Italian Pride "The Leaning Tower of Pisa"

The Leaning Tower of Pisa is not just some cranky Disneyland tourist attraction. It is an architectural gem and would be one of the most important monuments of medieval Europe even if it were not leaning. Standing in the Piazza dei Miracoli, it is part of the complex of four major gleaming white medieval buildings comprising: the Cathedral (Duomo), its bell tower (the Leaning Tower), its Baptistry and the Cemetery (Camposanto).
As with the other buildings in the Piazza, the bell tower was intended to represent the civic pride and glory of the wealthy city state of Pisa and as such it is beautiful, unique and enigmatic. In 1990 the Tower was closed to the public because of fears for its safety and in the same year a Commission was established by the Italian Prime Minister to implement stabilization measures. There can be no doubt about the importance of such an operation to Pisa, to Italy and to World Heritage.


The eight-storey tower is 53.3 m high above ground level and weighs 14,500 metric tonnes. Its masonry foundations are 19.6 m in diameter and have a maximum depth of 5.5 m below ground level. The foundations sloped towards the south at 5.5 degrees to the horizontal and in 1990 the seventh floor overhung the ground by about 4.5 m. Construction is in the form of a hollow cylinder surrounded by colonnades. The inner and outer surfaces of the cylinder are faced with tightly jointed marble but the material between these facings consists of mortar and stones in which extensive voids have been found. A spiral stairway winds up within the walls of the Tower. The stability of the masonry at second-storey level on the south side has been a matter of major concern.
The underlying ground consists of three distinct layers. Layer A is about 10 m thick and consists of variable soft silty deposits laid down in shallow water (lagoonal, fluvial and estuarine conditions) less than 10,000 years ago. Layer B consists of very soft sensitive marine clays laid down up to 30,000 years ago which extend to a depth of 40 m. This stratum is laterally very uniform. Layer C is a dense sand extending to considerable depth.
The water table in Layer A is between 1 m and 2 m deep. The many soil borings around, and even beneath, the Tower show that the surface of Layer B is dish-shaped due to the weight of the Tower above it. From this it can be deduced that the average settlement of the Tower is about 3 m, which shows how very compressible is the underlying soil.


The vertical axis of the Tower is not straight – it bends to the north. In an attempt to correct the lean, tapered blocks of masonry were placed at the level of each floor to bend the axis of the Tower away from the lean. Careful analysis of the relative inclinations of the masonry layers has revealed the history of the tilting of the Tower.

The internationally accepted conventions for the conservation of valuable historic monuments require that their essential character should be preserved, together with their history, craftsmanship and enigmas. Thus any invasive interventions on the Tower had to be kept to an absolute minimum and permanent stabilisation schemes involving propping or visible support were unacceptable – and in any case could have triggered the collapse of the fragile masonry. Any temporary stabilisation measure had to be non-invasive and reversible.
Temporary stabilisation of the foundations was achieved during the second half of 1993 by the application of 600 tonnes of lead weights to the north side of the foundations via a post-tensioned removable concrete ring, cast around the base of the Tower at plinth level. This caused a reduction in inclination of about one minute of arc and, more importantly, reduced the overturning moment by about 10%. In September 1995 the load was increased to 900 tonnes in order to control the movements of the Tower during an unsuccessful attempt to replace the unsightly lead weights with temporary ground anchors. The masonry problem was tackled in 1992 by binding a few lightly post-tensioned steel tendons around the tower at the first cornice and at intervals up the second storey.A permanent solution was sought that would result in a small reduction in inclination by half a degree, which is not enough to be visible but which would reduce the stresses in the masonry and stabilise the foundations. Given that the foundation of the Tower was on the point of instability and that any slight disturbance to the ground on the south side would almost certainly trigger collapse, finding a method of reducing the inclination was far from straightforward and gave rise to many heated debates within the Commission. Many possible methods of inducing controlled subsidence of the north side were investigated. These included drainage by means of wells, consolidation beneath the north side by electro-osmosis and loading the ground around the north side of the Tower by means of a pressing slab pulled down by ground anchors. None of these methods proved satisfactory.