Leaning Tower of Pisa: Why the Medieval Bell Tower Tilts

The Leaning Tower of Pisa is the free-standing bell tower, or campanile, of Pisa Cathedral in Tuscany, begun on 9 August 1173. Its foundations started to sink unevenly within a few years, and the tower was finished nearly two centuries later with a lean that builders could not remove. Today it is one of the most recognisable buildings in the world and a classic case study in soil mechanics and engineering rescue.

Background: Pisa at the height of its power

In the eleventh and twelfth centuries Pisa was one of the leading maritime republics of the Mediterranean, alongside Genoa and Venice. Its fleets traded with North Africa, Sicily and the Levant and took part in the Crusades. In 1063 a Pisan fleet attacked Muslim-held Palermo, and tradition holds that the booty helped pay for a new cathedral.

Work on the cathedral began in 1064 under the architect Buscheto, and the round baptistery followed from 1152, designed by Diotisalvi. A bell tower was the next element of this monumental group, built on a meadow at the north-western edge of the city. The ensemble, later called the Piazza dei Miracoli (“Square of Miracles”), became a UNESCO World Heritage Site in 1987.

The site had one serious drawback. It lies on an old river plain, where the ground consists of layers of soft clay, silt and sand deposited by rivers and the sea. Such soils compress slowly under heavy loads.

Construction: three phases over two centuries

Building started in August 1173. Contemporary inscriptions give the year as 1174, but Pisa used its own calendar, in which the year began on 25 March of the previous year, so the date corresponds to 1173 in modern reckoning. The design was a hollow cylinder of stone with a white and grey marble facing, encircled by arcades of columns on each storey.

The first architect is uncertain. Bonanno Pisano, a sculptor who cast bronze doors for the cathedral, has traditionally been named, but Diotisalvi and Gerardo di Gerardo have also been suggested. By about 1178, when work reached the third storey, the ground beneath was already giving way and the tower had begun to tilt.

Building then stopped for nearly a century, mainly because Pisa was engaged in wars with Genoa, Lucca and Florence. This long pause proved fortunate: it allowed the clay under the tower to consolidate and gain strength. Had the builders continued straight away, the structure would very probably have collapsed.

Work resumed in 1272 under Giovanni di Simone. By then the tower was leaning to the south. The masons tried to compensate by making the new storeys slightly taller on the south side, so the tower actually curves gently like a banana. The extra weight, however, made the tilt worse. Construction stopped again in 1284, the year Pisa’s fleet was crushed by Genoa at the Battle of Meloria.

The seventh storey was finished around 1319. The bell chamber at the top was completed in about 1372 by Tommaso di Andrea Pisano, who again built it angled to offset the lean. The tower eventually received seven bells, each tuned to a note of the musical scale.

Architecture and design

The tower is a Romanesque design that matches the cathedral and baptistery beside it. Above a ground storey of blind arcades rise six open galleries, each ringed by slender marble columns supporting round arches. The bell chamber at the top is narrower than the storeys below.

Structurally the tower is a hollow masonry cylinder. Its walls consist of an inner and an outer facing of stone with a core of rubble and mortar between them, and a spiral staircase runs up inside the thickness of the wall. At the base the walls are around 4 metres thick, narrowing higher up.

This construction proved both a strength and a weakness. The tower is heavy and stiff, which helped it survive centuries of tilting, but the masonry at the lower storeys is under great pressure on the south side. Modern engineers feared that the stone there might crush and fail even before the tower toppled.

Key people

• Bonanno Pisano: a twelfth-century Pisan sculptor traditionally credited as the first architect, although the attribution is uncertain.
• Diotisalvi: the architect of the baptistery, sometimes proposed as the tower’s original designer because of similarities with his other work.
• Giovanni di Simone: directed the second phase of construction from 1272 and tried to correct the lean with taller storeys on the south side.
• Tommaso di Andrea Pisano: completed the bell chamber in the 1370s.
• Michele Jamiolkowski and John Burland: geotechnical engineers who led the international committee that stabilised the tower between 1990 and 2001.

