September 25, 2011 in Nation/World

Mexico’s capital sinking

As aquifer under Mexico City drops, so do buildings
Tim Johnson McClatchy
 

The wooden door at the main entrance to the Basilica of Guadalupe in Mexico City, shown here on Sept. 12, is not square to the floor. That’s because as the city sinks, buildings tilt.
(Full-size photo)

By the numbers

50: Approximate number of structures that have been condemned in Mexico City since 2006 because of leaning

5,000: Approximate number of buildings considered unstable and at risk

380: Approximate number of fissures that have opened in the ground in the greater Mexico City area

MEXICO CITY – Walk into any of hundreds of homes or buildings in this huge capital, and you feel immediately that something is amiss. The buildings tilt.

“If you put a ball on the floor here,” Thierry Olivier said, sitting on the ground floor of his three-story building, “it will roll over there.”

By Olivier’s calculation, one corner of his 105-year-old building is 11 inches lower than the other. It lists like a tipsy cantina patron.

It’s a common phenomenon here, where many buildings are sinking, as each year Mexico City’s 21 million thirsty residents suck up water from the aquifer beneath one of the world’s largest metropolises. As the water level in the aquifer drops, the ground above it sinks.

But not evenly. Layers of soft clay beneath the city vary in thickness, and the ground sinks faster where clay dries out, grows brittle and collapses. That means that in some parts of the city, sidewalks buckle, window frames lean, subway lines need expensive repairs, and drainage canals no longer flow downhill.

As buildings turn and settle at a glacial pace, the humans who live in them also adjust, growing accustomed to living at a slight slope.

Engineers assert, however, that inhabitants face not only structural risks but also potential health problems as houses and apartment blocks incline.

“When a building tilts more than 1 degree, then I think it begins to become very uncomfortable,” said Enrique Santoyo Villa, an engineer who’s experienced at propping up and bolstering churches, monuments and other tilting structures.

“Tables aren’t stable. Liquids don’t look right when they are in big containers. … Windowpanes can break. Doors don’t close right,” Santoyo said.

Ancient Aztecs built the city of Tenochtitlan on an island in the middle of a large lake, making it the capital of their powerful empire. When Hernan Cortes and fellow conquistadors arrived in 1519 and conquered the Aztecs, the Spaniards built Mexico City atop the Aztec ruins, then drained much of the lake to control flooding.

Scores of colonial churches and other stone buildings in central Mexico City have survived frequent natural disasters but succumb to the soft clay underfoot, leaning or sinking into the ground.

Experts say parts of the metropolitan area have sunk by as much as 27 feet since the late 19th century, an average of 2.5 inches or so a year.

Some of the heaviest stone buildings, such as the opulent Palace of Fine Arts, have sunk 13 feet in a century. Its original ground floor is now a basement.

The tilt of other buildings is noticeable to the eye. A few list as a whole, while others, such as Mexico’s National Palace in the city’s Zocalo central square, undulate.

The city’s main cathedral and abutting Sagrario Church are a special case. The church is built partly atop the rigid remains of a giant pyramid to the Aztec sun god, so it sinks less than the larger cathedral, which seems to lean away in displeasure.

So acute was the cathedral’s tilt that Santoyo and other engineers, working in consultation with Italian experts who’d stabilized the leaning Tower of Pisa, spent six years and some $33 million to reinforce the foundation. The project was completed in 2002, correcting a 2.7 percent tilt to 2 percent, enough to stabilize the structure.

No building in the capital leans as precariously as the Basilica of Guadalupe, the central place of worship to Mexico’s patron saint. Construction of the basilica began in 1531 and lasted more than a century, but by the 1970s it had tilted so much that it was declared unsafe and a new, modern basilica was built next to it.

The city has condemned 50 or so structures since 2006 because of leaning, and another 5,000 or so homes and buildings are unstable and at risk, said Oscar Alejandro Roa, the director of prevention at the city’s Civil Defense Bureau.

In some of the buildings, he said, “You have a permanent feeling of vertigo.”

One of the dangers of the small earthquake swarms common to Mexico City, he said, is that “buildings shake at different rhythms.” Space between them is necessary so that they don’t bash into each other.

Given constant tremors and subsiding soil, engineering and architectural firms in the capital make a steady living off bolstering buildings.

As the ground shifts underfoot, some owners put their faith in engineers to correct any subsidence that may occur to their buildings in the future.

“Mexicans are probably the best foundation engineers in the world,” Olivier said, adding that he expects his building to last another century.

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