Speaking of leaning buildings, the first thing people think of is the Leaning Tower of Pisa in Italy. But in such a city, almost all buildings in the center of the city are tilted to varying degrees.
That is Mexico City, the capital of Mexico.
Like some coastal cities, Mexico City is also experiencing land subsidence problems. In recent years, some areas in the urban area have fallen by an astonishing 30 cm per year. In contrast, Venice, which is always warned of sinking into the sea, will take at least 20 years to descend to this distance.
So why is the settlement of Mexico’s capital so large, and how can it get out of this predicament?
City on sponge
Mexico City is located in central Mexico, at the same latitude as the Sahara Desert. This area belongs to the savanna climate, and the year is divided into two seasons, dry and rainy. The rainy season flows along the mountains to the center of the Mexico Valley basin, forming lakes.
Mexico’s impression is quite complex, from subtropical deserts to tropical rainforests, Mexico has it. Mexico City has a high altitude. Although it is in the tropics, it is relatively cool, with a suitable climate and abundant water sources. Therefore, Mexico City’s surroundings are very early There is human inhabitation. The most famous Aztec among the Native Americans lived here. The
early natives built a city on a small island on Lake Tescoco, the largest in the area at the time. This is a unique lake capital. Until the European colonists came to this continent.
It was the Spaniards who conquered Mexico. They conquered the Aztec civilization with cannons and ships, occupied the land and wealth of the indigenous people, and forced the indigenous people to convert to Catholicism. During the war, the island city on Lake Tescoco was razed to the ground. Then the Spaniards built a new city on this land as the capital of New Spain, a new country on the American continent, which is today Mexico City.
With the development of New Spain, the urban area of Mexico City continued to expand, and the original island could no longer bear the land requirements of a new Latin American city, so the Spanish decided to expand the island. The former aboriginal cities almost disappeared. Although there are some scattered traces, the cultural heritage has been cut off.
After the 16th century, the Spaniards filled most of the lake to use it as urban land. Streets and squares replaced dikes and canals. The lake was gradually drained in the process of filling and blocking. Today, we can no longer see the original Lake Tescoco and its water system, and some are just a super crowded city with a population of more than 20 million.
Just looking at the size of the population, Mexico City is undoubtedly a giant, and Cairo is equally eye-catching.
Although this has absorbed a huge number of people entering the city, it has many harms and brings many hidden dangers to the safety of urban residents. The first is the flooding disaster. After the lake was drained, due to lack of lake storage, rainwater and converging streams poured into the city after heavy rains, which flooded Mexico City for several years.
Therefore, Mexico City still retains some smaller waters. After all, the water flows to lower places and the result of completely hardening the road is disastrous.
Another hidden danger is even more serious, that is, the entire city sits on unstable backfills and lake beds. The clay of the original lake bed and the volcanic soil that the Spaniards used to fill the lake are different from the land we usually step on. It is extremely soft.
Volcanic soil has a characteristic-porous. Rainwater will penetrate the porous soil directly into the underground aquifer, and the full aquifer will support the upper soil.
The underground structure of Mexico City can be imagined as a sponge. When water is poured in, the sponge will absorb water and expand, and when the water is drawn out, the sponge will shrink. In addition, Mexico City is low-lying. There were originally huge lakes at the feet of these 20 million people, but there are still some smaller waters in the low-lying areas.
Mayan ruins of the temple of descent.
Therefore, once the water cycle in Mexico City has problems and the amount of water in the underground aquifer is not enough to support the soil and buildings on the ground, the entire city will sink, and it may eventually sink to the ground.
The danger does not directly come from the sinking of the city. If the whole city moves downwards at the same speed, nothing will happen, but the irregular shape of the aquifer causes the city to sink to the west when there is a shortage of water. piece. In this process, the underground pipeline will be destroyed, and the above ground structure will be torn vertically, which seriously threatens people’s safety.
Thirsty Central America
Although the underground structure of Mexico City is so unstable, as long as the aquifer has enough water, it can support the increasingly busy city on the ground. A city built on a lake bed does not seem to suffer from water shortages. However, climate change and human activities have jointly changed the ecological structure of Central America.
Most areas of Mexico have a savanna climate. The whole year is divided into two seasons: dry and rainy, and precipitation mostly occurs in the rainy season. The extreme climatic phenomenon caused by global warming has made Mexico’s rainy season precipitation more and more intense, while the rainy season has become shorter, and the dry dry season has been lengthened, and the dry season has less precipitation.
This caused excess precipitation in the rainy season to be lost before it penetrated into the aquifer, and insufficient precipitation to supplement groundwater during the dry season, and Mexico’s groundwater level gradually declined.
Simultaneously with the declining groundwater level, the population is gradually rising.
At the beginning of the 20th century, Mexico had a population of only about 14 million. Today, the population of Mexico City alone exceeds 22 million. Like other countries in Latin America, thousands of immigrants poured into the capital from all over the country looking for opportunities, allowing the city to grow from approximately 80 square kilometers in 1950 to 7,770 square kilometers today.
The city spread across the valley, almost erasing all remaining traces of the original lake.
The population of the Mexico City metropolitan area ranks among the top in the world, and the narrow and limited land is difficult to carry the operation of the huge capital. We can compare it with the same densely populated Tokyo metropolitan area in Japan. Tokyo also gathers nearly a quarter of the country’s population.
