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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 . Initial stabilization commenced with the installation of temporary lead counterweights totalling roughly six hundred tonnes onto the northern side of the foundation . In addition , engineers secured the structure with steel tendons linked to ground anchors , which provided essential protection against sudden seismic disturbances while technical teams prepared a permanent remedy . The decisive breakthrough came through the implementation of underexcavation , a method devised by geotechnical engineers to extract soil without disturbing the fragile masonry . Specialists drilled forty - one inclined boreholes beneath the elevated northern base , carrying out the controlled removal of small volumes of clay . As these cavities gently closed under the weight of the structure , the northern foundation underwent controlled subsidence , causing the tower to rotate back toward verticality . The constant monitoring of foundation movements by electronic sensors confirmed that this localized reduction of subsoil volume was both gradual and reversible , preventing sudden shifts in the center of gravity . By the completion of the project in 2001 , the intervention had achieved a reduction of the tilt by approximately ten percent , drawing the summit back by more than forty centimetres . This engineering triumph ensured the stabilization of the campanile for an estimated two to three centuries without altering its iconic silhouette . When you examine historic architecture in your own city , how should municipalities balance public safety against the preservation of structural flaws that possess cultural value ? Have you ever visited a monument where the visible intervention of modern engineering enhanced your appreciation of ancient craftsmanship ?
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