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 How do you restore a building famed for its fatal flaw? The Metropolis Observed August/September 2001
The panic-stricken government began drafting experts to save the structure. In 1990 Dr. Michele Jamiolkowski--a renowned professor of geotechnical engineering at Polytechnic of Turin who was already busy cleaning up Chernobyl and trying to save Venice--was surprised on opening his morning paper to learn that he had been appointed chairman of the international commission to save the tower. "This type of thing had never happened to me," he says. The tower was put on full life support while the commission debated how to save it. Eighteen tensioned cables were wrapped around the second story to prevent it from buckling at its aging joints. High-tech sensors--sensitive enough to register the infinitesimal changes in the edifice's tilt caused by a sunny afternoon--send more than 120 measurements every five minutes to a bank of computers. Despite the sophisticated monitoring equipment, it appeared that modern technology had no surefire cure. In addition, the mandate for Jamiolkowski's committee left very little flexibility. "The challenge was that there was a very low margin of safety," he says. "And it was unthinkable to do anything that would have a visual impact."
Italian prime minister Giulio Andreotti had in effect taken the tower away from the city of Pisa and put it under the control of the commission. Pisa's mayor and citizens were in an uproar about the closing of the structure, which is visited by nearly a million tourists a year. At the same time, many restoration experts were expressing alarm that the commission would act too hastily and wind up destroying the tower. James Beck, a professor of art history at Columbia University and the president of ArtWatch International, a watchdog group that monitors cultural monuments worldwide, says the commission's authoritarian approach reminded him of the fascist era. He charges it with unnecessarily "creating a background of fear that the tower was in imminent danger." But commission members say their fear was justified. By 1990, when Jamiolkowski was appointed, the top of the tower was 18.4 feet out of plumb, and its tilt was increasing at about .06 inches a year--twice its rate at the beginning of the twentieth century. The group faced two equally terrifying scenarios: the tower's aging masonry, aggravated by its lean, would buckle at weak points and it would collapse; or it would reach a point where the surrounding soil would no longer be capable of supporting it, and it would just tip over. The tower's signature stance is a result of its unfortunate location near the Arno River. It was built on some of the worst soil around, an uneven layer of river silt further compromised by an underlying layer of compressible clay. Though its architects were capable of designing one of the most beautiful towers of its day, their sophistication did not extend to laying foundations. The 191.5-foot structure, which weighs 14,500 tons, rests on a foundation that originally descended only about a yard into the ground. Work began in August 1173; by the time the middle of the fourth story was added in 1178, the edifice was already leaning slightly to the north. Then the builders stopped, but it is unclear whether it was because they ran out of money or because they were perplexed by the tilt. However, with a 100-year breather, the tower was able to settle to some degree on its bed of compressible soil. Work resumed in 1272, and this time the tower shifted southward. During this phase, which lasted only until 1278, the builders attempted to compensate for the lean by building the inner walls on the southern side progressively thicker to shift the weight back toward the vertical axis. This desperate attempt to compensate for gravity gave the tower somewhat of a banana shape. In 1360 the final story and bell chamber were completed--a top-heavy addition that greatly exacerbated the tilt. With time the lean increased steadily as the tower sank into the soft earth. By the early nineteenth century the base of its columns and its entrance were partially buried. The first ambitious effort to save the structure took place in 1838, when the architect Giardino della Gherardesca freed the buried portion by excavating a ditch, the bottom of which he filled with a thick layer of mortar. This measure removed the moist earth that had been weakening the structure and also made the interior accessible to visitors. But the removal of the supporting soil around the base to build the reinforced ditch (known as the catino) gave the tower a major jolt. Burland estimates that the tower jumped half a degree, or about 18 inches, at that time. Because the catino was constructed below groundwater level, a new problem was created: leaking groundwater, which had to be constantly pumped out. An attempt to fix this problem with 80 tons of grout in 1934 plugged the leaks but enhanced the tilt. It was one of those earlier attempts to save the tower that almost sabotaged Jamiolkowski's commission. In 1995, during a period the Italian press referred to as Black September, an attempt was made to substitute underground anchors for the pile of unsightly lead weights that were being used to prop up the tower. To install the anchors they started removing a small portion of the southern edge of della Gherardesca's catino, not realizing that during the 1934 intervention it had been fused to the original foundation by hundreds of steel pipes used to reinforce the concrete. Suddenly the tower lurched as far in one week as it had the entire previous year, and was stopped only by the addition of more lead weights. "This was the week during which I did not sleep," Jamiolkowski recalls. Finally the commission hit upon a relatively new method known as underexcavation, which had been used in 1990 to stabilize the Metropolitan Cathedral in Mexico City. The approach involves slowly removing soil from the tower's northern side, allowing it to rock back gently and reducing the tilt. But some members worried that the new method was too risky. The danger was that underexcavation could go too far and destabilize the tower. In January 2000, after much debate and testing of the new method, full-scale underexcavation actually began. Work proceeded extremely slowly, using 42 thin pipes that suctioned out only several wheelbarrels full of soil a day. By this spring it was clear that the commission's work has been a resounding success. The tower's tilt has been reduced by about 10 percent, or 20.2 inches, back to about where it was when della Gherardesca began fiddling with it in 1838. Pisan authorities regained control of the tower in June, and it will reopen to the public sometime in November. Jamiolkowski hopes it has been permanently stabilized--but at the very least, he says, the world shouldn't have to worry about it again until around 2200. |
