right: Mexico City is
gradually pulling the carpet from under its own feet. Far right: Rolando Justa, managing director
of Herrenknecht Mexico
length of 63km beneath the capital and the State of Mexico, as far as the State of Hidalgo. A consortium of five companies is commissioned with the realization
of this mega-project. Conagua searched for specialists. “We were looking for someone who can support us within our planned timeframe,” Jesús Medecigo remembers, “and we didn’t want the risk of relying on just one partner.”
Herrenknecht machines
Herrenknecht is supplying three of the six machines for this project and the new sewage duct should be completed by the autumn of 2012. The Mexican President, Felipe Calderón, was present when the first section of a Herrenknecht Earth Pressure Balance Shield was lowered into shaft zero in June last year. “There is a tradition in Mexican politics of building things that are visible,” said the President in his speech. “But this will be a highway no one can see and no one will ever drive on. But, still, it is vital.” It is also expensive – the construction budget exceeds US$1B. The
project includes more than an extension of the sewage system. It also provides for the construction of six sewage treatment plants to cover the needs of the entire city. Now, only 6% of the city’s sewage is treated. In future, using the treated water for non-drinking purposes will mean less water is removed from the underground reservoirs. “This will not raise the city up again,“ says Medecigo, “but it will prevent further subsidence.“ Geologist Federico Mosser was commissioned with exploring the
ground beneath the city. “Even our initial investigation showed us that 95% of the new tunnel’s route would take it through soft soil layers and only 5% through rock,“ he remembers.
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