The Stabilization of the Leaning Tower of Pisa

Abstract

The stabilization of the Tower of Pisa has proved to be an immensely difficult challenge to civil engineers and architectural conservationists. The Tower is founded on weak, highly compressible soils and its inclination has been increasing inexorably over the years to the point at which it was in a state of leaning instability. Any disturbance to the ground on the leaning side would have been very dangerous, ruling out conventional geotechnical processes such as underpinning and grouting. Moreover, the masonry was highly stressed and at risk of collapse. The internationally accepted conventions for the conservation of valuable historic monuments, of which the Tower is one of the best known and most treasured, require that their essential character should be preserved, with their history, craftsmanship, and enigmas. Thus any invasive or visible intervention in the Tower had to be kept to an absolute minimum. Stabilization of the Tower was achieved by means of an innovative method of soil extraction, which induced a small reduction in inclination not visible to the casual onlooker. This technique has provided an ‘ultra soft’ method of increasing the stability of the Tower, which at the same time is completely consistent with the requirements of architectural conservation. Its implementation has required advanced computer modelling, large-scale development trials, an exceptional level of continuous monitoring, and daily communication and control. The Tower was re-opened to the public on 15 December 2001.