leaning tower of pisa

LEANING TOWER OF PISA-BRIEF CONSTRUCTION HISTORY

The Leaning Tower of Pisa is perhaps the most well-known architectural oddity in the world.  The construction of the tower occurred in three phases, spanning nearly two centuries, as war and social unrest mired the construction.  The evident tilt of the tower was first noticed during the initial phase of construction which began in 1173 AD.  Engineers tried to compensate for the tilt by making the columns and arches of the third story slightly taller on the sinking side; however, political unrest halted construction before they could continue to the fourth story.  Additional adjustments were attempted on the fifth and sixth stories, but construction was once again halted before they could continue to the seventh.  Nearing the completion of the tower in 1372, the builders made a final attempt to compensate for the lean by angling the eighth (top) story bell chamber.  Regardless of these attempts to correct for the lean, the tilt continued to worsen throughout the centuries.  Careful monitoring, however, didn’t begin until 1911, when measurements revealed that the top of the tower was actually moving at a rate of around 0.05 inch a year.  While the periods of unrest were no doubt a bane to the tower builders, it is very likely that had these construction respites not occurred and allowed time for the underlying soils to settle, the tower would have toppled well before completion

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LEANING TOWER OF PISA-THE CAUSE

The exact cause of the tilt was not fully understood until 2001, when a serious stabilization effort (which began in the 1990’s) was completed.  It was known prior to the start of this stabilization effort that the tower had been built atop an inadequate foundation (which was only 3 meters thick); and was constructed on very soft silty soil.  Had these been the only factors at work, uniform settlement of the tower could have been expected; and the city of Pisa would play host to a significantly less famous (albeit more vertical) tower.  The 800 year old mystery was finally solved by John Burland, an English geotechnical engineer, who discovered that the primary cause of the tilt was a fluctuating water table which would perch higher on the tower’s north side, causing the tower’s characteristic slant to the south.

LEANING TOWER OF PISA-STABILIZATION EFFORTS

The Leaning Tower of Pisa has undergone many repairs and renovations designed to keep it standing despite the obvious tilt.  The first modern attempt at stabilization of the tower occurred in 1935, when engineers attempted to seal the base of the tower by drilling a network of holes into the foundation and then filling them with a cement grout mixture.  However, this only worsened the problem by slightly increasing the lean.  The failed stabilization did result in more cautious approaches by future preservation teams.

In 1990, the tower was closed to the public and apartments and houses in the path of the tower were vacated for safety.  This was partially spurred by the abrupt collapse of another Italian tower (Civic Tower of Pavia) due to masonry degradation.  City officials were concerned that if the Tower of Pavia could collapse simply due to masonry degradation, then collapse of the Tower of Pisa, with its more than 5° tilt, must have been eminent.  In 1991, an international team of geotechnical engineers, structural engineers, and historians were gathered in an attempt to save the famous landmark.  Still, denizens of the city considered it very important to retain the tilt of the tower, due to the role that this element played in promoting the tourism industry of Pisa.  The preservation team (led by John Burland) finally took action in 1992 when the first story was braced with steel tendons, to relieve the strain on the vulnerable masonry; and in 1993 when 600 tons of lead ingots were stacked around the base of the north side of the tower to counterweight the lean.  While the preservation team assured city officials that these were only temporary measures until a permanent solution could be found, the moves infuriated the general public of Pisa who viewed the measures as eyesores that would be another blow to the tourism industry, which had already declined by about 45 percent since the tower had been closed to the public.  In response, in 1995, the team opted for 10 underground steel anchors, to invisibly yank the tower northwards.  However, this only served to bring the tower closer to collapse than ever before.  The anchors were to be installed, 40 meters deep, from tensioned cables connected to the tower’s base.  In view of Pisa’s high water-table, the team froze the underlying ground with liquid nitrogen before any anchors were installed, to protect their excavations from flooding.  However, it was not taken into account that water expands when it freezes.  The groundwater pushed up beneath the tower and, once the freezing had ceased, created gaps for further settlement of the tower.  On the night of September 7, 1995, the tower lurched southwards by more than it had done in the entire previous year.  The team was summoned for an emergency meeting and the anchor plan was immediately abandoned and another 300 tons of lead ingots were added in a desperate attempt to prevent the loss of the tower.

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Consensus within the preservation team did not come easy, but all eventually agreed that soil extraction was the only viable solution that would be acceptable to all concerned parties, as it had the advantage of not touching the tower itself, thereby placating the art historians.  The process involved the installation of helical drills surrounded by hollow steel casings to remove soil from below the high north side of the tower.  This would create a condition of controlled, localized, subsidence; and allow gravity to coax the structure back upright.  Work began in 1999 and halted in 2001 after approximately 77 tons of soil had been removed and the tower had been straightened by 44 centimeters, returning to its 1838 inclination.  While more soil could have been removed, the soil extraction program reduced the stress on the vulnerable first story enough to be safe, yet also maintained the distinctive lean of the landmark.  The team estimated that it would take approximately 200 years for the tower to return to its pre-stabilization inclination, and the tower was reopened to the public in December 2001.

In 2003, Mr. Burland introduced a new drainage system beneath the piazza’s north side; upon discovering that the root cause of the lean was a perched water table upon the upper silt layer below the north side of the tower, which fluctuated during the rainy season, sometimes coming within 12 inches of the surface.  The new drainage system addressed this condition, and is hoped to permanently alleviate additional movement.  The inclination continues to be monitored daily and revealed that the tower did not move at all between 2003 and 2009.  This is the first time in its history that movement of the tower has completely ceased.  Thus, the tower has been deemed safer than ever, likely to the chagrin of tourists hopeful to have been present when the tower finally took the plunge.

LEANING TOWER OF PISA-TECHNICAL INFORMATION

The tower currently stands at a height of 55.86 m (183.27 ft) on the low (south) side and 56.7 m (186.02 ft) on the high (north) side.  The weight of the tower is estimated to be 16,000 tons.  The tower currently leans at an angle of 3.97°, but leaned at an angle of 5.5° prior to the stabilization efforts in the late 20th to early 21st centuries.

Fun Facts

  • Pisa got its name in 600 BC from a Greek word meaning “marshy land”.
  • The foundation of the cemetery, Campo Santo (located adjacent to the tower in the Cathedral Square), is made up of 53 shiploads of earth that were brought back from the Hill of Calvary in Jerusalem”
  • During World War II, the Allies discovered that the Germans were using the tower as an observation post.  A U.S. Army sergeant sent to confirm the presence of German troops in the tower was impressed by the beauty of the cathedral, and thus refrained from ordering an artillery strike, sparing it from destruction.
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