The Tower of Pisa (popularly known as the Leaning Tower and, in Pisa, the Campanile or the Tower ) is the bell tower of the Cathedral of Santa Maria Assunta , in the famous Piazza del Duomo ( a UNESCO World Heritage Site since 1987 ) of which it is the most famous monument due to its characteristic slope, a symbol of Pisa and one of the iconic symbols of Italy. It is a free-standing bell tower 57 metres high (58.36 metres considering the foundation level) [ 1 ] built over the course of two centuries, between the twelfth and fourteenth centuries. With a mass of 14,453 tonnes, [ 1 ] [ 2 ] the curved line predominates, with blind arches and six floors of loggias. The slope is due to a subsidence of the underlying ground that occurred already in the early stages of construction. [ 3 ]
The inclination of the building measures 3.97° with respect to the vertical axis. [ 4 ] [ 5 ] The tower is managed by the Opera della Primaziale Pisana , [ 6 ] the body that manages all the monuments in the Piazza del Duomo in Pisa. It has been proposed as one of the seven wonders of the modern world .
History
The construction
The Leaning Tower of Pisa in the 19th century
Work began on August 9, 1173. The foundations were left to rest for a whole year. Some studies attribute the project to the Pisan architect Diotisalvi , who was building the baptistery at the same time . Construction was stopped during the beginning of the third ring.
The analogies between the two buildings are in fact many, starting from the type of foundations. Others instead suggest Gherardi , while according to Vasari the works were started by Bonanno Pisano . [ 7 ] Vasari's thesis was considered valid especially after the discovery of a tombstone with the name of Bonanno near the bell tower, [ 8 ] walled up in the atrium of the building; furthermore in the nineteenth century an epigraphic fragment of pink material was found in the vicinity, probably a cast on which a metal plate was cast, which is placed on the jamb of the entrance door of the building. On this fragment one can read, reversed: "Pisan citizen named Bonanno". This cast was most likely related to the royal door of the Cathedral, destroyed during the fire of 1595 .
The first phase of the works was interrupted halfway up the third floor, due to the subsidence of the ground on which the base of the bell tower stands. The subsidence of the ground, consisting of soft normally consolidated clay , is the cause of the leaning of the tower and, although to a lesser extent, of all the buildings in the square.
Work resumed in 1275 under the guidance of Giovanni di Simone and Giovanni Pisano , adding three more floors to the previous construction. In an attempt to straighten the tower, the three
added floors tended to curve in the opposite direction to the slope. The bell tower was completed in the middle of the following century, adding the bell chamber .
The restorations
Since its construction the overhang has substantially increa h stability or even reduction of the slope. During the nineteenth century the bell tower was subject to important restorations, which led, for example, to the isolation of the base of the tower. The works, carried out under the direction of Alessandro Gherardesca , contributed to definitively disproving the theory, supported by some scholars of the time, according to which the bell tower had been thought to be leaning since its origin. [ 9 ]
Lead counterweights in 1998
In fact, soil tests carried out during the restoration revealed the presence of a notable quantity of underground water which made the ground yielding. To deal with this problem, large quantities were sucked up from the subsoil with the help of pumps, but this favoured the phenomenon of subsidence and the consequent increase in the slope of the tower. [ 10 ] In the last decades of the 20th century the inclination had undergone a significant increase, so much so that the danger of collapse had become concrete. In 1993 the displacement from the top of the axis to the base had been estimated at approximately 4.47 metres, [ 11 ] or approximately 4.5 degrees . [ 12 ]
During the consolidation works, which began in 1990 and ended at the end of 2001 , the slope of the bell tower was reduced by encircling some floors, temporarily applying steel tie rods and lead counterweights (up to 900 tons) and under-excavation, bringing it back to what it presumably had 200 years before. Furthermore, the base was consolidated to allow the tower to be kept safe for at least another three centuries, thus allowing access to visitors. Starting in 2004, the restoration of all the external stone surfaces and the restoration and preparation of the internal rooms began. Some of these interventions were carried out thanks to the funds of the Lotto game , in accordance with the provisions of law no. 662/1996 [ 13 ] .
The stone restoration
The tower in the 1990s