Mayan Pyramid in Yucatan, Mexico.
Due to pollution, the tap water cannot be drunk directly, and more people choose to raise funds to rent trucks to transport drinking water. In Mexico City, there are about 500 water trucks traveling between the suburbs and urban areas every day, carrying drinking water from deep wells in the suburbs, and rushing to relieve the city’s thirst.
Tokyo is located in a subtropical monsoon climate zone and is also on the coast of an island country with abundant precipitation. Many rivers in Kanto flow into Tokyo Bay, and they are also affected by typhoons. Floods are also possible, but the large amount of water does not mean that the city can retain water to prevent subsidence. Even with such superior congenital conditions, it once caused ground subsidence due to the excessive exploitation of groundwater, which reached its peak in 1968. This year, the entire city of Tokyo dropped by an average of 24 cm, and the city’s groundwater extraction reached 1.5 million cubic meters per day. Later, the Tokyo government introduced a bill to restrict pumping, reducing the rate of settlement to less than 1 cm per year.
However, Mexico City is obviously unable to stop pumping groundwater. Its surface rivers and shallow groundwater cannot meet the needs of the city’s operation. It has to obtain more water from distant reservoirs and deeper underground.
In fact, when it comes to the water pipes in Mexico City, the locals are very proud, and they think this is one of the miracles of modern water conservancy projects. The total length of pipelines that transport water from outside the city to the city center exceeds 12,000 kilometers, and the depth of water wells for drinking and extraction by residents in the suburbs generally exceeds 300 meters.
What’s more, the growth of this city is far from stopping, the boundaries of the city are still expanding, and the water source will also be expanded further and deeper. The energy required to extract and transport these water resources is huge, and what is even more heartbreaking is that this already thirsty city is facing inevitable water waste. Due to the uneven settlement of the ground, some buried water pipelines are squeezed and damaged. These pipelines often cannot be repaired in time, causing 40% of the water to drip and leak during the transportation.
This forms a vicious circle: water is taken from deep underground due to lack of water, and part of the water taken out due to ground subsidence is not used and is lost by evaporation. People can only continue to ask for water resources from deeper underground. , Resulting in more serious land subsidence.
The Mexican city government has invested a lot of money in order to ensure the city’s water supply. However, urban residents’ water has not been effectively guaranteed. There are still 20% of residents who can’t open the tap to have water. They can only turn on the faucet all year round to welcome that there may be only one hour of water supply that week.
Due to pollution, water from taps cannot be drunk directly, and more people choose to raise funds to rent trucks to transport drinking water. In Mexico City, there are about 500 water trucks that travel between the suburbs and urban areas every day, carrying drinking water from deep wells in the suburbs, and rushing to alleviate the city’s thirst.
There are also large slums in this big capital, and different classes naturally have different water treatments, but even so, the life may be much better than in the countryside.
Fill a torn wound
The entire Mexico City sank 9 meters in the last century, and continued to “sink” down at an average annual rate of 6 centimeters for the first 20 years of this century. However, “average” is a confusing term. In fact, in some neighborhoods, the settlement can reach 30 centimeters a year. Uncontrollable ground subsidence put the building in danger. Hundreds of churches and houses in the city center were abandoned or demolished due to ground collapse; Mexico City’s Metro Line 2 also became up and down due to irregular ground subsidence. .
The most obvious evidence is the Independence Monument, the symbol of the founding of Mexico.
A large mural in the Episcopal Cathedral.
Classic exhibits in the Mexican Museum of Anthropology.
Mexico Constitution Plaza.
As the tallest monument in the world, it stands in the center of the turntable of the open boulevard in the city center. The gilded angel on the top of the building looks down on the traffic underneath. It is the pride of Mexicans.
When the Mexican president cut the ribbon for it in 1910, the base of the monument had only 9 steps. However, with the serious settlement of the ground in this area, 14 large steps must be added to the base in order to make the monument remain connected to the street. connection.
The height of the newly added steps is the height of the ground sinking in this area.
Land subsidence has now threatened the lives and property of Mexico’s urban residents. The most crowded neighborhood in the capital, “Iztapalapa”, is home to about 2 million people, and it is also the most serious water shortage in the city. Block. There were also news reports that the road surface of this street suddenly cracked during the day, and a teenager accidentally fell into a huge crack and died.
But the current population of Mexico City is increasing by 350,000 per year. According to Alan Morgan, a geologist at the University of Waterloo, the groundwater level in the surrounding areas of the capital drops by about 3 feet per year. Since the precipitation rate is lower than the evaporation rate, this decline is irreversible. If the current annual extraction of groundwater is If it remains unchanged, it is expected that within 50 years the shallow underground aquifers in the capital will be completely evacuated.
By then, Mexico City will undoubtedly be worse. The entire city’s building foundations, wells, streets and sewer systems will be destroyed to varying degrees, and the city may also be paralyzed.
The only “benefits” it brings may be the creation of a number of jobs-now Mexico City has a dedicated team of workers responsible for filling the cracks in the building due to ground subsidence. According to statistics, the maintenance team can block the cracks every year. There are 40,000 cracks.
When building cities on small islands in the lake, perhaps even the best prophet of Aztec would not have thought that the current capital would be 2 meters below the surface of the water at that time. Such a thirsty, crowded, and dirty sinking city is full of crises in an increasingly extreme climate. Maybe a heavy rain will restore it to the way it was before the European colonists came.