Balancing inclines by Lucia Capitani

Destined to collapse with the passing of time, a solution to the mystery may now have been found. The Leaning Tower of Pisa, one of the Seven Marvels of our times, is famous for its five-degree tilt from the vertical, so that the top hangs around 4 metres over the base. The Tower first began to lean southwards at the end of the 12th century, when work on the fourth storey was under way, and over the centuries the building has sunk 3 metres in the ground. The entire area is in fact subject to a gradual subsidence of the soil and the ground underneath is like a water reservoir, given the proximity of the sea, which has since moved back and is now at a distance of over six miles from the town. Work was interrupted and then resumed around a century later. When the seventh circular storey was reached the Tower subsided again, making further building impossible. Finally, thanks to a correction to the top part of the building by means of an artifice still visible to the naked eye, almost 200 years after the foundations were laid the belfry was completed. While the Tower continued to subside, rotating by 1.2 millimetres per year, in 1990 an international committee of prominent experts was called in to save the world’s most famous bell tower. The Tower seemed fated to collapse because of the nature of the soil, composed of marine and river sediments, and also because of the strain on the building due to its inclination. The problem was so complex it required ten years of studies. Scientific tests revealed the risk of breakage at the level of the first cornice, under special stress, and plastic-coated steel cables 15 millimetres in diameter were wrapped around the critical point to prevent further widening of the cracks and maintain the integrity of the masonry. To reduce the lean, in 1993 a circular girder supporting a series of weights, sixty lead ingots weighing 10 tons each, was installed on the north side to counterbalance the inclination: the Tower was saved, but a state-of-the-art monitoring system capable of receiving up to 220 different signals every 5 minutes was set up to keep it constantly under control. The lingot strategy made it possible, for the first time in history, to stop and actually reduce by a centimetre the inclination of the Tower. At this point it became necessary to intervene exploiting the same balancing principle but eliminating the visual impact of the lead weights. A system of anchorages positioned at a depth of up to 45 metres was fixed to a concrete ring, this time located below the ground. As work on the underground ring proceeded on the north side, to the south and the east experts were forced to pump liquid nitrogen into the ground to freeze the earth. Then disaster struck: the Tower’s inclination suddenly increased and another 270 tons of lead had to be placed on the north side. The project was abandoned. Once the danger had passed, a definitive solution was found: the soil extraction method, involving the gradual removal of soil from under the north side of the Tower, causing it to subside a little and counterbalance the south side.
To strengthen the structure, cement was injected in the masonry and slim steel rods were inserted. Tie rods were used to secure the Tower, forming a sort of “lasso” that would tighten around the body of the building in case of danger. In 1999 soil extraction began: 41 drills gradually removed 38 cubic metres of soil and the Tower was moved back half a degree, crucial for its stability. The lead weights, the circular girder, drills and tie rods, being no longer needed, were dismantled one by one. In 2001 the Tower was opened to the public, but the restoration of its stonework is yet to be completed. The next step was to measure the temperature of the stone in different seasons of the years, observe the effect of the wind, monitor alterations caused by pollution and the presence of plants and lichen and even analyse the course of rainwater inside the building. To order, update and compare all the data gathered in years of studies the Scuola Normale di Pisa, subsidised by the European Community, has created a special data processing system called Akira that provides a perfect picture of the whole Tower, stone by stone, using state-of-the-art technology. Work on the Tower’s masonry began in July 2000, under the supervision of the Central Institute of Restoration, and is expected to be completed in 2006. In order to keep the Tower open to the public, an innovative scaffolding runs up the building without touching the marble surface, like an external lift, so that work can be carried out on only a single storey at a time, leaving the monument and its restoration exposed to the curious gaze of tourists.

Lucia Capitani