Why the tower tilts

The answer lies underground. The tower weighs about 14,500 tonnes, yet its ring-shaped foundation is only about 3 metres deep and roughly 19.6 metres across. Beneath it lie soft, compressible clays, and the soil under the southern side is slightly weaker than under the northern side.

As the walls rose, the foundation pressed into the ground, and the south side settled more. Once a tall building starts to lean, its centre of gravity shifts toward the low side, adding pressure there and encouraging further tilting. Engineers call this “leaning instability”: the problem is not that the stone is weak but that the soil cannot hold the tower perfectly upright.

Over the centuries the lean increased slowly. In 1838 the architect Alessandro Della Gherardesca dug a walkway around the base so visitors could see the decorated plinth; this disturbed the ground and caused a sudden increase in tilt. In 1934 cement grout was injected into the foundation under the Fascist government, which also made matters worse.

Saving the tower

By the late twentieth century the tower was leaning by about 5.5 degrees, with its top roughly 4.5 metres out of vertical, and the tilt was increasing by about a millimetre each year. The Italian government had sought solutions since the 1960s. In 1989 a medieval civic tower in Pavia collapsed suddenly, killing four people, which added urgency.

On 7 January 1990 the Leaning Tower was closed to the public. An international committee chaired by the Italian geotechnical engineer Michele Jamiolkowski, with the British engineer John Burland among its leading members, studied the problem. The bells were tied, and steel bands were wrapped around the lower storeys to stop the masonry cracking.

In 1993 about 600 tonnes of lead ingots were placed on a concrete ring on the north side as a temporary counterweight, later increased to around 900 tonnes. The decisive measure, carried out between 1999 and 2001, was soil extraction: drills slowly removed small quantities of earth from beneath the north side, allowing that side to settle gently. Temporary steel cables held the tower during the work.

The method straightened the tower by about 45 centimetres, returning it roughly to its position in the early nineteenth century. The tower reopened to visitors in December 2001. Monitoring since then has shown that it is stable, and engineers have said it should remain safe for at least 200 years. The aim was never to make it vertical, since the lean is the reason millions come to see it.

By the numbers

Myths and facts

• Myth: The tower was designed to lean. Fact: It was meant to be vertical; the tilt began during construction because of the soil, and builders spent centuries trying to compensate.
• Myth: Galileo proved his law of falling bodies by dropping balls from the top. Fact: The story appears only in a later biography by his pupil Vincenzo Viviani, and there is no contemporary evidence that the experiment took place.
• Myth: The tower is the only leaning building in Pisa. Fact: The bell towers of San Nicola and San Michele degli Scalzi in Pisa also lean, for the same geological reasons.

Timeline

• 1064: Construction of Pisa Cathedral begins.
• 1173, 9 August: Foundations of the bell tower are laid.
• c. 1178: Work reaches the third storey; the tower begins to tilt and building stops.
• 1272: Construction resumes under Giovanni di Simone.
• 1284: Work halts again after Pisa’s defeat at the Battle of Meloria.
• c. 1319: The seventh storey is completed.
• c. 1372: The bell chamber is finished by Tommaso di Andrea Pisano.
• 1838: Excavation of a walkway around the base increases the lean.
• 1990, 7 January: The tower is closed to the public for stabilisation work.
• 2001, December: The tower reopens after soil extraction reduces the tilt.

Why it matters

The Leaning Tower turned an engineering failure into a symbol of Pisa and Italy. Its long construction mirrors other medieval projects that took generations, such as Cologne Cathedral, and it stands in the city where Fibonacci’s Liber Abaci was written while the tower was still unfinished.

For engineers, the rescue of 1990 to 2001 is a landmark in geotechnical science. It showed that careful measurement and a slow, reversible intervention could stabilise a fragile monument without changing its character.

The Leaning Tower of Pisa reveals both the ambition and the limits of medieval builders, who raised a marble tower on ground that could not fully support it and then adapted their design over two centuries. Its modern rescue shows how science can preserve heritage. Few buildings so clearly show how geology shapes history.