The Geotechnical Stabilization of the Leaning Tower of Pisa

The Geotechnical Stabilization of the Leaning Tower of Pisa

The construction of the freestanding bell tower in Pisa began in 1173 on an unstable foundation of alluvial silt and marine clay. Differential settlement of the unconsolidated subsoil became apparent during the erection of the third tier, initiating a gradual southward tilt that accelerated over subsequent centuries. By 1990, after the catastrophic collapse of the historic Civic Tower of Pavia, the Italian government ordered the immediate closure of the monument to the public. Structural analysis revealed that the inclination had exceeded five and a half degrees, placing the masonry under severe shear stress and raising imminent fears of complete structural failure.

The mitigation of this collapse required an unprecedented synthesis of civil engineering and historic preservation. An international commission evaluated various intervention proposals, rejecting chemical soil injection and underpinning with heavy masonry because previous attempts in the twentieth century had inadvertently worsened ground displacement.