Saving a Sinking Cathedral

MEXICO CITY â€”  The majestic Metropolitan Cathedral was built as a symbol of the Spanish crown’s domination over its new colonies in the Americas, showing the power of the Roman Catholic Church over pagan Aztec rituals.

The conquistadores built the cathedral right on top of the ruins of the pyramids they had systematically demolished after their conquest of Mexico in 1521.

Yet ever since, it has seemed as if the defeated Aztec warriors were fighting back--reaching up from their graves to drag the most glorious icon of the capital of New Spain back into the earth. In this century alone, downtown Mexico City has sunk more than 25 feet as the water-starved ground has compressed; because some parts have sunk faster than others, one end of the cathedral settled nearly 8 feet deeper than the other, like a listing ship.

As the ballooning Mexico City population devoured the capital’s ground water throughout this century, the cathedral grew ever more lopsided, until it snapped in 1989 from the cumulative stresses, opening a dangerous crack across its spine. At that point, its gnarled naves and pillars were strapped with scaffolding and reinforcement beams.

Yet Mexican authorities never considered the prospect of failing to save the cathedral, which took 250 years to build and which art historian Manuel Toussaint described in 1944 as “the most notable monument of the colonial period in the Americas.�

Today, the 435-year-old cathedral has nearly been righted thanks to an ingenious rescue operation. Instead of needing last rites, the church will soon be healthy enough to throw away its structural supports and regain its original unfettered splendor.

And in the process of saving the cathedral, Mexican engineers have developed a methodology that offers hope of rescue for crippled buildings around the world, including the Leaning Tower of Pisa.

Faced with budget constraints and the need to keep the cathedral functioning during the restoration, the team in charge of the project came up with a solution that was counterintuitive and comparatively cheap: Rather than try to prop up the sinking front end, the engineers would remove subsoil from the other end--and ever so carefully lower it back toward relative equilibrium.

If that sounds straightforward, imagine the scale of the project: Inch by inch and year by year, the engineering team has repositioned a church that weighs more than 127,000 tons and is more than 400 feet long. Since excavation work began in 1993, the entire cathedral has been ratcheted more than 3 feet toward level, nearly half the distance it had settled askew.

“It is a process of observing and adjusting constantly,� said architect Sergio Zaldivar, the project leader. “We are working against 400 years of deterioration.�

The cathedral’s pollution-stained gray stone facade, with its two huge bell towers added in the late 1700s, broods over Mexico City’s stark, concrete Zocalo, said to be the largest plaza in the world after Red Square in Moscow.

Even under scaffolding, the cathedral remains a focal point for the bustle and swirl of life in the Zocalo and the surrounding congested downtown streets. And despite Mexico’s almost total racial mixing over the centuries, the contentious relationship between Spaniard and native Mexican is still played out in the Zocalo every day.

As if to taunt the priests saying Mass inside, indigenous dancers and drummers perform in the square within earshot of the cathedral. Often, native Maya in traditional dress give soapbox lectures to passersby, urging them not to forget their Indian roots and to beware of the Catholic Church.

Along the western fence of the cathedral, plumbers, electricians and other artisans line up neatly with their toolboxes on display, offering their services. Hawkers crowd the nearby sidewalks selling pirated cassette tapes and T-shirts of Subcommander Marcos, the leader of the leftist Zapatista rebels in the southern state of Chiapas.

Neither the cathedral nor any other Mexican church building still belongs to Catholic authorities after the seizure of church assets under the 1857 constitution. And the church is not involved in the restoration work, which is being carried out by the Mexico City government. The latter is responsible for the cathedral and other historic monuments.

Varied Sinkage Rates a Problem for Engineers

Mexico City’s varying sinking rates have caused damage to scores of historic buildings over the centuries, and frequent earthquakes have added to the structural havoc. Mexican engineers have built a reputation for their skill in shoring up stricken monuments--though none posed such a challenge as the cathedral.

The heart of the cathedral’s structural problem is that different sections of the building are sinking at widely different rates. By 1989, the heavier bell-tower end of the cathedral had settled nearly 8 feet deeper than the rear of the church, and the eastern tower had subsided nearly 3 feet more than the western tower.

This bewildering range of settling rates led to dangerous buckling throughout the structure as well as cracking walls and roof vaults, causing the pillars to lean and creating waves in the floor that can make visitors dizzy as they stroll the naves.

Conquistador Hernan Cortes personally ordered that New Spain’s first, humble cathedral be built in the center of the defeated Aztec capital, Tenochtitlan, in 1524. That church was replaced by the present cathedral, which was intended to be as grand as its towering counterparts in Seville and Granada in Spain.

Despite warnings from advisors that the ground was unstable, construction began in 1563. The church started to sink shortly thereafter.

The builders had to alter the design several times and were forced to add more than 3 feet in length to the front facade to keep it in balance.

Most of the main work was completed by the 1770s, but further changes were made until the final consecration ceremony in 1813, three years after Mexico declared its independence from Spain. During its lengthy construction, the church absorbed disparate styles, from baroque to neoclassical. It is a stylistic hodgepodge that somehow achieves a sense of harmony.

Toussaint applauded the finished cathedral this way: “It is moving, it is dramatic, it is a monument that contains in itself the whole history of a people--the religious, passionate and tragic history of that people.�

The idea for the new solution came from Enrique Tamez, an engineer who had used computer analysis to help lead the diagnostic procedure--and who understood the complexity of the variegated subsidence under the cathedral and the contortions the structure was experiencing.

