Feat of clay keeps Pisa’s tower at full tilt

THE peculiar conditions which caused the Leaning Tower of Pisa to veer to one side are also the reasons for its survival for eight centuries, British and Italian experts have discovered.

Engineers have been puzzled as to how the tower, which leans dramatically, could have survived four major earthquakes since building started in the 12th century. The answer is that it sits on a relatively soft, pliable strata of clay and sand. The soft foundations caused subsidence which gives the monument its five-degree tilt, but it also absorbed the energy from major seismic events, preventing the cylindrical tower from toppling over.

A research team of 16 engineers from Britain and Italy identified a phenomenon known as dynamic soil-structure interaction. They said the unique combination of the “height and stiffness of the tower” and the “softness of the foundation soil” has enabled the tower to survive. During major seismic events, the tower “does not resonate with earthquake ground motion” because the soil “causes the vibrational characteristics of the structure to be modified substantially”.

“Ironically, the very same soil that caused the leaning instability and brought the tower to the verge of collapse, can be credited for helping it survive these seismic events,” said Prof George Mylonakis, of the University of Bristol, a member of the team.