Why the Leaning Tower of Pisa leans, and how it was stabilised

The short answer

The tower leans because the ground under it gave way while it was being built. Tuscany's tourism site attributes its inclination of about four degrees to subsidence of the ground beneath it that began in the early phases of construction.[1] It is the bell tower of Pisa's cathedral and one of the four monuments of the Piazza dei Miracoli, as a 2009 engineering paper on the tower puts it.[3] The square, under its formal name Piazza del Duomo, was inscribed on the World Heritage List in 1987.[2]

Tuscany's tourism site dates the start of construction to August 1173 and says the tilt began as early as the third storey, after which the builders carried on in the opposite direction to the lean. The same page says building went on, with two long interruptions, for about two hundred years.[1]

The sources do not agree on exactly when the lean began. Against the tourism site's third storey, the 2009 engineering paper says the tower started leaning southwards during the second construction stage, and Imperial College's consultancy arm says it first tilted in 1173, when the second floor was built.[5][3] XTOUR reports the three accounts as their sources give them.

Who built it

Tuscany's tourism site names three people. It attributes the founding of the building as a whole to Bonanno Pisano, the author also of the cathedral's bronze doors, and says the original design is presumed to be by Diotisalvi, who was building the baptistery at the time. The same page gives the bell chamber, on the seventh ring, to Giovanni di Simone in the fourteenth century.[1] XTOUR reports the Diotisalvi attribution as the site's presumption, not as a settled fact.

Why it kept leaning

The 2009 paper by John Burland, Michele Jamiolkowski and Carlo Viggiani, three geotechnical engineers, describes the tower's motion as "leaning instability": at a critical height, the overturning moment produced by a small increase in tilt equals or exceeds the resisting moment the foundation can supply. The same paper says that in 1990 the tilt was increasing by about six arc seconds a year, and that, besides creep, the most likely cause of the tower's progressive seasonal rotation was a groundwater level that fluctuated with the heavy rainstorms of September to December.[3]

The 1990 to 2001 rescue

In 1990 the Italian government appointed an international committee for the safeguard and stabilisation of the tower.[3] The authors' 2003 paper describes it as a multidisciplinary body of experts in art, restoration and materials alongside structural and geotechnical engineers.[4] Imperial College's consultancy arm says Burland was selected by the Italian government as part of a 14-strong committee.[5]

The first step was temporary. The 2009 paper says that in the second half of 1993, 600 tonnes of lead weights were placed on the north side of the foundation through a removable post-tensioned concrete ring, reducing the overturning moment by about ten per cent, and that the load was raised to 900 tonnes in September 1995.[3]

The lasting fix was underexcavation. According to the 2003 paper's abstract, after weighing several approaches the committee chose to remove small volumes of soil from beneath the north side of the foundation in a controlled way, and did so between 1999 and 2001.[4] The 2009 paper describes the extraction tubes installed just beneath the north side, the start of full underexcavation in February 2000 and the final extraction on 6 June 2001.[3]

How much it moved

The engineers' paper gives the correction as an angle. By the final extraction the tower had been rotated northwards by about 1,800 arc seconds, the half a degree the committee had aimed for, and with later drainage works the reduction reached 1,880 arc seconds by mid-2002, about ten per cent of the tilt at its 1993 maximum.[3] Imperial College's consultancy arm gives the result as a straightening of 45 centimetres, after 38 cubic metres of soil had been removed.[5] The two figures measure different things, an angle at the foundation and a distance, so XTOUR reports each as its source states it.

The sources also differ on when the tower reopened. The 2009 paper says it reopened to the public in December 2001 after being closed since early 1990, while Tuscany's tourism site says it reopened in the summer of 2001.[1][3] XTOUR has read both and does not choose between them.

What XTOUR does not tell you

This explainer covers the tower's engineering history, not a visit. XTOUR does not publish ticket rules, age limits, climbing conditions or opening hours for the tower; the Opera della Primaziale Pisana publishes ticket and opening-time pages on its own site.[6] The guide to Pisa's Piazza del Duomo sets out what those pages say. For Pisa as a place, see the Pisa destination page and the guide to choosing between Florence, Pisa and Siena.