Tilting the Tower of Pisa
Special to The Washington Post
Wednesday, December 9, 1998; Page H05
For almost eight centuries, curious travelers have flocked to Pisa, Italy, to see its famous landmark. Tilting southward more than 5 degrees from the vertical, the 195-foot Tower of Pisa is about 15 feet out of plumb. But the Italian government finally has approved a plan to rescue the imperiled structure.
Photographs do not convey the dramatic extent of the tower's lean, nor do they reveal the odd, banana-like shape that is a result of its construction history.
Work on the tower was begun during the early Renaissance when newly emerged republics, grand duchies and seats of ecclesiastical authority in Italy competed with each other in building campaigns. The first tower campaign began in 1173 and stopped in 1178 at the fourth story. Records indicate that it was already leaning then, thanks to comparatively soft ground on the south side.
So when construction recommenced in 1272 for another six years, more stone was added to the low side in an attempt to even things up. The bell chamber at the top was completed in about 1370. Apparently, several generations of craftsmen and stonemasons were not particularly proud of their accomplishment because nowhere in the tower did they record their names.
Closed to the general public since 1990, the tower is unstable, although further movement has been stopped with a series of temporary measures instituted in 1992. At that time, the government feared that the tower would fall before a permanent stabilization method could be developed.
That year, Italy's prime minister named another commission of experts -- structural and geotechnical engineers, architects, art historians and archeologists -- to develop and implement a permanent method. It was the 15th such panel in this century -- in a nation that has had 56 governments since 1945.
Essentially, two interdependent problems are involved. One is soil condition, which has been studied by taking thousands of borings under and around the tower. The structure sits on silty sand, beneath which are layers of clay to about 130 feet.
Initially, the tower leaned because the ground on the south side is more compressible than on the north side. In addition, Pisa is near the Mediterranean coast and only about six feet above sea level, so the water table is quite high. Because of unregulated pumping from deep wells during the early 1970s, the water table and soil behavior changed again, increasing the tower's inclination.
The second problem is the fragile structure. The tower is a hollow cylinder faced with striking white marble on inner and outer surfaces. Between the facings is rubble and mortar, and tests have revealed large voids. The thin marble facings are not bonded to the fill material, so the marble bears nearly all of the weight.
Because the tower is leaning and because of the inconsistent nature of the structure, its weight, or load, is distributed unevenly. On the south side, the stone is being compressed while, on some parts of the north side, stone is being pulled apart.
Masonry structures generally can withstand considerable compressive force to a certain height. But they cannot handle strong pulling (tensile) forces well. Recent studies indicate that, in some places, the tower's marble shell is in as much danger from structural as from soil failure.
Over the years, many solutions have been proposed. No one has suggested bringing the tower to a fully upright position, which probably is neither possible nor desirable. The Leaning Tower is a major international tourist attraction in a city that might otherwise receive comparatively few visitors.
Now, however, the tower's immediate vicinity looks more like a construction site than the Plaza of Miracles, as the area containing the tower, a cathedral, a baptistery and a cemetery is called. Around the tower, engineers are moving from one remedial solution to another.
The previous solution involved depressing the ground on the high side. In May 1993, a concrete ring was built around the base in hopes of using it to anchor cables. Four months later, workers began placing lead counterweights on the ground there, pressing it down. About 1,000 tons of ingots have managed to stabilize the foundations temporarily.
Meanwhile, the masonry has been secured by binding steel cables, lightly prestressed and covered with plastic, around the outside to the second story, much like wrapping rubber bands around a bundle of pencils. With those measures in place, the commission bought time to devise a permanent solution to be implemented this winter.
Reaching a consensus within the panel was time-consuming and stressful, with extensive and heated examinations of each proposal. At last, the Italian members opted to pursue the scheme of a British member, John Burland, a professor of geotechnical engineering at London's Imperial College.
Scheduled to take about a year, the plan involves erecting a stationary A-frame structure on the north side and extending cables from it to a sort of sling or harness around the midsection.
This is to hold the structure in place while crews gradually begin to remove small amounts of soil from the high north side. Presumably, this will cause that side to subside or sink slightly, rotating the tower back toward the north by about one-half of 1 degree.
Before the A-frame idea, some experts had proposed an elaborate reinforcement scheme intended to get rid of the unsightly mass of lead weights. It entailed freezing the ground to stabilize the wet soil in order to place anchors for 10 planned cables. Unhappily, when the freezing effort ended after 36 hours, the tower began to rotate south a fraction of a degree. The search for a solution continued.
According to Burland, the goal "is to reduce the inclination by about 10 percent. That would not be visible to the naked eye. This will be done very gently, very slowly in a very controlled sort of way."
Will it work? Critics, notably James Beck, an art historian at Columbia University, say the tower should have been left alone. The lead ingots have permanently altered soil characteristics on the north side and, if removed, may bring down the tower.
In his London lab, Burland has been creating computerized and physical models of the tower during the righting process, trying to predict how the structure would respond as soil is removed.
Based on these studies, Burland is quite optimistic. Of course, obtaining perfectly accurate information about every aspect of the structure and foundations of the 800-year-old tower is impossible.
In the Plaza of Miracles, it may indeed take one to keep the tower from falling.
Jane Morley, communications manager and project historian with Quinn Evans/Architects in the District, recently returned from examining the tower project site in Pisa.
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