The Leaning Tower of Pisa
Structural Engineer, June 2000

Structural Engineer (PISA, Italia, June 2000 )

The much-celebrated Leaning Tower of Pisa is actually leaning a bit less than it did just one year ago. An international team of engineers is at work (once again), trying to keep the 827-year-old structure from collapsing under its own weight. Their goal is not to raise this cathedral bell tower to a perfect 90-degree angle but only to move the tower a small amount - not enough to discourage tourists from visiting Pisa, Italy, however. Thus far, the operation has been a success, but these engineers have their work cut out for them as this tower has a history of teasing its builders with its instability. Construction began in August of 1173 and continued (with two long interruptions) for two hundred years. Despite the disjointed construction schedule, builders stayed true to the design of the original architect whose identity remains unknown.

Many efforts were made during its erection to stop the relentless tilting but to no avail. Countless columns and other parts have been fixed or replaced over the centuries, and the current tower project involves removing soil from the north side to ease the structure slightly back toward the vertical. Many reasons have been offered over the centuries as to why the Tower of Pisa persists in leaning. Early constructors theorized it was caused by the quality of the building materials. The current consensus, however, is that the soil is to blame. Recent aerial photographs have revealed the fact that a river once ran through the Piazza dei Miracoli where the tower now stands. The subsoil of the entire plain of Pisa is composed of geologically-recent lagoon and marsh deposits, and the irregular stratifications of sand and clayey silts is believed to be the cause of the inclination. From the beginning, builders tried to compensate for the tower's tilt. The correction made in 1360, during the construction of the belfry, is one example. To make up for the tilt at ground level, two additionaI steps were added to the south side. Other historical interventions include 600-ton lead weights, pumping water from the soil, and high-tension anchors placed underground (see chart for a time line). When the current project is completed, the Leaning Tower of Pisa should stand as much as 20 inches straighter than when engineers began removing soil just over a year ago. Their task involves drilling down into the soft soil on: the north side of the tower, As the soil is extracted, it leaves a cavity that closes quickly, and as a result, the ground above it subsides and takes the tower with it, According to the Italian media, the tower should reopen to the public in June 2001.

For more on the engineering history of engineering history of Italy,
check out the "In Civil History" in the June issue of CE News.