To carry out the excavations, the engineering team dug 32 wells under the cathedral and the adjacent Sagrario church, built in the 1700s as a lateral extension of the cathedral. The concrete-lined wells are 10 feet in diameter and reach as far as 75 feet below ground. The wellheads are just below floor level and can be reached only through the eerie corridors of the cathedral’s crypts.

At the bottom of each well, dozens of portholes give access to the muddy clay. Using hydraulic drivers, the crews push steel pipes horizontally deep into the clay. The pipes fill up with clay and then are slowly pulled back into the bottom of the well and hauled to the surface.

Thus, while parishioners attend Mass and tourists stroll overhead, workers in hard hats are busily pushing wheelbarrows full of mud to the music of compressors and extractor fans. This activity occurs in narrow, low corridors whose walls are lined with as many as five levels of caskets of prominent Mexico City families.

Moving Nearly an Inch a Month at Top Speed

The soil-extraction process is painstakingly complex. Taking too much clay from the wrong spot, and not coordinating the many simultaneous extractions, would risk overcorrection that would cause even worse damage. At one point, the excavations were so effective that the church was being moved nearly an inch a month.

Zaldivar said his team is on the verge of declaring the project a success and stopping the excavations, at least for the next few years. The original goal was to win back 39 inches of the 2.7-yard differential between the two ends of the church. Having moved the church 38 inches, the team is just about there.

The cost has been about $2.5 million a year, which Zaldivar notes is “less than a highway overpass� and was far cheaper than any of the other options considered--such as installing 1,500 pilings up to 200 feet deep. The Italian government, keenly interested for the potential application of the approach at Pisa, contributed a sophisticated monitoring system. (The Pisa committee last year agreed to a similar excavation approach.) Otherwise, the expertise and technology have all been Mexican.

A stream of foreign specialists from areas with similar problems, including Germany and Brazil, has visited to study the concept and its execution, Zaldivar said.

The sinking of the city results from over-exploitation of the ground water in the Valley of Mexico, especially this century. Not only has the area’s population grown to about 18 million, but the percentage of those people with access to running water has risen fast, hugely increasing consumption.

Seventy percent of the city’s water comes from wells tapping the ground water (the rest is pumped in at great cost from surrounding areas), and the wells have lowered the water table perilously throughout the valley. The central city--a lake bed in the Aztec era--has subsided nearly 26 feet as the drier subsoil has compressed.

“This is an apocalyptic barbarity,� Zaldivar said of the city’s self-inflicted water problems. He has to deal with the consequences, not only for the cathedral but for many other monuments, including the National Palace, which is visibly bulging and twisting despite the more than 200 pilings placed underneath its foundation to shore it up.

“In the Mexican mentality, the cathedral represents the Mother Church. It is the liturgical center, the seat of the archbishop,� said Father Ruben Avila, who has served here since 1985, shortly after a fierce earthquake knocked down hundreds of buildings and killed more than 8,000 people in the capital.

Surrounded by the 55 miles of scaffolding that shrouds the interior of the church, priests celebrate Mass several times a day.

“The cathedral is now in intensive care, full of tubes and machinery, as if it were staying alive on a respirator,� Avila said. “But, thank the Lord, the cathedral will soon be able to show its fine face once again.�

Shift in Demographics Alters Neighborhood

Avila said that the parishioners are mainly poor people from the area’s decaying neighborhoods. Once-elegant palaces in the surrounding streets have been transformed into overloaded boardinghouses for street hawkers and other itinerants, many of whom come to the capital from Mexico’s poorest villages to scrape a living from the city’s markets.

“At first, people were afraid [of the restoration work]. There were many interpretations, even that the cathedral was going to fall down,� Avila said. “So the people didn’t come for a while. But now they have returned, and the cathedral is a sea of people again, even if all its splendor is not yet shining.�

Avila noted that the cathedral is as significant for its artistic treasure-trove as for its architectural importance. Along the outer two naves are 16 beautifully crafted chapels. The intricate Altar of the Kings, reaching 90 feet high, is regarded as the world’s finest wood-carved and gold-foil-covered retablo.

Nobody is using the word “miracle� to describe the rescue effort. But among the many statues of saints scattered through the church that attract prayers of the faithful each day, one seems especially appropriate: St. Judas Tadeo, with his “prayer for difficult cases.�

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Mending a Cathedral

Loss of ground water brought on by overpopulation caused the tower end of the 435-year-old Metropolitan Cathedral in Mexico City to sink 8 feet deeper than the back end. Instead of raising the sinking front end, engineers are removing subsoil from the back end. Since the excavation began in 1993, the back side of the cathedra has been lowered 38 inches, one inch from the goal.

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Removing the Soil

Thirty-two wells were dug under the cathedral and adjacent Sagrario church. The wells are accessed by workers through corridors in the cathedral’s crypts.

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Each concrete-lined well is 10 feet in diameter and reaches as far as 75 feet below ground.

1. Crews use hydraulic drivers to push steep pipes through dozens of portholes into the muddy clay.

2. Five-foot section of pipe filled with clay is drawn back into well.

3. Hydraulic pulley is used to haul pipe section to surface, where it is emptied.

Leveling Out

Engineers used computer analyses to determine how much soil to remove to restore the cathedral to near-level condition. The goal was to eliminate almost half of the 8 feet it sank.

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Cathedral is more than 400 feet long and weighs more than 127,000 tons.

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The rate of sinkage varied at different points beneath the structure.