Built on 3 m of fine sand over weak clay. Differential settlement on the south side caused the tower to tilt; engineers stopped construction for almost a century to let settlement plateau.
Foundations were sized for typical bearing pressure but the bell tower was placed on a heterogeneous soil profile. The south side settled into the soft Pancone clay layer faster than the north. By the time builders reached the third floor, the lean was already 0.2°. Construction halted for ~95 years, during which time consolidation slowed enough that work could resume — but the tilt continued for centuries until 20th-century stabilisation works.
Insufficient site investigation: the clay layer's low shear strength (ϕ ~ 24°, c ~ 35 kPa) was unknown. Modern bearing-capacity formulas with shape and depth factors would have predicted FoS < 1.5 even at the third-floor load.
Multi-layer soil bearing capacity with consolidation settlement check at the third-floor load level.
Open Geotechnical Analysis