Taking shape: terminal building at Mexico City's new airport. Taking shape: terminal building at Mexico City's new airport. milenio

Soil is a challenge in building new airport

'It's very muddy,' says an engineeer as work proceeds to slow the site's sinking

Since Aztec times construction projects in the Valley of Mexico have faced unique challenges because of the lake-covered topography and spongy underlying soil.

Those same challenges remain today.

But at Mexico City’s new international airport, which is now beginning to take shape, work designed to slow down the sinking of the site is almost half finished.

The biggest progress to date has been the laying of a foundation designed to bring greater stability to the soft earth that was once covered by the waters of Lake Texcoco.

The system is called deep foundation or piling, with vertical piles inserted at depths of four to 11 meters to increase the ground’s capacity to bear the enormous weight to which it will be subjected.

A total of 5,564 concrete piles measuring between 15 and 21 meters long will be used and 2,612 of them are already in place, representing 47% completion of the construction stage.

According to Miguel Solís — a supervisor for Gami Ingenieria e Instalaciones, the company that manufactures, transports and installs the piles — land in the area is sinking at a rate of 1.5 to two centimeters a month, up to 24 centimeters a year, but the deep foundation system will reduce that by 50% and ensure that the inevitable sinking happens in a uniform fashion.

“The airport is going to continue sinking [but] the piles help to increase the bearing capacity so that the building doesn’t sink as quickly. The concrete slab will work independently, like a boat, without being bound to the piles.  It’s called a flotation system. The terminal building is going to float on the old Texcoco Lake.”

The company is also working on a similar foundation system at the ground transportation center.

The perimeter of the x-shaped terminal building – conceived as an allusion to Mexico — has also been marked out and a 1.5-meter-thick slab of hydraulic cement reinforced with steel is expected to be poured next week over an area measuring 1.5 kilometers by 550 meters.

It was announced in January that a consortium led by Carlos Slim’s construction company had been awarded an almost $4 billion contract to construct the terminal.

Work has also begun on the construction of an air traffic control tower to the north of the terminal building and again the major challenge is preparing the land to bear significant weight.

The ground will be excavated to a depth of six meters where a 65-meter-diameter foundation slab will be placed to support the 90-meter tower.

Abel Casados, an engineer working on the tower construction, explained to the newspaper Milenio that due to the special characteristics of the soil at the site, the excavation will take place in two stages and geotechnical instruments will be used to monitor the behavior of the ground on a daily basis.

Casados added that the water content of the soil “is three or four times more salty than seawater,” which can affect the durability of the piles. So pozzolan and other additives are used in the structures to increase their resistance.

“It’s one of the main challenges of the ground.  This soil is one of a kind in the world.”

Still, he remains confident that the project will be a success.

“It has been questioned a lot but Mexican engineering has all the experience [required] to carry out [the work to meet] all of these challenges.”

The muddy ground is one. “For every soil particle we have three water particles. It’s soil with 300% dampness; it’s very muddy.”

The US $13-billion airport is expected to begin operations in 2020 with three runways and a passenger capacity of 50 million per year before eventually reaching a capacity of 120 million passengers annually with six runways.

The existing airport is struggling to cope under increasing strain from passenger numbers, making the new project vital for the ongoing prosperity of one of the world’s largest cities.

Source: Milenio (sp)

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