The Leaning Tower of Pisa

The Leaning Tower of Pisa is a hollow, cylindrical structure with internal and external facings of hewed marble, and the cavity in between these is filled with coarse rubble and mortar (called ‘pouch’ masonry).  The stonework mass is surrounded by arcades of arches and columns, which stand on a drum base and are topped by a bell chamber.  The structure is therefore made of eight segments, called tiers.  A spiral staircase of 293 steps, set in the thickness of the stonework, leads to the bell-chamber.  The tower is 58.5 meters high form the plane of its foundation and weighs 14,500 tons.  The foundations are 19.6 meters in diameter; the mean pressure on the ground is 50.7 tons per square meter.  This figure is very high in ratio to the poor quality of the ground of the foundations.  The South side (leaning direction) of the second tier is submitted to a great amount of stress on the stonework; consequently, there is risk of fracture due to a crushing effect, and this could cause the Tower to collapse suddenly.

To lessen this risk, the critical area was encased in 1992 with gently taut, fine steel cables sheathed in plastic.  The final stages of solidification of the structure are now (1999) in progress; once these have been completed, the steel bands will be removed.

For as far as 40 meters deep, the ground surrounding the foundations of the Tower is of very poor quality.  The weight of the Tower has deformed the clay ‘platform’ to the extent that it has caused a basin-shaped depression under the construction itself.  In fact, the changeability of the ground in the foundation area is the actual cause of the inclination of the Tower.  To date, the degree of inclination is approximately 10% (5.5 sexagesimal degrees, or 20,000 arc seconds; the seventh cornice overhangs the first by 4.5 meters).  Calculations show that since the beginning of this century the inclination has constantly increased, at an average rate of 5 to 6 arc seconds per year (which corresponds to slightly over one millimeter per year increase in the overhang).  The Tower began inclining during its construction, which took place in three stages (with an interruption of about one century between each).  Between the years 1173 and 1178, Bonanno directed the work as far ad half way up the fourth tier, and from 1272 to 1278 Giovanni di Simone brought it up to the seventh cornice; during its latter period it began leaning towards South, as may be seen in the corrections imposed by the builders.  The bell-chamber was added on between 1360 and 1370 by Tommaso Pisano, son of Andrea, simultaneously to further counterbalance of the inclination which had accumulated in the meantime; in fact, the floor of the bell chamber has six steps on the South and only four on the North.  It has been calculated that the inclination of the Tower had already reached 1.6 degrees or approximately 6,000 arc seconds, by 1360.

Only a slight margin of safety separates the Tower from the risk of capsizing and collapsing.  In order to decrease this risk, ingots of lead, weighing a total of just under 1,000 tons, were attached to the monument on the North side in 1993.  This temporary expedient has decreased the inclination by about 50 arc-seconds (meaning over one centimeter less overhang) and arrested the progressive increase in inclination. The ultimate solidification of the Tower will entail a decrease in inclination of approximately a one-half degree (1,800 arc seconds), around 35 centimeters less overhang.  This means that the Tower will be brought back to the position it held three centuries ago, and in which it will remain stable.  This decrease will not be visible to the naked eye – hence the Tower will practically not change in appearance.

The decrease in inclination will be produced by the removal of earth towards the North of the monument and from underneath the Northern border of its foundation, by means of inclined, 15 centimeter diameter perforations, furrowed with special drills.

A first series of holes has already been drilled between February and May 1999, achieving a decrease in inclination of about 120 arc seconds (or, over 2 centimeters reduction in overhang).  At the same time, the removal of lead ingots began.  Prior to this procedure, two pairs of steel cables were installed and attached to the third tier as a preventative measure; if the necessity should arise, stabilizing forces may be applied with these cables.

The complete procedure foresees the removal of earth through 41 holes and the entire operation should last about 18 – 24 months; throughout the procedure all the lead ingots will be gradually removed and, at the end of the work, the steel cables will also be dismantled, leaving the monument free of the obstacles and ready to be admired by the whole world.