HISTORIC NOTE: Underexcavating the Tower of Pisa: Back to Future

Abstract

The stabilization of the Tower of Pisa is a very difficult challenge for geotechnical engineering. 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 is about to reach leaning instability. Any disturbance to the ground beneath the south side of the foundation is very dangerous; therefore the use of conventional geotechnical processes at the south side, such as underpinning, grouting, etc., involves unacceptable risk. The internationally accepted conventions for the conservation and preservation of valuable historic buildings, of which the Pisa 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 intrusive interventions on the tower have to be kept to an absolute minimum and permanent stabilization schemes involving propping or visible support are unacceptable and in any case could trigger the collapse of the fragile masonry.

In 1990 the Italian Government appointed an International Committee for the safeguard and stabilization of the Tower. It was conceived as a multidisciplinary body, whose components are: experts of arts, restoration and materials; structural engineers; geotechnical engineers. After a careful consideration of a number of possible approaches, the Committee adopted a controlled removal of small volumes of soil from beneath the north side of the foundation (underexcavation). The technique of underexcavation provides an ultra-soft method of increasing the stability of the tower which is completely consistent with the requirements of architectural conservation.

The paper reports the analyses and experimental investigations carried out to explore the applicability of the procedure to the stabilization of the leaning tower of Pisa. All the results being satisfactory, a preliminary stage of underexcavation of the tower has been carried out in 1999; the results obtained ....