 Frankfurter Allgemeine Zeitung 2000 (PISA, Italia, Sunday 18 December 2000 ) |
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By Georg Küffner The Leaning Tower of Pisa was, is and will remain a spectacularly crooked architectural monument. The souvenir sellers and pizzerias around the Piazza dei Miracoli in Pisa need not fear that current measures to straighten it will cost the tower its attraction and them their livelihoods. Quite the contrary. Although the stabilization effort is slowly nearing completion, the changes are imperceptible to the naked eye. The deviation of the tower's vertical position measured by the distance from its base to a point on the ground, indicated by a plumb line dropped from its highest ledge, has been corrected by a modest 30 centimeters (12 inches). The goal is to reduce the tilt by about 10 percent. This would steady the future lean of the tower by about 4 meters (13.2 feet), which, if experts have correctly gauged the tower's movements over past centuries, will leave the tower about where it was 300 years ago. |  A crane places new steel cables anchoring Pisa's Leaning Tower. (AP Photo) |
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The brave assumption is that the tower might still top Italy's list of favorite tourist destinations well into the middle of this millennium. The stabilization effort has proceeded so well that authorities now hope the tower can be reopened to the public next summer, 10 years after it was closed.
Before such a step can be taken, however, the campanile must not only be straightened, its interior structure must also be strengthened. The renovation work includes many individual steps, each of which must be precisely coordinated with the others. The overriding goal is to employ only protective measures. That eliminates all straightening techniques that involved the foundation directly. A successful procedure developed in Germany for straightening smokestacks that have been undermined by hard coal mining was, therefore, not even considered. That procedure would have involved sawing the entire structure and lifting it hydraulically. Instead of this, an international committee of experts examined three gentler methods whose feasibility had been studied for several years prior. They soon eliminated Stuttgart Professor Fritz Leohardt's idea of bolting down the side opposite the tilt by using what are known as earth anchors, while simultaneously compressing the soil under it. To test the practical feasibility of this technique, a concrete ring was laid around the tower's foundation. Lead weights were placed on the side opposite the lean. Although the tower reacted very well to this treatment and straightened up a bit, this method was rejected for use in long-term renovation. However, the effort was not entirely in vain. Some of the original 900 metric tons (990 tons) of lead weights are still in place on the concrete ring and are being used to support the current work. They will be gradually removed as straightening progresses. |
 (Photo: Georg Küffner) |
Another idea not implemented was that of Professor Carlo Viggiani, a soils engineer at the University of Naples. He proposed that the water component of the clay soil under the portion of the foundation protruding from the ground be removed, using electrical osmosis to reduce its volume. This, in turn, would cause the tower to sink back into the soil, thus correcting itself.
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Although this procedure never got beyond of the experimental stage, the committee was convinced of the validity of its underlying concept. Straightening the tower was simply a matter of removing some of the soil from under the foundation on the side opposite the direction of the lean. Moreover, using what they termed the "lost earth method" did not involve breaking any new ground. The technique had already been used to level the soil under the foundation of Mexico City's cathedral.
To avoid incurring any unnecessary risks, the method was tested on a nine-meter tall and seven-meter wide concrete model tower, with conditions for the test closely approximating the actual ones. The test tower was built just 500 meters away from the leaning tower and on the same pudding-like, ground-waterlogged soil as Pisa's landmark. A drill was used to extract small quantities of soil from under the 560-ton test tower, while a lavish array of sensors monitored changes in its position. The results of this method, suggested by London Professor John Burland, were so satisfactory that it won the bid for the renovation work. Beginning in February 1999, tiny doses of sandy, clay mud were extracted from under the leaning tower. Within five months, a total of 7.5 cubic meters (26.25 cubic feet) of soil had been extracted through 12 bore holes from under the point on the north side, where the centuries-old structure penetrates the soil. The observable result was a vertical positioning of some 80 seconds of arc. The site was then left alone until February 2000 to allow the tower to further compress the soil in the drilling zone. During that time, the tower righted itself by another 50 seconds of arc. Measured by plumb line deviation, the campanile has straightened itself by about 3 centimeters since construction work began. After these first efforts produced such good results, the decision was made to apply the technique on a greater scale. A total of 41 holes were drilled beneath the north side of the tower. Every day, the wagon drill was applied to about six of the holes. During each shift, about 200 to 300 liters (52 to 78 gallons) of mud were brought to the surface. At the end of each shift, the tower's own weight closed the holes overnight. Ever so slightly, the north side of the tower subsided into the ground as the opposite side began to rise. The construction fence attracted the attention of curious tourists to the drilling work. But what they found even more interesting were the "suspenders" to which the tower had been attached. These holding cables leading north above the Opera del Duomo (cathedral opera) were attached to steel cables wrapped around the tower's second story. Whether they could restrain the tower if it should begin to lean dangerously or even tip over was more than dubious. Some experts maintained that putting tension on the "suspenders" would put lateral thrust on the point in the structure where its stone and mortar construction was at its weakest, thus causing the tower to collapse. The torre pendente (leaning tower) does not consist, as its white marble exterior might lead one to think, of a solid structure of stone blocks carefully laid on top of one another. Quite the contrary. Some 800 years ago, the tower's master builders made the 2.7 meter-thick walls out of a "sandwich," consisting of inner and outer stonework shells that are between 15 and 30 centimeters thick with a filling between them of sand and rocks to which a lime mortar matrix was added. It is not unusual to find cavities of up to 30 centimeters in diameter in the filling. Thanks to this sandwich-like construction, the thin exterior skin must bear the main load of the tower's approximately 15,000 metric tons. Individual stones proved unequal to the task and burst. In other words, the tower not only leans, it is in danger of structural collapse. Seven years ago plastic-coated steel cables were tightly wrapped around the tower just beneath the first circular walkway to minimize that danger. The main threat to the tower is at a point not far from the "buttons" of the suspenders. Here, the marble shell coming up from below is displaced by nearly 2 meters, so the stonework exterior shell that continues upward is not properly supported from below. The interior filling, therefore, has to carry a large part of the tower's weight at this point. This places strong outward horizontal forces on the exterior shell. To reduce these forces, the tower must be straightened a bit. Every degree of arc, which lifts the tower back toward the vertical, reduces these destructive stresses. To maintain its future attractiveness as a tourist attraction, the tower will never be made straight as an arrow. That possibility is strictly ruled out by its banana-like curvature. The tower not only leans, it is crooked. Even in the early stages of its construction, the south side began to subside into the soil significantly faster than the north side. The master builders counteracted that by warping the structure as they built it. This is why the belfry at the top of the tower has six steps on its south side and only four on the north. The builders were forced to resort to this artifice because the tower was already leaning that much by the time this final stage was begun. If by next spring the Leaning Tower of Pisa is pushed back to the north by the 40 centimeters the plans call for, it will be in a more stable condition than at any time in the last three centuries. Contributing to that stability will be the 70 tie rods that have been carefully drilled through the south side wall and tightened. In addition, 50 square meters of the crumbling interior materials on that side of the tower have been filled and solidified with a cement matrix. It remains to be seen whether these measures will actually give the tower the long-term new lease on life that everyone would happily grant it, after all the efforts of the past years. The tower's behavior will always be hard to predict. For Paolo Hieniger, a doctor of engineering who has coordinated and supervised the renovation work for the commission during the last seven years, the tower is and will remain a peculiar creature. © Frankfurter Allgemeine Zeitung 2000 All rights reserved. Reproduction in whole or in part is prohibited. |
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