City on a Lake

Did you know that Mexico City, the capital of Mexico, is built on a lake? Roma Agarwal in her book, Built, points out that it started off as a small island but:

“The city now spreads far beyond its original site, but the center of town, which contains most of the historical Aztec and Spanish buildings, sits on that lake.”

So why was the city built on/near a lake in the first place?

“The site of the city was determined by a (Aztec) vision.”

But the Aztecs just built on that island and near it. The Spanish came in 1521 and “cut down trees around the lake, causing mudslides and erosion that made the lake be shallower”. Chaos, devastation and flooding. Eventually, the lake was filled with soil to “allow the city to expand”.

 

Which brings us to the topic of soil. Since this is a book on how things are constructed, Agarwal gets into the details of piles (“columns put deep into the ground to help support the structure above them”). The type of pile and how well it works depends on the nature of the soil, specifically how sticky it is, how much friction it offers.

 

So an engineer needs to know the type of soil. But there’s more:

“It’s not enough to meet the soil, ask how it’s feeling on the day you start building… It has a history and a character that an engineer must consider.”

The Aztecs had built their pyramids, thereby exerting a lot of pressure on areas where the soil thus became “consolidated and compacted”, while other areas “which hadn’t been weighed down, remained light and less dense”.

 

All of which is why Mexico City is sinking: in the past 150 years, it has sunk by over 10 meters, “more than a three-story building”. But it also means some areas, the ones where the soil was more “consolidated and compacted” sank less while others sank more. Unfortunately, Mexico City’s enormous Metropolitan Cathedral, lies on both types of soil, and so it has been tilting. A massive attempt to straighten the tilt was started but stopped midway in 1998. The tilt had been reduced but not eliminated, and the soil is now being “watched” via sensors:

“It has become a place of science as well as a place of worship.”

 

(In case you’re wondering, yes, this “groundbreaking work” is being replicated at the Leaning Tower of Pisa).

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